Tufts OpenCourseware
Author: Fulcrum Institute Development Team

Challenge Solutions/Discussion

In discussing these two questions we have to think about what happens on a molecular level both above the pot of boiling water and in the pot of water itself. When water boils the rate at which some fast moving molecules escape the liquid water is equal to the rate at which some fast moving molecules above the water reenter the water.

Challenge 1 Solution

The boiling point of water is 100° C at sea level. At sea level atmospheric pressure is substantially greater than it is in Denver which is at an altitude of more than 5000 feet. On a molecular level this means that in Denver there are fewer molecules of air [really fewer molecules of nitrogen and oxygen] zipping around and colliding with one another above the surface of a pot of water. This, in turn, makes it easier for the fast moving water molecules that escape from the liquid water to get away and remain in the vapor phase. This means that the average speed of a water molecule escaping the hot water in Denver is lower than the average speed of a water molecule escaping a pot of hot water at the same temperature at sea level.

By the same token, if the pressure on the water is greater than atmospheric pressure, as it is in the power plant of a nuclear submarine, the opposite will be true.

Challenge 2 Solution

The reason you can cool a cup of hot coffee by blowing across the top of it is that you are clearing the region above the coffee of some of the faster moving water molecules that have escaped from the liquid coffee. This disturbs the equilibrium that we discussed above. If you don’t blow across the coffee then approximately the same number of fast moving molecules leave the liquid per second as reenter the liquid from the vapor above. If you blow across the coffee more fast moving water molecules leave the liquid per second than reenter it. This lowers the average speed of the molecules in the coffee, and thus its temperature.