What Happens when Water Boils?
As water boils, the water turns into steam (also known as water vapor or water gas.) You can probably watch this happen if you pay close attention when you boil your pot of water. First, the water begins to form bubbles at the bottom of the pot near the heating device (see if this happens for you.) Then, the bubbles begin to rise until the bubbles begin to pop off the surface of the water and seemingly evaporate into the air. What's happening here?
As the energy from the heating device goes into heating the water, the water temperature rises. You'll notice this happening on your thermometer. But at a certain temperature, the heating energy goes only to turning the water into a gas. When that point is reached, your thermometer reading will not get any higher. At that point, all of the energy from the heating device is used to transform the water from its liquid state to its gaseous state (that's when you start seeing the bubbles form.) When you reach point where the temperature of the water does not increase any further, you will have found the boiling point of your water. Interestingly enough, no matter how much longer you keep the pot of water boiling, it will get no hotter, because at this point all the energy from the heating device is being used to change the water into gas and not for increasing the temperature. Why is this?
As you heat up the water, you are breaking down the liquid molecules so that it can turn into a gas. Bubbles are created in the water closest to the bottom of the pot first. But the pressure of the outside air will squash those bubbles at first because they don't have enough pressure inside them to stand up to the outside air pressure. As more energy goes into making those bubbles though, they will begin to be able to stand up to the outside air pressure. When they get to the point where they can stand up to the outside air pressure, you'll see massive bubbles coming off of your water, the temperature of your water will top off and your boiling point will be reached.