At each moment, some molecules manage to jump out of your pot, but at
the same time some other water molecules jump back in. There is an
equilibrium.
What happens when a fluid boils?
You probably know that the fluid consists of many very small particles, so-called molecules. These little guys run around very quickly. All the time, they try to jump out of the fluid. But, when you have a pot of water on the stove, they cannot do that very well, because they cannot pass through the metal of the pot, and because they cannot overcome the atmospheric pressure of the air.
At each moment, some molecules manage to jump out of your pot, but at
the same time some other water molecules jump back in. There is an
equilibrium.
But when you heat the water, you make the molecules jump around faster. Eventually, many of them manage to jump out of the water into the air. This is the moment when your soup boils. Here in Haugesund, this happens at a temperature of 100 degrees Celsius.
Perhaps you know that the atmospheric pressure of the air depends on
altitude. On the Mount Everest, for instance, water boils at a temperature
that is less than 100 degrees C, because the air pressures with less force
against the surface of the water in the pot.
Now what is the vapor pressure? If you put your pot with water into a vacuum, then some of the water molecules will jump out immediately, and form an atmosphere. Again, you have an equilbrium between molecules that are out of the pot, and the rest. The atmosphere formed by the guys that are out has a characteristic pressure, the vapor pressure. It depends very strongly on temperature.
When the vapor pressure of a fluid becomes equal to the pressure of a
surrounding gas, the fluid boils. So, when you soup boils, it means that
its vapor pressure is now equal to the atmospheric pressure in your kitchen.
Now you know enough to go to the exercise!