Category Archives: Science
Nothing else that we see in nature is quite like it. It is essential to our survival and it is the Great Destroyer at the same time. It looks beautiful and magical; it’s ephemeral and yet leaves what it touches permanently changed. As a child I stared into fire wondering what it was or what the heck was going on there. When the ancient philosophers began to think about what all the stuff in the world was made of, it is unsurprising that many chose earth, water and air. But fire? Including fire as the fourth element seemed just the thing to keep the stuff in the world from being static, and in this they weren’t all that far off the mark. For a period of time, in the not too distant past, a popular theory about fire was that it consisted of something called phlogiston that existed in all combustible things, and that the flame and heat were the result of it’s being released. As strange as this may seem today, this idea was promoted by such great scientists as Joseph Priestley (1733-1804), the discoverer of oxygen. The truth is more interesting I think though, for fire is not a thing at all, but a process.
One of my great aims with this blog is to be able to explain science in a way that communicates my wonder at the natural world to non-scientists. The natural world is beautiful, and not just the way it appears and plays with our senses, but in understanding it. This beauty of understanding can reach down into our soul and help sustain us in an often difficult life. What appears below may appear technical at points, but if you stay with it, these concepts are enlightening (literally in this case). You may get excited when you see something later, in a totally different context, and you understand it because of a few simple and basic concepts. It’s worth the effort!
The ABCs of Fire
In order to underestand fire, there are three physical concepts that you will need to understand: light (a part of “electromagnetic radiation”), blackbodies, and a very simple bit of chemistry. “Electromagnetic radiation” and “blackbodies” may sound over your head, but I hope it won’t be soon. I pleed that you inform me if and where I lose you in the comment section below. I will amend this essay until it is comprehensible to all!
Today, physicists know a great deal about the way light behaves. They can predict nearly exactly what it will do and use it for all kinds of amazing purposes that make our lives easier. What it actually is, however, is a bit more difficult to know because what is happening with light is so utterly small that it lives far below our senses and it is literally the fastest thing possible in the Universe. Sometimes light acts like a “wave” in that it oscillates back and forth in its tiny world, and sometimes it acts as a “particle,” in that it comes in small packages of energy we can call “photons”. Whether light is a wave or particle has been one of the most contentious debates in all of science history. Aristotle thought of it as a wave while Democritus thought it was a particle. Later, with more evidence in hand, Isaac Newton went with the particle idea, while Christian Huygens showed it acted like a wave. The wave theory then gained the upper hand as scientists saw they could predict how light would behave if they used math that presumed it was a wave. Then came Einstein who proved it was a package of energy in 1905 (a simple thing for which he won the Nobel Prize – he never won the prize for his revolutionary work on Relativity). Enter the quantum physicists stage left. Lots of math. Amibguity (which is often called “duality”) on whether the new math supports either side or something else entirely. For us, let’s be content for now to think of light as a particle went it fits our needs and as a wave when it fits other needs. That’s pretty much what scientists did when they made your Ipad.
A: Light and the “Electromagnetic Spectrum”
Perhaps the most interesting thing about light to take away from this essay on fire, is that what we see as visible light is just a tiny portion of a huge spectrum of “electromagnetic radiation”. DO NOT be afraid of that word. Electromagnetic radiation is just a small packet of energy, called a photon, that travels as a wave. The distance from one wave “top” to the next is logically called “wavelength”. It so happens that the shorter the wavelenth, the more energy the photon particle carries and the longer the wavelength the less energy the photon carries. We will call electromagnetic radiation “light” in the remainder of this essay, to sound less technical, and then say that “visible light” is that little portion of light (EM) that our eyes are sensitive to and that we can see. It is time to look at a picture of the Electromagnetic Spectrum (or spectrum of light).
From this diagram it is obvious that the light we see is only a small portion of the light there is. This little sliver is expanded above to show how the colors of the rainbow are really light waves of different wavelenths. This range is very small, – 375 to 750 billionths of a meter for visible light. Helpful for our ability to see objects, much of this part of light tends to bounce off the objects around us. If you see an orange chair, you are seeing the light in the orange wavelenth that is bouncing off; the rest of the light in the visible range hitting the chair is absorbed.
Not a coincidence, if you ask me, our eyes have evolved to “see” only that part of the light spectrum that is at the peak wavelength that our sun gives off. Aliens who evolved around a different sun might have eyes that aren’t sensitive to our sunlight and they might be blind here.
What about light at other wavelenths? Is that useless to us just because we can’t see it? Oh no!! Light at a bit longer length is absorbed by our skin and we feel it as warmth. This is infrared light. A bit longer and you find light that we called “microwaves” and this wavelength is useful to warming our food and sending your cell phone signal. A bit longer yet and we find light that transmits radar, TV, then FM and AM radio. Obviously, at the long wavelenths of these radio waves, the light passes right through things we know as solid objects like your house, otherwise, your radio would not pick up a signal. Also, our eyes are certainly not senstive to this light. Can you imagine if you could “see” cell phone, TV and radio signals? Our vision would be a chaotic unhelpful mess!
