How Everything Works
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Water, Steam, and Ice
Page 7 of 10 (98 Questions and Answers)

1173. What happens to ice when it is left in the freezer—does it evaporate? I have noticed that over time the ice cubes shrink? — J & K
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When you leave ice in a frostless refrigerator, it gradually sublimes and shrinks away to nothing. Sublimation is equivalent to evaporation, except it involves a solid converting directly into a gas. The surface of an ice cube is a busy place, with water molecules landing and taking off all the time. If more water molecules land than leave, the ice cube will grow in size. If more water molecules leave than land, the ice cube will shrink. The water molecule landing rate is determined by how much moisture there is in the air. In a frostless refrigerator, the air is extremely dry, meaning that it contains very few water molecules. Thus the landing rate in a frostless refrigerator is very low and the ice cubes shrink. If you watch the ice cubes in an older style refrigerator, you will find that they grow over time because the air in that refrigerator is moist and the landing rate is high. Incidentally, this sublimation of water molecules from ice is why snow disappears gradually even when the weather remains cold and is also how freeze drying of food is done.

1182. How is the skin better hydrated by vapor as opposed to liquid water? Wrapping yourself in a damp sheet is more effective at treating the dryness of eczema than taking a bath. — CW
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When your skin is immersed in pure water, the only molecules that ever collide with its surface are water molecules. That might seem to be the ideal situation for keeping skin moist, however such immersion can have other unintended consequences. First, any water soluble atoms, molecules, and ions that can move to the surface of your skin will dissolve away in the surrounding water and you'll never see them again. Second, any water soluble atoms, molecules, and ions that can't move to the surface of your skin will draw water into your skin by way of osmosis—the pure water will flow into your skin cells in an attempt to dilute the dissolved particles inside those cells. After a relatively short time, the cells of your skin will contain many more water molecules than before and your skin will look all wrinkly. This flow of water soluble materials out of your skin and water into your skin may not be so wonderful for your eczema.

When you wrap yourself in a wet cloth, you are ensuring that the relative humidity near the surface of your skin will be close to 100%. Air molecules will still be present around your skin but now there will be essentially no net transfer of water between your skin and the surrounding air—water molecules will leave your skin for the air at roughly the same rate as water molecules return to your skin from the air. In effect, you are stopping evaporation from your skin and very little else. Stopping evaporation from your skin will also cause it to accumulate moisture, but this time the new moisture will come from within your body. Water molecules that would have left your skin had it been surrounded by dry air are now staying in your skin, where they add to the moisture in your skin. Overall, you skin will contain more water but it will not have lost as many water-soluble chemicals and it will not have water driven into it by osmotic pressure. It may be this more gentle moisturizing effect that makes wrapping yourself in a damp sheet more pleasant for your eczema than immersing yourself in water.


1183. We heated a cup of water in a microwave oven for 2-1/2 minutes and then added a spoonful of sugar to it. A rush of tiny bubbles ensued. Did the sugar crystals nucleate boiling water molecules that were trapped by surrounding cooler molecules or did they nucleate the release of dissolved air? — VC
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When you heated the water in the microwave oven, you raised its temperature above its boiling temperature, yet it did not boil. While the water was hot enough to boil—that is, any steam bubble that formed in this hot water would have a pressure at least equal to atmospheric pressure and would not be crushed by the surrounding air—the water was having a difficult time forming steam bubbles. For a bubble to appear, several water molecules must simultaneously break free of their neighbors to form a bubble nucleus. Once this nucleation has occurred, additional water molecules can evaporate into the bubble, making it grow. This nucleation is rare in pure water near its boiling temperature; in most cases it is assisted by hot spots at the bottom of a pot on the stove or by imperfections in the container holding the water. But when you heat water in a glass or glazed ceramic container in a microwave oven, there are no hot spots or surface imperfections to nucleate the bubbles. The water superheats above its boiling temperature. When you add sugar crystals to this superheated water, the crystal's sharp edges and points assist the nucleation of steam bubbles and the water boils violently.

Your suggestions for why the bubbles appear raise two interesting points. First, in a thermal system such as hot water, you can't identify some molecules as being boiling hot and others as being cooler—temperature is a property of the entire system and not of individual molecules. However, at a given instant, there are molecules with more energy than their neighbors and it is these energetic molecules that may break free of their neighbors to form a bubble nucleus.

Second, water often contains dissolved gases and these gases come out of solution when the water is heated. While many of the gas molecules leave through the water's surface, some of them may leave as bubbles from within the water. This gas bubble formation requires nucleation as well, which is why these bubbles often appear on the inner surfaces of a metal pot on the stove—flaws in the pot's surface assist bubble nucleation. But these gas bubbles aren't what you observed; there just isn't that much dissolve gas. You can prove that the bubbles you observe are steam: repeat the experiment several times with the same water. Each time you heat the water and add sugar, it bubbles wildly—something that wouldn't be possible if you were simply releasing dissolved gases from the water.


1184. I fight a constant battle with mildew in the Pacific Northwest. I can buy solid chemicals to put in my closets, which take water out of the air, eventually creating a bucket full of water. Do these devices actually lower the moisture content of the air or do they just make me feel like I'm doing something? — MD
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How much effect these drying agents have depends on how much air they're exposed to. Water molecules are continuously going back and forth between the air and everything exposed to that air—your clothing, your hair, the walls of your home, the contents of a saltshaker, and the drawers in a wooden bureau. The water molecules land on and take off from every surface, like busy miniature airports. The rate at which water molecules land on an object depends on how humid the air is. The rate at which water molecules leave that object depends on how hot the object is and on how tightly water molecules cling to it.

