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Warming, melting, and freezing water

Water can be made warmer by placing it over a fire, by heating it with an immersion heater, by vigorously stirring it, by exposing it to the Sun, or by bringing it in contact with a hotter body—even indirectly (Movie 1).

Ice placed in a warm environment melts (Movie 2). Alternatively, we can actively heat ice in a test tube placed in warm water and monitor the temperature of the mixture of ice and water, and of the water bath (Fig. 1). It is found that the temperature of a cold block of ice first rises to 0°C. Then the ice-water mixture stays at a constant 0°C until all the ice has melted. Only then, upon further heating, does the temperature of the water rise as expected. The temperature of the water bath decreases all the time (Fig. 2).

When water is placed in a cold environment (considerably colder than 0°C), the reverse happens. The temperature of the water drops to 0°C. Now ice starts to form, and the temperature of the substance stays constant at 0°C. Finally, when there is only ice, the temperature continues to sink below the freezing point.

Interpretation

To make a body of water warmer, we need to add heat (*). Heat can be transferred from outside from another body, it can be produced by a fire or an immersion heater and then transferred, it can be produced inside by stirring the water, or by absorbing sunlight. The effects of producing heat or just transferring it from another body are the same.

Obviously, heat is needed to melt ice. First, the heat coming from the warm water raises the temperature of the cold ice to 0°C. Then the temperature of the substance in the test tube (Fig. 2) stays constant for quite a while before starting to rise. During the phase of constant temperature, ice transforms into water.

Here we have an example that demonstrates very clearly that heat and temperature are totally different concepts. Wile the temperature stays constant, the quantity of heat of the substance in the test tubes increases.

(*) Note: The technical term for quantities of heat is entropy. What we call heat, and what is officially called entropy by engineers and physicsist is closely related to what scientisits before 1850 called caloric. When presenting phenomena in terms of standard language, it makes sense to keep using the word heat for what we normally call entropy.