On the other side of visible light we find light that has shorter wavelength and is more energetic. First up, we find ultraviolet light which is responsible for giving us a sunburn. Much of this light from the sun is absorbed by the ozone layer high up in the stratosphere. If the ozone layer were to suddenly disappear, we would certainly need to be wearing lots of sunscreen or skin cancer for the damage it would do would be rampant. If the ozone layer had never existed, however, I suspect that our skin would have evolved some protective measure by now, probably in the form of more pigments like melanin that would absorb it before it could do damage. It’s no accident that people indigenous to the equatorial regions have darker skin. More energetic than ultraviolet-rays, light is then called X-rays. This light is useful at finding cavities, where bones are broken, and where the bomb is hidden in that terrorist’s luggage. Too much X-ray light can cause internal damage to our DNA, not just to the skin layer. Finally, gamma rays are the most energetic form of light, given off by such high intensity sources of energy as the explosion of supernovae. They can be lethal to humans in a short time, but luckily, our atmosphere absorbs almost all of it. In space, however, we do not have this protection. The space suits and space stations humans live in out in space have to be specially designed to absorb this really high intensity light before it gets to the body.
So just how does light (EM) arise in the first place? Well, first, did you know that there is such a thing as the coldest temperature a thing can get? The temperature that a thing is depends on how fast the molecules, atoms, and even electrons in a thing are moving. There is no exact limit to how much movement that these things at the atomic level can have, so there is no one highest temperature. But there is a limit to the coldest temperature: this is simply where all atomic motion comes to a stop. This is called Absolute Zero which is defined as 0° Kelvin, -273.15° Celcius, or -459.67° Fahrenheit. There is nothing we encounter here on Earth that is that cold, however (except possibly approaching it in a laboratory). Everything in our world that we know of as “stuff” jiggles around at the atomic level, and so, it is above Absolute Zero. One type of movement that happens at this level is that electrons jump up and down from different energy levels. If an atom receives a photon of light energy at a specific wavelength, an electron jumps outward to a higher energy level. If the electron then jumps back down to where it was, it releases the same wavelength of light back out. Now, as I described in my essay on “Water,” each element has electrons that can only live at specific energy levels, or “orbitals”. So, if you were looking at an oxygen atom in isolation, there would be only specific wavelengths of light it could absorb or emit. This is how it is possible for astronomers, looking at light emitted from the farthest reaches of star’s atmosphere, can tell what elements are there: the elements present emit light in only specific wavelength bands. But in our world (and deeper in a sun) the atoms are not in isolation. They are constantly bouncing off each other, and the energy that is absorbed and emitted when this happens varies all over the map. So for substantial matter, not isolated atoms, the light energy gets smeared out into a spectrum that depends more on the object’s temperature. Any object that acts this way is called a blackbody. The sun is a blackbody with a surface temperature of 5778° Kelvin and it give off light that is largely in the visible spectrum as noted at the beginning of this essay. Here is a diagram which shows the amount (intensity) of electromagnetic radiation (light) for a blackbody at different temperatures.
At the temperature of the earth and the human body, most of the light given off is in the infrared range. Although you and I don’t glow when the light is turned off, if we had special glasses that were sensitive to infrared light then we could see each other. The army has developed special glasses that are designed to see exactly the spectrum of light that something about our body temperature gives off; a good way to see the enemy at night! Here are two pictures of a man with a plastic bag around his arm. On the top we see him with the lights turned on, as normal. On the bottom, however, the lights are off and it is pitch dark. The camera taking this picture is sensitive to light that a blackbody the temperature of 73.6°F to 93.4°F gives off (near that of the man). Notice that the infrared light from his arm travels right through the bag. His right temple is the warmest thing in the picture.
So I hope this discussion of blackbodies helps to understand where light comes from. Before I put these ideas together to create fire, we need to understand one final concept, and this I promise to keep simple.
The Chemistry of Fire
When two substance react with one another to create new substances, heat can either be absorbed or given off. Let’s look at these.
1. Substance A + Substance B + HEAT ENERGY = Substance C + (maybe other substances)
We can see that heat energy is taken from the environment when this type of reaction occurs. The environment becomes colder. If you held the substances in your hand while this was occurring, the Substance C would feel cold because it has taken heat from your skin. This type of chemical reaction is called an endothermic reaction (endo = in, within; therm = heat). Endothermic reactions cannot occur spontaneously because work must be done to gather the heat into the reaction. Consider that you plug your refrigerator into the electrical socket to make the machinery do this work so it can take the heat from the food.
2. Substance A + Substance B = Substance C + (maybe other substances) + HEAT ENERGY
In this case heat energy is released into the environment. This is called an exothermic reaction (exo = out). Exothermic reactions CAN occur spontaneously when the two substances touch, they may even explode!, or they may need a bit of energy to get them started.
To the right is a diagram of an exothermic reaction where a little energy is needed to get it started. An example of such a reaction would be when the oxygen in the air reacts with carbon compounds in paper (which comes from wood). One of the most famous novels by American author Ray Bradbury was titled “Fahrenheit 451,” which is the temperature at which paper spontaneously bursts into flames (book burning featured prominently in the story). At this temperature there is enough energy for the reaction between oxygen in the air and the carbon to just go. At lower temperatures, however, you will need something to get it started. Once the reaction is started, however, remember that it is releasing energy (the ∆H in the diagram) into its immediate surroundings, and as long as the temperature on the surface of the paper is over 451°F, the reaction will keep going. Do you see where this is headed?
