Tag: water steam and ice

I’m helping on a lesson plan for grades 3-12 where students make ice cream. Adding salt to the ice makes the ice colder. I’m having trouble explaining why we put salt on the roads to melt ice, but in making ice cream the salt actually lowers the temperature of the ice. — N

These two observations—that salt melts ice and that salt makes ice colder—are actually consistent with one another. When you add salt to ice, you make a relatively ordered mixture—pure crystalline ice and pure crystalline salt. This orderly arrangement is looked on unfavorably by nature; given a chance, nature tends to maximize randomness. There is a much more disorderly arrangement available—salt water—and nature tends toward disorderly arrangements. When you put the salt and ice together, nature’s tendency toward randomness begins to drive the system to rearrange. The ice begins to melt so that the salt can dissolve in it. Although the melting of ice requires energy, the randomness this melting and dissolving produces makes this process take place. The energy needed to melt the ice is extracted from the remaining ice and that ice gets colder. When you’re making ice cream, some of the energy needed to melt the ice also comes from the ice cream mix, so that it gets colder, too. If there is enough salt around, the ice will melt completely to form very cold salt water—the desired result with salt on a slippery sidewalk. The salt water remains liquid well below the normal freezing temperature of water because forming ice crystals would require the salt and water to separate from one another—an orderly and therefore unlikely event. In short, nature’s trend toward disorder causes salt to melt ice, even though that melting lowers the temperatures of everything involved well below the freezing temperature of pure water.

Why does water freeze at very low pressure? I saw an experiment in which a small amount of water first boiled and then froze solid when exposed to a vacuum. — BLG, Old Bridge, NJ

Water molecules are always leaving the surface of liquid water and when they do, they carry away more than their fair share of the water’s thermal energy. Placing the water in a vacuum speeds this process because (1) it prevents those gaseous water molecules from returning to the liquid water, in which case they would return the thermal energy, and (2) it makes it possible for bubbles of water vapor to remain stable inside the liquid water even at low temperature, so that the water can boil. Overall, the main effect of putting the water in a vacuum is that its molecules leave rapidly and don’t return. Since each leaving water molecule takes away more than its fair share of thermal energy, the water molecules that remain behind become cooler and cooler. You experience this effect when evaporating water from your skin makes you feel cold. In the present case, this cooling is so effective that the remaining water cools all the way to water’s freezing point and the water begins to crystallize into ice. Water molecules continue to leave the surface of ice, a process called sublimation, so that even the ice gradually gets colder in the vacuum.

Why and how does water conduct electricity? — SM, Murrysville, PA

Water molecules are electrically neutral and do not accelerate in response to electric fields. For that reason, a liquid consisting only of water molecules wouldn’t conduct electricity. However, real water contains things other than water molecules. Even in completely pure water, about 1 in every 10,000,000 water molecules is found to have dissociated into a hydrogen ion (H⁺) and a hydroxide ion (OH^–). These electrically charged ions do accelerate in response to electric fields and they make it possible for even the purest water to conduct electricity weakly. Adding impurities, particularly ionic impurities such as salts, makes water an even better conductor of electricity.

A company claims that if you place their sealed liquid-filled plastic ball into your washing machine, you can eliminate the need for caustic detergents, improving the ecology and saving the planet. The claim is that this ball changes the ionic charge of the water and “magically releases” the dirt from your clothing. Is it possible to use ions to clean as well or better than detergent? — RO, Garden City, MI

I’m afraid that this claim is nonsense and, like the stone in “stone soup,” the ball does nothing at all. The old-time medicine show didn’t really disappear, it just evolved into a more modern form. Since the ball doesn’t add or remove chemicals from the water, it can’t alter the numbers of neutral and ionic particles in the water. But ions have very little to do with how water cleans clothes anyway. Water is already a wonderful solvent for salts and sugars, so you can clean many soils from your clothes with just water alone. But water is a poor solvent for oils and fats because oil and fat molecules don’t bind well to water molecules. That’s where detergents come into play—they form shells called micelles around the oil and fat molecules and render those molecules soluble in water. Without detergents, you’ll have trouble cleaning oils and fats from your clothes. Since oils and fats aren’t affected one way or the other by ions, even the ball’s claimed activity won’t help them to dissolve in the water.

In his Lectures on the Elements of Chemistry, Joseph Black discussed his difficulty in understanding latent heat. He performed an experiment where water in a tube was brought below freezing without a phase change. The water remained in this equilibrium as long as the tube of water was not disturbed. When it was disturbed, the water instantly turned to ice, releasing enough heat to raise the temperature of the ice to 0° C. Please explain why the system remained in equilibrium until it was acted upon by some external motion. — EDH, Annapolis, MD

The water in Black’s tube was in an unstable equilibrium state known as supercooled water. Supercooled water tends to spontaneously convert into ice. When part of this supercooled water does convert to ice, it releases enough latent heat energy to raise its temperature and that of the remaining water to 0° C, thereby terminating the phase transition before all of the water has become ice.

