Phase changes

Answer the following questions in your head:

  1. A pot of water on the stove is getting hotter and hotter. At what temperature will it begin to boil?
  2. You walk into a kitchen and find a pot of boiling water on the stove. What is the temperature of the water?
  3. Sketch a graph of the temperature of a pot of water on the stove as it is heated.

It is common to answer these questions in this way: A. 100 oC (or 212 oF), B. 100 oC, C. a graph that looks like the following:

T v time. shows a constantly increasing temp





If those were the answers you gave, you may begin to realize that they can’t all be correct. If the temperature continues to rise (as the graph shows) then there is no way to predict what the temperature of the boiling water in question B would be.

The truth is that the graph for question C should look like this.

T v time. graph increases and then flattens during phase change





In fact, the temperature stays the same during any phase change. Thus, melting ice stays at 0 oC until all of the ice has melted, just like boiling water stays at 100 oC until all of the water has boiled away.

This leads to a simple question—why? Why doesn’t the temperature change during a phase change?
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