Putting it All Together: Fire!
Now we have all the elements in place to explain what I wanted to know when I looked at fire as a boy. You probably think we have too much information, but I hope you will see why I explained the first bits.
Let’s consider a candle flame and a wood match. Neither the match or candle catch on fire at room temperature. If you rub a match just right, however, the friction adds a bit of heat to the tip which ignites at much less than the 451°F of paper because the chemicals there ignite more easily. The fire from this tiny explosion then provides enough energy to start a reaction between the oxygen in the air and the carbon in the wood. (Notice how this energy is called “activation energy” in the above diagram.) Being exothermic, this reaction keeps releasing heat, which then causes more oxygen to react with the wood, and on and on, until the wood is used up. That is, it becomes self sustaining. The energy released as the match stick reacts with air can then be used to make the candle wick (a combination of cotton and wax) spontaneously ignite by placing the burning match near it. This temperature is not all that far from the 451°F where paper ignites. Now just like the match, the reaction in the wick also releases heat, and that reaction also becomes self sustaining. It takes much longer for all the material in the candle to react with the air than the match does, and so candles are more useful to us; matches exists only to provide the activation energy for other things to catch fire.
OK, so now we have an ongoing reaction between the wick and the air occurring at the surface of the candle. As this exothermic reaction continues, the heat released has to go somewhere, so where does it go? Simply into the air in the immediate vicinity. Now let’s recall what a blackbody is. A blackbody emits more or less energy at different wavelengths depending on its temperature. Normally things like a candle and the air are cool enough that they don’t give off enough light at a wavelength we can see. As they get hotter, however, they will eventually get to the point where they begin emitting light in the part of the electromagnetic spectrum that is visible. This is why the flame shines and lights a room; because it is hot. The reason the flame flickers is because YOU ARE ACTUALLY SEEING THE AIR THAT IS USUALLY INVISIBLE, and that’s how air moves. Normally, we can feel the air blow across our skin, but we don’t usually see it.
You might wonder why the air does not simply glow in a sphere around the place on the wick surface where the reaction is happening and the heat is being released. This is because hot air expands and becomes lighter. That hot air, hot enough for us to see, rises up, and the surrounding cooler air rushes in to replace it providing both fresh oxygen for the ongoing exothermic reaction and at the same time molding the air into the familiar birthday candle flame shape shown in the picture at the start of this essay. About an inch above the reaction process happening on the wick surface, the air cools off enough that it becomes invisible to us again.
And that’s it folks. It is possible that many pyromaniacs, be they lovely experts in fireworks displays or heinous arsonists, don’t really understand what they are doing. But for me, at least, it is beautiful to know.
Water? An essay on water? Why? Do you mean that substance I wash my hands and clothes with? That fogs up my windshield on the inside when it falls from the sky on cool nights on the outside? That flows from my eyes when I feel strongly about something? That I ski across in it’s liquid form on summer days and ski over in it’s slippery solid form in winter? That courses through my body and makes me desire it ruthlessly when I don’t drink enough of it? That I don scuba gear and dive deep down into to see an entirely different world it rules? That floats in the sky and rains down to nourish the food I eat or to destroy entire towns with it’s amazing power? That substance that keeps land as far north as Europe warmer in winter than it should be? That was the matrix out of which myself and all other life sprang? Yes. Water. That substance.
Ack! No! A Wee Bit of Chemistry
Oh, I know! Many of you think you are not good at science and hate chemistry especially. Listen carefully. You were not taught properly. Please let me attempt to share a few basics that will help you see the world more beautifully in general, and water in particular. Here goes.
All matter that we know, things we can touch and that weigh something, are composed of atoms. Atoms are protons and neutrons in a nucleus surrounded by a swarm of electrons. Protons have a postive charge and weigh something, neutrons have no charge and weigh the same as protons, electrons have a charge equal and oppostie to the proton, but weigh little. Do you feel uncomprehending already? Good!! Because there certainly is something that looks like hocus pocus here; Harry Potter magic. By “having a charge” that means it possesses a force which can reach across space (a “field”) and attract something of an opposite charge or repel something of the same charge. The force here is called the “electomagnetic force.” By having a weight it means that there is a different force we call gravity that reaches across space (again) to attract any other thing that can carry the gravitation force. The magic here is that these forces reach across space somehow, sometimes called “action at a distance,” and what your chemistry teacher never explained to you is that no one knows how it is possible to do this. Not even Einstein knew how this happened, and though we can describe these forces in detail with mathematics, we do not know how they reach across space. There is probably something fundamental that we just don’t understand, but it certainly looks magical. To me, this is wondrous and beautiful. OK. Now, there are 98 types of atoms that exist naturally and these are called “elements”. Elements are the smallest thing that cannot be naturally broken down any further and yet retain it’s unique characteristics. This is determined by the number of protons in the nucleus. For example, hydrogen is the lightest element with only one proton, oxygen has eight protons, and iron has 26 which is why it is relatively heavier. There seems like more than 98 types of stuff in the world though, and that is correct because these elements can combine in different ways to form compounds, also called molecules. Water is such a compound; it has one oxygen atom connected to two hydrogen atoms: H2O. Now there are a few different ways that elements can combine, or react with each other, or simply influence each other by being close. This is important:
Types of Bonds Between Atoms and Molecules
The covalent bond is the strongest bond elements use to combine to form compounds, and it is how the hydrogen atoms are attached to the oxygen atom in water. This is how it works, simply (yes, it gets deeper; if you find yourself asking why, email me!). Around each nucleus in an atom there are “shells” that can contain a certain number of electrons. (find out why and win the Nobel Prize). The first shell is full with 2 electrons. The second with 8 electrons, the third with 18, and the fourth with 32, etc… (that covers the first 60 elements). The first and second shells are stable only if they have two or eight electrons respectively. The third shell is stable with 8 or 18 electrons. Now let’s look at water, H2O, with two hydrogen and one oxygen atom. Stay with me here. Oxygen with its eight protons, has eight electrons to be electically balanced, but what about it’s shells? Two in the first shell and six left over in the second shell. But it wants eight in the second shell!!! Bad. This is what makes one plain oxygen atom reactive. How about the hydrogen? They only have one proton and one electron. They are missing one in their first shell. If one hydrogen atom gets together with an oxygen atom they can share one electron and the hydrogen will be stable but the oxygen is still missing one. This is called hydroxide and it is still reactive. Now, if another hydrogen atom comes along to share an electron it too is now stable as is the oxygen now. This is water. And water is pretty stable. This type of sharing of electrons, as in water, is an example of the covalent bond.
