Category: water steam and ice

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.

How do you make an energy converter to convert water into energy? — SB

I’m afraid that there is no simple way to convert water into energy. People have been trying to use fusion to extract the nuclear energy stored in the hydrogen nuclei in water. But while billions of dollars have been spent on research, there is no viable scheme for this process for controlled fusion in sight. The stars are powered by hydrogen fusion, but people on the earth aren’t likely to be using it as a source for peaceful energy any time soon.

How does dry ice work to freeze things? — JH

Solid carbon dioxide or “dry ice” sublimes into gaseous carbon dioxide at a temperature well below 0° C. Since it takes energy to separate the molecules of carbon dioxide from one another, the dry ice absorbs heat as it sublimes and takes that heat out of any warmer objects nearby. Those nearby objects become colder and colder as the heat leaves them and eventually they begin to freeze.

How does fog form? — KB

The interface between a droplet of water and the air around it is a busy place. Water molecules are constantly leaving the droplet to become water vapor in the air and water molecules in the air are constantly returning to the droplet as liquid water. What determines whether the droplet grows or shrinks is the difference between these two rates. If more water molecules return to the droplet than leave, the droplet will grow. If more water molecules leave the droplet than return, the droplet will shrink. How often water molecules leave the droplet depends on the droplet’s temperature. How often water molecules return to the droplet depends on the moisture content of the air.

This dynamic balance of growth and shrinkage occurs right in the middle of the air all the time. Tiny water droplets form by accident, even in reasonably dry air, but in most cases they quickly shrink back to nothing because the leaving rate is higher than the returning rate. However, when air that contains lots of moisture experiences a decrease in temperature, the returning rate can exceed the leaving rate. When that happens, the tiny droplets that appear by accident don’t immediately disappear. Instead, they grow larger and larger. Depending on the altitude, we call the white mist that results clouds or fog.