# Thermal Energy, Heat, and Work

## Phase Change and Latent Heat


### Section Key Terms


### Phase Changes

So far, we have learned that adding thermal energy by heat increases the temperature of a substance. But surprisingly, there are situations where adding energy does not change the temperature of a substance at all! Instead, the additional thermal energy acts to loosen bonds between molecules or atoms and causes a phase change. Because this energy enters or leaves a system during a phase change without causing a temperature change in the system, it is known as latent heat (latent means hidden).
The three phases of matter that you frequently encounter are solid, liquid and gas (see ). Solid has the least energetic state; atoms in solids are in close contact, with forces between them that allow the particles to vibrate but not change position with neighboring particles. (These forces can be thought of as springs that can be stretched or compressed, but not easily broken.)
Liquid has a more energetic state, in which particles can slide smoothly past one another and change neighbors, although they are still held together by their mutual attraction.
Gas has a more energetic state than liquid, in which particles are broken free of their bonds. Particles in gases are separated by distances that are large compared with the size of the particles.
The most energetic state of all is plasma. Although you may not have heard much about plasma, it is actually the most common state of matter in the universe—stars are made up of plasma, as is lightning. The plasma state is reached by heating a gas to the point where particles are pulled apart, separating the electrons from the rest of the particle. This produces an ionized gas that is a combination of the negatively charged free electrons and positively charged ions, known as plasma.
During a phase change, matter changes from one phase to another, either through the addition of energy by heat and the transition to a more energetic state, or from the removal of energy by heat and the transition to a less energetic state.
Phase changes to a more energetic state include the following:
1. Melting—Solid to liquid
2. Vaporization—Liquid to gas (included boiling and evaporation)
3. Sublimation—Solid to gas
4. IonizationGas to plasma
Phase changes to a less energetic state are as follows:
1. Condensation—Gas to liquid
2. Freezing—Liquid to solid
3. Recombination—Plasma to gas
4. DepositionGas to solid
Energy is required to melt a solid because the bonds between the particles in the solid must be broken. Since the energy involved in a phase changes is used to break bonds, there is no increase in the kinetic energies of the particles, and therefore no rise in temperature. Similarly, energy is needed to vaporize a liquid to overcome the attractive forces between particles in the liquid. There is no temperature change until a phase change is completed. The temperature of a cup of soda and ice that is initially at 0 
   stays at 0 
   until all of the ice has melted. In the reverse of these processes—freezing and condensation—energy is released from the latent heat (see ).
The heat, Q, required to change the phase of a sample of mass m is
 
   (for melting/freezing),
 
   (for vaporization/condensation),
where  
   is the latent heat of fusion, and  
   is the latent heat of vaporization. The latent heat of fusion is the amount of heat needed to cause a phase change between solid and liquid. The latent heat of vaporization is the amount of heat needed to cause a phase change between liquid and gas.  
   and  
   are coefficients that vary from substance to substance, depending on the strength of intermolecular forces, and both have standard units of J/kg. See  for values of  
   and  
   of different substances.
Let’s consider the example of adding heat to ice to examine its transitions through all three phases—solid to liquid to gas. A phase diagram indicating the temperature changes of water as energy is added is shown in . The ice starts out at −20 
 , and its temperature rises linearly, absorbing heat at a constant rate until it reaches 0 
   Once at this temperature, the ice gradually melts, absorbing 334 kJ/kg. The temperature remains constant at 0 
   during this phase change. Once all the ice has melted, the temperature of the liquid water rises, absorbing heat at a new constant rate. At 100 
 , the water begins to boil and the temperature again remains constant while the water absorbs 2256 kJ/kg during this phase change. When all the liquid has become steam, the temperature rises again at a constant rate.
We have seen that vaporization requires heat transfer to a substance from its surroundings. Condensation is the reverse process, where heat in transferred away from a substance to its surroundings. This release of latent heat increases the temperature of the surroundings. Energy must be removed from the condensing particles to make a vapor condense. This is why condensation occurs on cold surfaces: the heat transfers energy away from the warm vapor to the cold surface. The energy is exactly the same as that required to cause the phase change in the other direction, from liquid to vapor, and so it can be calculated from  
  . Latent heat is also released into the environment when a liquid freezes, and can be calculated from  
  .

### Solving Thermal Energy Problems with Phase Changes


### Practice Problems


### Check Your Understanding


### Section Summary

1. Most substances have four distinct phases: solid, liquid, gas, and plasma.
2. Gas is the most energetic state and solid is the least.
3. During a phase change, a substance undergoes transition to a higher energy state when heat is added, or to a lower energy state when heat is removed.
4. Heat is added to a substance during melting and vaporization.
5. Latent heat is released by a substance during condensation and freezing.
6. Phase changes occur at fixed temperatures called boiling and freezing (or melting) points for a given substance.

### Key Equations


### Concept Items


### Critical Thinking


### Problems


### Test Prep Multiple Choice


### Test Prep Short Answer


### Performance Task


### Test Prep Extended Response