The ionic bond is the next strongest bond. In this type of bond one atom doesn’t share an electron with another atom – they completely give it up. One familiar molecule that forms this way is table salt, NaCl, or sodium chloride. Sodium has eleven protons and electrons – one too many electrons to fit into the first and second shells. Chlorine has 17 protons and electrons and is lacking one electron in its third shell that would make it stable. So the sodium atom completely gives up one electron to the chlorine atom. Now their outer shells are stable, but the atoms have electromagnetic charges: sodium now has 11 protons and just 10 electrons so it has a positive charge of +1. Chlorine now has a -1 charge. These two atoms are now electromagnetically (magically!) attracted to each other. But not as strongly as if they were sharing the electrons; they can be broken apart fairly easily.
The hydrogen bond is the last bond we will discuss here and it is the weakest. Some atoms, like nitrogen, oxygen and fluorine have a great ability to attract electrons, for reasons beyond the scope of this discussion. So in the water molecule the oxygen atom kind of “hogs” the electrons it is sharing covalently with the hydrogen atoms. Furthermore, the two hydrogen atoms don’t
position themselves on opposite (180 degrees apart) sides of the oxygen atom as one might, perhaps, suspect. But are just 105 degrees apart. This asymmetry, combined with the way oxygen monopolizes the electrons, causes the “hydrogen” side of water to be a bit positively charged and the oxygen side to be slightly negatively charged, as seen in the picture at right. This asymmetry of charge is why water is called a dipole molecule, and it is responsible for many of water’s interesting qualities. Notice how if another water molecule came along a hydrogen would like to be near the oxygen atom; electromagnetically, opposites
attract. This is the hydrogen bond. Water is the only known substance which is capable of forming four of these hydrogen bonds; two hydrogens attracted to the oxygen (in addition to the two covalently bonded ones) and an oxygen to each of the hydrogens. This is shown in the figure on the left which illustrates how the 105 degree angle between the hydrogens results in a tetrahedral (pyramidal) shape when water molecules gather together. In addition, also notice how if a salt molecule came along with it’s negative and positve sides, water might interfere. Stay tuned.
Water Vapor, Liquid Water and Ice
We are just about to get to the more interesting impacts that all this chemistry has on water as we know it, so hold on just a bit longer. The title of this paragraph indicates the three main states that a substance can be in, and in this case, water. Whether water is a vapor, a liquid or a solid depends on how much kinetic (jiggling) energy the molecule has. Temperature is just a measure of the average jiggling of all the molecules. In hot water vapor the molecules are jiggling madly, while in cold ice they are more sedate. There is one other thing that effects what state water is in and that is the pressure it is under. Let’s see how this works. If a water molecule has a great deal of energy and is jiggling strongly all about, it is not going to want to form hydrogen bonds with its neighbors. It will be banging about solitarily and in this form water is a gas, or vapor. Water vapor is invisible. Cool it down, specificaly to 212˚F, and it starts to form hydrogen bonds and it becomes what we know as liquid water. In liquid water, the hydrogen bonds are constantly breaking and reforming and even the covalently bonded hydrogen atoms are occasionally breaking away from the oxygen and bonding with a new oxygen. The water molecules are slipping and sliding all over one another. Just to make sure you are paying attention, here is a question. When you see steam rising out of a teapot or a cloud floating in the sky, are you looking at water vapor or a liquid? Yes, these are liquid forms of water; tiny droplets floating and buffeted about by the air. Look closely at a boiling teapot sometime. For a short distance above the spout you see nothing, then the steam appears as the vapor condenses to a liquid. This is the same as what happens in the sky when clouds form. As the liquid water continues to cool off, to 32˚F, the
hydrogen bonds become much stronger because the hydrogens have less energy to break away, and a cyrstalized form of water emerges which we know as ice. Because of the unusual shape of the individual water molecule and the tetrahedral shape that occurs when they bond together there is quite a bit of space inside the structure of ice. This is illustrated by the picture on the right. We’ll see how this impacts fish.
