Climate and Global Change
At what temperature does
water freeze?
Introduction
To form raindrops that are 100 times larger in diameter than cloud droplets, a prospective droplet would have to collect many, many cloud droplets. Raindrops are 1,000,000 times larger in volume than cloud droplets. In addition to collecting smaller cloud droplets, another of the ways this tremendous growth occurs is the Bergeron-Findeisen or ice process. This process is active for “cold” clouds, i.e., for clouds with tops tall enough to reach temperatures below freezing.
For liquid droplets to freeze, the droplets need to have a nucleus to begin the process. These nuclei are referred to as “freezing nuclei.” Without a freezing nucleus, a water droplet would not freeze until it reached temperatures as cold as -40°C, i.e., until it became much colder than the “advertised” freezing temperature of 0°C. There are not enough freezing nuclei in the atmosphere. Between 0°C and -40°C, clouds are frequently a mixture of ice and liquid, at temperatures below (i.e., heights above) -40°C, clouds are all ice.
At temperatures below 0°C, liquid water is called supercooled. In the temperature range between 0°C and -40°C, ice crystals grow at the expense of liquid drops, i.e., the supercooled droplets evaporate and the ice crystals grow by deposition.
Freezing of Ice Experiment
Fill a beaker with a well-mixed mixture of ice, water and salt. The mixture should be mostly ice with some water so that you have good contact to the test tube you are going to insert into the mixture. Recall that this mixture will cool to temperatures below 0°C. Fahrenheit used a salt, water and ice mixture to calibrate his thermometers and determine the zero on the Fahrenheit temperature scale. We use salt to keep ice from forming on sidewalks. This works to lower the freezing temperature from that of pure water to the freezing temperature of saltwater. It is for the same reason that we add salt to the ice when we make homemade ice cream.
Measure the temperature of the ice, water and salt mixture. Record your reading below.
Ice, water and salt mixture temperature = ______
If the temperature you measured is not at least -10°C, add some more salt to the mixture, stir and re-measure the temperature.
Clean a test tube and a thermometer with distilled water, no detergent. Partially fill a test tube with distilled water. Place the test tube with a thermometer in the beaker containing the ice, water and salt mixture. While stirring the mixture in the beaker, watch the temperature reading on the thermometer. Try not to disturb the water in the test tube while you are stirring the mixture in the beaker. Record the lowest temperature the thermometer reaches before the distilled water freezes.
Lowest temperature = ______
What happened to the temperature just after the distilled water froze?
What do you believe would have happened if we had not used distilled water? If the water had had impurities in it, would this have affected the results of the experiment?
Remove the thermometer and test tube from the ice-salt-water mixture and let it warm until the ice in the test tube has thawed. Rinse the test tube and thermometer again and partially fill the test tube with distilled water as before. Put the test tube back into the ice-water-salt mixture and allow it to cool again. When the temperature gets close to the lowest temperature you measured before (e.g., within about 2°C), drop a shaving of ice into the test tube.
Describe what happened?