When heat is added to a pure body of water at 100° Celsius the temperature does not change. What happens instead is that the water beings to boil. This example is known as a change of state or a phase change. The transition between phases of water involves large amounts of energy compared to the amount of energy required to raise the temperature by 1° C.
Why it matters
- Phase changes in pure water occur at a specific temperature. At 1 atm, water freezes at 0° C and boils at 100° C. The energy required to change water from a liquid to a solid is 333.7 kJ/kg while the energy required to boil water is 2257 kJ/kg. The amount of energy needed to change the phase of water to a gas from a liquid is 540 times the amount of energy needed to raise the same amount of water 1° C.
- The reason so much heat needs to be added for a phase change can be understood by looking at the molecules of water. When you look closely at the molecules in a liquid, the molecules are relatively close together and they have a little room to jiggle. As you increase the temperature, the atoms continue to jiggle more and more, which causes an increase in the temperature.
- At some point, the temperature stop changing and it reaches a plateau. This is where the molecules have moved as far apart from each other as they possibly can. Any additional energy that is added to the system goes into overcoming the attractive forces the molecules have on each other. When the attractive forces are overcome, some of the water molecules are able to escape which results in the water boiling.
Can You Apply It?
Using the information provided above, answer the following questions.
- If adding heat to the system increases the temperature of a material, what does this say about the definition of temperature and how it related to kinetic energy?
- Why doesn't the temperature of water increase pass 100° C when you start adding heat to it?
- If a molecule of water is able to escape from a body of water, what happens to the average kinetic energy of the system? Does it increase or decrease?