There is another way the hydrogen bonds profoundly affects water: it makes it able to store a lot of heat. Scientifically, it is said to have a “high specific heat capacity.” But it can be explained easily in layman’s terms. You might think that heat and temperature are the same thing, but they are not. Whereas, temperature is an average measure of how fast the molecules are jiggling, heat is the amount of energy it takes to make them jiggle in the first place. The hydrogen bonds in water make the molecules hold themselves together more strongly and it is harder to make them jiggle. You have to put a lot of heat into water to make it feel warmer: it takes a lot of energy to warm up a pot of water to boiling and a lake after winter before you can swim in it. Imagine a pail of water in the driveway next to your car under the full sun. Which in your experience will feel hot more quickly? Furthermore, there is something else called the “heat of fusion (freezing)” and “heat of evaporation”. Let’s start with an ice cube and gradually give it heat energy. Making the change from ice to water at 32˚F requires an extra boost of heat energy absorbed. Then you need exactly the same amount of heat for each degree increase in temperature up to the boiling point. To convert from liquid to gas then requires an even bigger heat energy boost than the ice to liquid transition. You may be confused. Let’s think of some real life examples with the weather. Suppose it is 35˚F and it is snowing. The snow will begin to melt while falling and this change will suck up a lot of heat, the heat of fusion, from the surrounding air. This will quickly cool the air to 32˚F where the melting will stop. Now imagine a thunderstorm. It is hot until the rain starts to fall. Then as the rain evaporates on its way down it absorbs a lot of heat, the heat of vaporization, from the surrounding air and this air again cools off (this time to the dew point). Once the sun comes back out the air resists warming much because of all the heat of vaporization absorbed converting the water back to vapor. After the ground is dry, then the air warms quickly. None of these dramatic effects, which you may not have noticed before, but I hope you do next time, would not be possible if it weren’t for the hydrogen bonds.
I mentioned above that temperature reflects only an average of the jiggling of the molecules. If ice has a temperature of 25˚F for example, there may still be a few molecules here and there that are jiggling wildly. They may be jiggling so wildly in fact. that these peculiarly energetic ones near the surface may jump right past liquid into a vapor. This conversion from ice directly to a vapor is called sublimation. I’m sure that you have had the experience of a snowstorm followed by many dry days. But while I’m sure you noticed yourself shoveling out of the driveway, I wonder if you noticed the snow slowly disappearing in the following days. Given enough time, the snow would disappear completely.
How Fish Survive Under a Frozen Lake
If you look back at the crystal lattice structure of the ice shown above, notice that there is a quite a bit of space between the tetrahedrally connected molecules. But in liquid water the molecules are just slipping over each other. Water is, in fact, unusual in that the solid form is less dense than the liquid form. Most substances get denser and denser as they get colder, but because of water’s asymmetric shape and the hydrogen bonds, ice will float on top of liquid water. In fact, water happens to be most dense at 39˚F.
So what does this have to do with fish? Imagine a lake as winter approaches and the air above it cools. Gradually, some of the heat from the top layer of the water escapes to the air and this layer cools. As it cools, it gets denser (like most substances) and sinks to the bottom. Assuming winter conditions above, this process continues until the entire lake water has cycled from top to bottom and all the water is at 39˚F. But this is as dense at it gets. Then as the top layer gets colder it gets lighter and remains on the top. When it reaches 32˚F it begins to freeze and form ice, which floats. In really cold winters some heat does escape through the ice layer and the ice thickens, but in general, ice, with the empty space between the molecules and the hydrogen bond structure, is a very good insulator. The fish remain unfrozen and swimming freely at a balmy 39˚F on the lake bottom. Imagine if water were like most substances and got denser as it froze. The entire lake would become a solid block of ice with well preserved, and dead, fish. In my essay “The Man in the Moon” under Memoirs, I tell of times in the past when the surface of the earth froze over all the way to the equator. If the ice had reached all the way to the bottom it is unlikely life would have survived, or at least life beyond a single cell. The evolution of intelligent life (does that mean us?) would have had a nearly impossible hurdle to overcome.
Boiling an Egg in Denver and the Importance of the ‘Triple Point’ to Life on Earth
Before I remarked that the freezing point of water was at 32˚F and the boiling point was at 212˚F. But unfortunately it is a bit more complicated than that. That’s at sea level, where the air pressure is equal to the weight of the entire atmosphere above it (yes the atmosphere weighs something – about 14.7 lbs/sq. inch in fact!). Obviously, as you increase your altitude you leave some air below you, there is less air above you and the air pressure decreases. Now in a pot of tap water at about 75˚F the “75”, remember, only represents the average jiggling of the water molecules. There are always some moving more slowly (colder) and some faster (hotter). Some, if they are on the surface, even have enough energy to jump right out into vapor. The combination of all these molecules ready to jump out and become vapor even at an average temperature of 75F is called it’s vapor pressure. At 212˚F, and at sea level, the vapor pressure of the water is equal to the atmospheric pressure and the water starts to vaporize, or form bubbles even within the liquid.