But in the experiment you describe, the supercooled water was having trouble nucleating the initial seed ice crystal on which the remaining water could crystallize. Given enough time, that water would have spontaneously formed a seed crystal and the growth of the ice crystal would have proceeded rapidly after that. However, Black accelerated the formation of the seed crystal by shaking the tube. A defect at the surface of the tube or a piece of dust then acted as the trigger and helped the seed ice crystal form. The water then crystallized rapidly around this seed crystal. After the ice had formed, the water was truly in equilibrium.

Is it true that water that has been previously boiled will boil faster than water that hasn’t been boiled? — HE, Haddonfield, NJ

I don’t think so. The only effect that bringing water to a boil has on the water is to drive dissolved gases out of solution. Once the water returns to room temperature, it’s essentially the same as it was before it was heated to boiling, except that it contains very little dissolved air. It may be that this absence of dissolved air will allow the water to boil slightly faster the next time around, but I doubt that you’d be able to detect a difference.

When you walk on snow when it is cold (-20° C), the snow squeaks; but when it is relatively warm (-5° C) the snow doesn’t squeak. Why? — PW, Alberta, CA

Near ice’s melting temperature, the surfaces within warm snow become more and more liquid-like. These liquid-like surfaces not only allow the warm snow to stick together as firm snowballs, but they act as lubricants so that the snow is particularly slippery. At much lower temperatures, the snow’s surfaces are much more solid and they slide uneasily and noisily across one another. The cold snow squeaks because it hasn’t “been oiled.”

How does a snow making machine work? — IB, Blue Ash, OH

A snow-making machine simply sprays a fine mist of water high into the cold air overhead, so that that mist can freeze into tiny particles of ice before falling back to the ground. If the air is cold enough, the mist will solidify before it hits the ground and before it has time to evaporate into water vapor. This freezing process isn’t as simple as it sounds because water can’t turn into an ice crystal without a seed on which that crystal can grow. Forming a seed crystal is a random process in which a couple of water molecules accidentally arrange themselves in a crystalline lattice. In snow making, each water droplet has only a few seconds in which to freeze and it can easily take that long for a seed crystal to form. However, people have found that adding certain chemicals or other materials to the water before spraying it into the air can speed the formation of seed crystals and dramatically increase the fraction of water that becomes artificial snow.

Our problem concerns temperature. At different temperatures, solubility of compounds varies. If we extract water from a pond at two degrees Celsius and then test it at room temperature, our reading isn’t going to be accurate. On the other hand, it isn’t practical for us to perform out tests outside. The substances we are testing are nitrites, nitrates, ammonia, pH, hardness, oxygen level, phosphates, temperature, and ORP. — J&E, Missouri

If you collect pond water at 2° C and then bring it into a room at 20° C, there will be a few subtle changes in the water’s contents. While the amounts of various dissolved materials can’t change unless atoms move in or out of the water, how they interact with one does change somewhat with temperature. I would be very surprised if anything that’s dissolved in that pond water comes out of solution when you warm it to room temperature, so if all you want to do is to determine the concentrations of various dissolved materials, go ahead and do it at room temperature. You might have to be careful with dissolved gases, because it’s relatively easy for gas molecules to enter or leave the pond water without your noticing that it’s happening, but the nitrites, nitrates, hardness, and phosphates aren’t going anywhere. Ammonia can leave as a gas, so you should be a little careful with it. I don’t know enough about ORP (oxidization reduction potential) to say anything about it. But you’ll have to be very careful with oxygen concentration because you can modify this just by pouring the water through air and making bubbles.

However, to be sure that the contents of the pond water are interacting with one another just as they were in the pond, you should cool the water back down to 2° C before making any measurements. This is particularly important for pH measurements, since water’s pH decreases slightly with increasing temperature.

What is pH and why is it so important to my garden pond and spa? — NW, California

pH is a measure of the concentration of dissolved hydrogen ions in water. When a hydrogen atom loses an electron and becomes a hydrogen ion—a proton—it can dissolve nicely in water. Actually, this proton sticks itself to the oxygen atom of a water molecule, producing a hydronium ion (H₃O⁺) that is then carried around by shells of water molecules. The higher the concentration of hydrogen (or hydronium) ions in water, the lower the water’s pH. More specifically, pH is negative the log (base 10) of the molar hydrogen ion concentration. That means that water with a pH of 6 has ten times as many hydrogen ions per liter as water with a pH of 7.

Pure water naturally contains some hydrogen ions, formed by water molecules that have spontaneously dissociated into hydrogen ions (H⁺) and hydroxide ions (OH^–). Pure water has enough of these hydrogen ions in it to give it a pH of 7. But if you dissolve acidic materials in the water, materials that tend to produce hydrogen ions, the pH of the water will drop. If you dissolve basic materials in the water, materials that tend to bind with hydrogen ions and reduce their concentration, the pH of the water will rise. Water with too many or too few hydrogen ions tends to be chemically aggressive and we do best in water that has a pH near 7.