The landing and leaving processes are in perpetual competition and the fastest one wins. If the air is humid and the object is cold or attractive to water molecules, the landing process dominates and water condenses out of the air and onto the object. If the air is dry and the object is hot or doesn't bind water molecules well, taking off dominates and water evaporates from the object into the air.

Your problem is that the air in your closets is very humid and landing is winning—too much water is condensing on your walls. To stop this condensation, you either have to heat the walls, so that water molecules leave them faster, or reduce the humidity of the air, so that water molecules land less often. Putting a material that binds water molecules into your closets changes the balance of landing and taking off—water molecules that land on this material don't return to the air often so the humidity of the air diminishes. With less humidity in the air, the rate at which water molecules land on the walls also diminishes.

But this drying effect only works if the air in the closet is trapped there. If your closet exchanges air quickly with outdoor air, the water molecules removed by the drying agent will be quickly replaced with new water molecules from outside. In effect, you will be trying to dry the great outdoors, a hopeless task. To make the most of this drying agent, you should let it work on as little air as possible by sealing the closet and slowing the exchange of air with outside. Better yet, replace the drying agent with a dehumidifier. A dehumidifier accumulates water molecules from the air by presenting the air with a chilled surface. Water molecules land on the cold surface and then don't have enough energy to return to the air. They are trapped by the cold rather than by chemical binding.


1205. If a given volume of water is placed in a container and frozen, will it weigh more, less, or remain the same relative to when it was in a liquid state? — RL, Denver, Colorado
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Freezing water has virtually no effect on its weight—as long as the same number of water molecules remain in the container, the overall weight of the container and water/ice won't change significantly. But water does expand as it freezes, so the container will become more full as the ice forms. Water's expansion upon freezing makes ice less dense—less mass per volume—than liquid water. This decrease in density explains why ice floats on water and why pipes often break as the water inside them freezes.

However, you'll notice that I said "freezing the water has virtually no effect on its weight." In reality, the water does lose a tiny fraction of its weight. That's because to freeze the water, you must remove some of the water's energy. As Einstein pointed out with his famous formula E=mc2, energy and mass are related to one another and since mass acquires weight when it's near the earth, so does energy. Because the thermal energy in liquid water has a tiny weight, when you remove some of this thermal energy from the water, the water loses some of its weight. But don't expect to measure this weight loss with a common scale—the weight change is on the order of one part in a trillion, a factor that's presently beyond the precision of even the most advanced research measuring devices.


1221. Suppose that I fill a rigid container with water and that this container will not expand or contract as its temperature changes. Will the water turn to ice when I cool it below 0° C? — PL, Taikoo Shing, Hong Kong
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Since water normally expands as it forms ice, the rigid container will prevent it from freezing at 0° C. If the container was completely filled with water at room temperature, then an "empty" region will appear inside the container when you first begin to cool it toward freezing. That's because water contracts as you cool it toward 4° C. The "empty" region isn't really empty, it contains gaseous water vapor. But once the water's temperature drops below 4° C, the water begins to expand as it cools. It will first expand into the "empty" region, but when that region becomes full the water will no longer be able to expand. Instead, its pressure will begin to rise dramatically. This elevated pressure is what will ultimately prevent the water from freezing at 0° C—high pressure depresses water's freezing temperature. Although the water will eventually freeze, you'll have to cool it far below 0° C for that to occur.

1222. When water boils the "air bubbles" rising from the bottom of the pan seem to be created spontaneously out of nothing. I told my son that they are not air bubbles but rather water vapor. Is that correct? — JG, Austin, TX
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Yes, these bubbles contain water vapor, not air. The reason that you don't see them until the water reaches its boiling temperature is that a bubble containing only water vapor isn't stable at lower temperatures—the surrounding air pressure will crush it. But once the water temperature is high enough, the water vapor bubbles are stable and they grow while rising to the top of the water.

1231. Why do carbonated beverages "burn" your throat? — TS
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When carbon dioxide gas (CO2) dissolves in water (H2O), its molecules often cling to water molecules in such a way that they form carbonic acid molecules (H2CO3). Carbonic acid is a weak acid, an acid in which most molecules are completely intact at any given moment. But some of those molecules are dissociated and exist as two dissolved fragments: a negatively charged HCO3- ion and a positively charged H+ ion. The H+ ions are responsible for acidity—the higher their concentration in a solution, the more acidic that solution is. The presence of carbonic acid in carbonated water makes that water acidic—the more carbonated, the more acidic. What you're feeling when you drink a carbonated beverage is the moderate acidity of that beverage "irritating" your throat.

1245. How does a rice cooker know when to turn off? — JS, Tokyo, Japan
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The rice cooker turns off when there is no longer enough liquid water on its heating element to keep that element's temperature at the boiling temperature of water (212° F or 100° C). As long as the element is covered with liquid water, it is hard for that element's temperature to rise above water's boiling temperature. That's because as the water boils, all of the thermal energy produced in the heating element is converted very efficiently into chemical potential energy in the resulting steam. In short, boiling water remains at 212° F even as you add lots of thermal energy into it.

But as soon as the liquid water is gone (and, fortuitously, the rice is fully cooked), there is nothing left to keep the heating element's temperature from rising. As more electric energy enters the element and becomes thermal energy, the element gets hotter and hotter. A thermostat, probably a bimetallic strip like that used in most toasters, senses the sudden temperature rise. It releases a switch that turns off the electric power to the rice cooker.


1252. Is there rain above the clouds? — JM, Arlington Heights, Illinois
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No. If you are above the clouds, then the sky above you is free from droplets of condensed moisture. While that doesn't mean that there is no water overhead, that water must be entirely in the form of gaseous water molecules. Since rain forms when droplets of condensed moisture grow large enough to descend rapidly through the air, the absence of any condensed droplets makes it impossible for full raindrops to form. In short, no clouds overhead, no rain.

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