It starts to boil. Obviously then, vapor pressure and atmospheric pressure become equal at a lower temperature as you rise in altitude. The approximate value is that the boiling point of water decreases 2˚F for each 1000 ft. rise. Denver, the Mile High City, at 5280 ft., hence has a boiling point of about 10˚F less, or 202˚F. You better boil your eggs longer there because you won’t be able to get your water boiling hotter than this except in a pressure cooker. In fact, cooks in Denver should we wary of all their cooking recipes! Interestingly, in a soda bottle the pressure of the air in the little bubble at the top if far greater than sea level pressure. So when you take off the top the pressure on the soda suddenly decreases, and all kinds of bubbles form. What it is doing actually is boiling. Only in this case, the vapor is carbon dioxide and not water vapor.
The diagram above is called a phase diagram and shows the relationship of the solid, liquid, and vapor forms of a substance to the pressure and temperature. There are a couple of interesting things to point out here which I don’t mention above. One is the triple point, which is the place where solid, liquid and gas can all exist at the same temperature and pressure. For water this is at about 32˚F and about 1/100 of sea surface air pressure. On earth, this is quite high in the atmosphere, but is important to the hydrologic (water) cycle because water vapor is so easily converted to ice crystals in large amounts there. In the general scheme of things, astronomers (or astrobiologists) look for planets where the triple point of water could occur because it indicates a “habitable zone” around a star for life as we know it. All three phases of water have played a role in the evolution of life on earth. Now look for a point on the graph called the “critical point”. This is a place so hot and under so much pressure that a new phase of matter comes into play which is somewhere between vapor and liquid. Hydrogen bonds don’t form well, so it’s not quite a liquid. Yet, the molecules are so tightly packed together they still slip and slide around each other as if they were a liquid. Surely, there is no place on earth where this would occur correct? No! In the deepest parts of the ocean, under the great weight of the water above it, are thermal vents where the water is super heated from the lava just below. This new phase occurs there. And there are serious evolutionary biologists who have studied the possibility that life on earth began in such a place. Certainly, there are critters that live there now.
Before we leave this scary looking diagram I’d like to shout out to any skate or skiing enthusiasts who might have wondered why ice is slippery. When an ice skate or ski (or foot or tire) places pressure on the ice surface and, furthermore, rubs over it, two things happen. The extra pressure causes the water to become denser, and we have already seen how water is densest at 39˚F which is above freezing. On the diagram we can see how the line between liquid water and ice drifts slightly to the left at increased pressure, showing a lower freezing point. Furthermore, the friction across the crystallized ice surface adds heat energy and breaks the top layer of hydrogen bonds. So what you have is a thin layer of water over a sturdy surface of hard ice – most excellent for slippage! For those who live in northern climates you may also have noticed that it is easier to drive across “crunchy” snow on very cold days than it is to drive across slushy snow on days when the temperature is warmer.
Water and Life
Another reason why astrobiologists, often look for water as one of their first orders of business is because water is known as the great solvent. This is due to it’s electric polarity discussed above. Any other substance such as sugars or salts which also have a strong electric charge become easy solutes (that which is dissolved) and are miscible, meaning easily mixed. Think of it this way. Table salt, NaCl, is formed by an ionic bond which we said is stronger than a hydrogen bond. So salt is able to squirm it’s way into water easily, breaking the hydrogen bonds along the way so that the very negative oxygen side of the water molecule can capture the positive sodium (Na) atom, and the negative chlorine (Cl) can hug the postive hydrogen side. Just a little stirring and the table salt is even distributed in the water. It is probably easy to see how this is crucial to life. Our blood, mostly water, carries all these ions throughout our body where they perform crucial functions. Sodium is crucial to blood pressure, the transmission of nerve impulses, and the transport of nutrients across cell walls. Potassium (K+) is also used in nerve transmission and aids in the concentration of urine in the kidneys so just the right amount of water is left in the body. Of course, sugar is vital to energy. These are just a few of a great many examples of the ions that water’s polarity carries throughout the body, which are then used to keep us alive. Fats and oils, on the other hand, have no charge and do not mix easily (they are immiscible) with water because they cannot break the hydrogen bonds. This is why it is so hard to mix oil and vinegar. Fats are also vital to human metabolism, however, and they too must find a way to get transported around the body in our (mostly water) blood. To do so, they wrap themself in a charged protein covering until they get to the place where they are needed. Interestingly, some vitamins and drugs have a charge, dissolve in water, and are removed easily by the kidneys. Two examples would be vitamin C and codeine. Other vitamins and drugs are soluble in fat, are not so easily transported to the kidneys, and last longer in the body. Examples of these include vitamin A, carotene, and marijuana (THC). This is why it is possible to overdose on vitamin A, but highly unlikely for vitamin C. It is also why people who consume too much carrot juice turn orange (the carotene stays in their fat tissue) and why marijuana shows up in a drug test long after a person has smoked pot, but that codeine abusers might pass a drug test more easily after a short period.
Besides it’s polarity, water helps to sustain life by it’s high specific heat as well. One of the most important things for our physiology is that our internal conditions remain the same; this is known as homeostasis. Temperature is one of these conditions and it should stay around 98.6˚F for our biochemistry to work properly. We all know how we feel when we get a fever, but of course, being too cold on the inside is equally detrimental. Water, which comprises about 65 – 70% of the human body is a stabilizing force for temperature homeostasis because it is so slow to heat up or cool down. Also, remember the large amount of heat absorbed when liquid water converts to vapor? When we get hot and sweat, much this heat of vaporization is taken directly off our skin and helps to cool us back down. Animals like dogs that don’t sweat must find other ways of getting rid of extra heat and in this case they pant. Panting causes evaporation of water out of the mouth and lungs.
