Heat and Phase Transformations: Super-Cooled Water Instant Freeze
[Shopping List: bottled water in plastic bottles (carbonated water works best); ice; rock salt]
- Prepare an ice water bath, measure the temperature which should be very close to 0°C.
- Add rock salt, mix thoroughly, and remeasure temperature. The temperature should drop well below 0°C now, perhaps as low as -6°C if you've added enough salt.
- Add several unopened water bottles to the ice-water-salt bath. Allow 1-2 hours for the water in the bottles to reach the same temperature as the bath. Be careful not to disturb the bottles as they cool, and you should find that most of the bottles do not freeze even though the temperature is well below the normal freezing point..
- Carefully remove a bottle, observe that it is completely liquid with no ice, then smack the bottle sharply with your other hand or on the table top. It should instantly begin to freeze, continuing for several seconds until the entire bottle is frozen. Look closely, often the crystal growth is very beautiful and slow enough to easily see form.
- A small amount of water will remain liquid, and the ice will have more of a slush consistency, but it is frozen.
What's Happening: Normally pure water freezes at 0°C (32°F). In an ice-water bath as long as there is plenty of both phases present, the temperature will remain 0°C as freezing and melting are in equilibrium. When a soluble impurity is added, regardless of the material, the equilibrium freezing point will be lowered (this is called a colligitive property because it is independent of the solute). The more impurity dissolved, the lower the freezing point is depressed. This allows us to prepare a stable temperature bath a few degrees lower than the normal freezing point of water.
Even pure water will not necessarily freeze at 0°C. It is not enough to simply lower the temperature, freezing also requires nucleation sites where ice crystals can more easily form. This is somewhat similar to growing crystals in saturated solutions, where a seed crystal is often used to help start the crystallization. Thus water can be lowered to a temperature well below the normal freezing point if no good nucleation sites are present. This is called super-cooled water. [Note that the water we are using is not actually pure, but the small amount of ingredients dissolved in the water depress its freezing point only a very small amount, and don't provide many good nucleation sites, so it is still actually super-cooled.] When we smack or shake the bottle we create a lot of gas bubbles (especially if the water is carbonated), which can easily serve as nucleation sites to start the freezing. Once started, the ice crystals grow and spread rapidly until nearly all of the water is frozen. The ice crystals formed usually have a needle-like structure. When water freezes it gives up heat to form ice (called the latent heat), and this heat must go somewhere, so it actually warms some of the other water molecules (or even melts some of the ice), so that not all of the water will end up frozen unless this extra heat is removed in some other way (such as an electric freezer would do).
Variations: If you open the super-cooled water bottle very carefully, it might not freeze yet. Carefully put a thermometer into the bottle and verify that the temperature is well below the normal freezing point. Now initiate freezing by smacking or shaking the bottle, or by dropping a small piece of ice (a seed crystal) into the bottle. As the water freezes you will see the temperature rise until it reaches 0°C, then stabilize. This rise in temperature is due to the latent heat of the ice that is forming warming the remaining water, but it is still cold enough to freeze, so it does, even while the temperature is actually rising. Another interesting demonstration is to pour super-cooled water into a glass or dish. If the glass or dish is not too warm, the water may actually freeze as it hits the glass, with the freeze front moving up and into the bottle as you continue to pour it.
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