Surface Tension, Raindrops and Bugs That Walk on Water
As I’m sure you have noticed by now, it is impossible to explain the curiousness of water without an appreciation for hydrogen bonding. Well, it arises again with the subject of water’s surface tension and cohesiveness. But what are these things exactly? I’m sure you have noticed how a drop of water on the kitchen counter does not immediately flatten out and spread across the surface. It sits there like a little half sphere with a curved top. This is cohesiveness. You have also seen bugs that can walk across the surface of the water without breaking through. This is surface tension and these two concepts are related. Let’s look at the picture below right. Under the surface of water, the hydrogen bonds form in all directions around a molecule. This gives strength to the fluid and results in its cohesiveness, or ability to hold itself together. On the surface, however, there are no molecules above to form hydrogen bonds with, and hence, they form extra strong bonds with those molecules on the sides and below. This creates a thin film of extra cohesive water on the top which bugs (and Jesus?) can exploit to walk across.
Should the bug be unlucky enough to poke a leg through the top, however, it’s sunk! Now, knowing all that you do about water by now, what do you think happens to cohesiveness and surface tension as water warms up? Got it? The extra jiggling of the molecules makes the hydrogen bonds more fragile and both of these qualities decrease. Bugs have a harder time walking across water as it warms, and a drop of hot water on the counter will flatten out compared to a colder drop.
One of the most significant ramifications of water’s cohesiveness is it’s ability to form raindrops. Without this quality, the change from water vapor to liquid droplets in the sky would be much harder, and rain would not be produced as readily. As it is, there is a limit to how big a raindrop (which actually forms a pancake shape as it falls against the wind, unlike the classic shape we normally think of and as it is depicted in the picture at the beginning of this essay) can get. The largest drops ever recorded, under perfect conditions, are about 7mm in diameter. What are perfect conditions? One is a strong updraft which counteracts the wind that would occur if the drop were to start falling and provides fresh moisture, and the other is colder air, which we saw increases cohesiveness. Normally, a raindrop reaches maximum size at about 4mm where is starts to fall and is either broken up by the wind as it grows larger, or smashes against other drops.
Soap – Wash your hair! Clean your clothes!
Have you ever wondered how soap works? Well, we are ready to explain that! First lets look at a picture of a
smallpiece (a “micele”!) of soap (see right). Each molecule of soap has two parts: a negatively charged end that is attracted to the positive hydrogen side of a water molecule, and an end without any charge at all (non-polar) that likes other molecules without a charge like oil or grease. When lots of soap molecules get together the non-polar ends get adhere to each other and form a sphere with a negatively charged surface as shown in the picture. Normally, oil and water don’t like to mix. But soap acts as an emulsifier, which is something that can mix two things that normally stay separated. It can mix them because the inside of the micele of soap grabs some oil and the outside floats happily in the water. Soap is also a surfactant, which is something that reduces the surface tension of water and helps it to penetrate other substances. Now oil and grease tend to collect bits of dirt and grime like on my face after a long day. As I lather up my face with soap and water, the inside of the soap bits collect the oil, dirt and grime, the loss of surface tension helps it penetrate my pores, I rinse it off with more water, and voila, clean face. There may be a type of soap that works in a bit different way, but if there is, I don’t know of it.
The Hydrologic Cycle, Ocean Circulation and Why Europe is so Warm
We saw above how the circulation of water throughout the body, in the form of our blood (but also in lymph and interstitial fluid), is vital to our life because of it’s ability to carry so many nutirents and waste where they should go. But the circulation of water is important on other scales too. The temperature of our planet, so well positioned in the “habitable zone” around the sun, is nearly perfect for the circulation of water, in all it’s three forms, throughout the ecosystem.The diagram above is so comprehensive in illustating this circulation on the local scale that it almost makes explanation unnecessary. However, let me point out a few interesting points about this picture and which are less known. First, take note of the groundwater. You may not think of this as that important, but you certainly would not be here without it. In places with significant precipitation it gathers in aquifers, creates the possibility of wells, springs, and due to it’s high specific heat keeps the temperature more than 10ft. below ground at a stable 50˚F throughout the year, just as it helps keep our internal body temperature stable. About 20% of all the fresh water on earth is ground water, though only about .61% of all of earth’s water is since so much is in the salt water oceans. Now notice where it says “Fog drip”. Fog is really a cloud on the ground, so fog drip is a bit like rain, only the water vapor condenses directly onto things instead of forming drops midair. Interestingly, fog can provide enough water to sustain life.
The Atacama Desert in northern Chile is known as the driest place on earth, it literally never rains, and yet there are plants there that sustain themselves because of the frequency of fog that rolls in off the cold Pacific Ocean.
Now take a gander at the “Evapotranspiration” label. This is the total amount of water vapor entering the air from the ground/lakes (evaporation) and through the leaves of plants (transpiration). Transpiration from vegetation is more significant than one might suspect and can significantly increase the humidity where foliage is dense. This brings me to one of my favorite concepts in nature: that of a watershed. A watershed is any land area where precipitation can either re-enter the air through evapotranspiration, or find it’s way, through rivers or ground water, into a body of water named for that specific watershed. An example of this is the Great Lakes watershed shown below (which shows both the watershed for all the Great Lakes and also for each specific lake. In general, if evapotranspiration and drainage through rivers is greater than the precipitation falling in this area, the water level will fall, and vice versa. Notice how close the Great Lakes watershed comes to the edge of Lake Michigan near Chicago. All water falling just to the west does not end up in Lake Michigan but finds its way into the Mississippi River watershed.
On a global scale, the circulation of water in the oceans acquires significance for climate across the entire planet. Due to water’s high heat capacity, acting as a heat sponge (as you know well by now), cities located on the ocean tend to have more moderate climates than those cities located within the continent. This is especially true for cities on the western side of continents in the mid-latitudes than on the eastern side, since weather, in general, moves west to east. Compare Seattle to Chicago or London to Moscow and the difference in their winter temperatures. However, the surface of the ocean moves in regular patterns. In the North Atlantic and North Pacific, this pattern tends to be a giant clockwise gyre. To see the surface circulation of the oceans in real time press here for a really cool video. Though the circulation may vary from day to day one thing I’m pretty certain you will see and that is a river of (warm) water that comes out of the tropics in the Caribbean and heads north up the coast of the U.S. and then out across the North
Atlantic. This is commonly called the Gulf stream and it is responsible for the transport of lots of heat over to the European Continent during winter. Next time you are near a globe check out how far north the city of London is. London has a wet and chilly winter but trace a line to the west over to the North American continent. Imagine how cold London would be if it were located that far north in Canada. If not for the Gulf stream, I doubt this blog would even be written in English. In the North Pacific, we see a different effect of this ocean gyre and it is much sadder. Any trash that finds its way into this gyre tends to follow the path of least resistance and end up in the middle of the gyre. This patch,
discovered as recently as 1988, has grown to twice the size of Texas, and contains at least 46,000 pieces of floating plastic/sq.mile. This is indeed the world’s largest landfill.
Finally, there is indeed an even larger circulation of water on earth. No, the water does not just circulate and spin upon the ocean’s surface. It also overturns, dropping down to the ocean floor in spots and then crawling along the ocean floor until later rises back to the top. This is the thermohaline circulation (thermo = heat; haline = salt). What causes it to rise and fall is the density of the water, just as we observed in the lake before, only this time the situation is complicated by the salt content. We have seen how salt, or sodium chloride is easily and evenly mixed in water, and we have also seen how sodium with eleven protons and chlorine with seventeen protons are heavier than oxygen with eight and hydrogen with one. So saltier water is heavier. Imagine the warm water of the jet stream moving along the surface towards the North Atlantic. It will slowly give off heat energy and cool off. Some may end up freezing, and when salt water freezes it pushes the salt out, leaving the unfrozen water behind even saltier. What started as light warm tropical water is now cold and salty, and here, in the North Atlantic near the coast of Greenland is one of two places where the ocean water sinks to the bottom. The other is in the southern hemisphere in the Weddell Sea off the coast of Antarctica. Once on the bottom the water circulates around the globe, just as it did on the surface, only this time it is highly influenced by the topography of the ocean floor. Then, obviously, if the ocean water is sinking somewhere, it must be rising somewhere else. This upwelling, however, is more diffuse and occurs in much more than in two places, and it is difficult to measure, although some of this bottom water has been on the ocean floor for as long as 1600 years. One good sign of upwelling is how productive the fishing is; where water rises from the sea floor it brings up nutrients that have fallen to the bottom and which ocean critters can feed off of. To see a video of the thermohaline ocean circulation press here.
Global Warming and an Ending Thought
The thermohaline circulation could have a mitigating effect on global warming in the short run as upwelled water absorbs some of the heat, as it does so well, and the sinking water takes some of the carbon dioxide we burn off to the bottom. This would only work for so long however – until this warmth resurfaces or the oceans reach a saturation point for carbon dioxide. The fear among many scientists today is that global warming may drastically alter the thermohaline circulation or shut it down entirely. The two areas in the oceans where the surface water falls to the bottom are also the great drivers of this movement. But as the Arctic Ocean melts, as it has done in a drastic manner in the past ten years, and the glaciers shrink on Greenland, more fresh water is added to the area in the North Atlantic where it sinks. This fresher, less salty water, is less dense and is less driven to sink. It is impossible to tell what all the effects this might have on the climate, but it is likely to be big.
You have made it to the end of “Water”. But let me leave you pondering one last thought. We have seen water circulate through our bodies, in two dimensions upon the ocean surface, and in three dimensions as it falls to the ocean floor. But what about the circulation of water in four dimensions – through time as well? For, through time, the chaotic diffusion of water would be great. I wonder if any water molecules in my body now were once part of the massive glaciers that dug out the Great Lakes, that nourished plants of the Tethys Sea that sunk to the bottom where they became part of the petroleum that I fueled my car with today, or that flowed through the blood of a tyrannasaurus rex, or the Buddha, or Jesus Christ, or my great grandmother Anna Gustafson or maybe even yourself!
What part of this essay was new and most interesting to you?