PS-3.7    Explain the processes of phase change in terms of temperature, heat transfer, and particle arrangement. 
Key Concepts: Phase change (in terms of energy)     Temperature change (in terms of energy)    Boiling point    Freezing/Melting point
Heat Energy
As learned earlier, the Kinetic Theory of Matter states that matter is composed ( made of) particles called atoms. These particles are in constant motion, and the motion of the particles is proportional to the heat energy they contain. Remember, the faster they move, the more heat energy they contain ( and the higher the temperature! ).
To understand how and why phase changes take place in matter, it is important to understand the difference between kinetic energy and another form of energy, potential energy. The kinetic energy of particles, is proportional to how fast they vibrate. The potential energy of atoms or molecules is proportional to the distance between the particles. This diagram summarizes this relationship. Each diagram shows 4 Nitrogen atoms. The atoms in diagram "A" have less potential energy than the atoms in diagram "B" because they are closer together.

Lower Potential E               More Potential E
Closer together                          Further apart
Also note that even though the atoms in diagram "B" have a higher potential energy, both sets of atoms are at the same temperature. We know they are at the same temperature because both sets of atoms have the same amount of vibratory motion, and we know that vibratory motion = kinetic energy = temperature.
It is important to note that temperature changes result from adding or removing heat energy, thus changing the kinetic energy of particles, but when phase changes occur, changes in energy do not result in a change in temperature. When phase changes occur, any energy added or removed from the particles of a substance only result in a change in the distance between particles. In other words, phase changes result from a change in potential energy.
The following diagram shows how this works. The graph below shows the results of heating a 400g piece of ice for 10 minutes.
                                                                                   
Here is a description of what is happening in this heating curve graph:
Line a:  Between -50oC and 0oC, there is only one phase of matter present, solid ice. With the added heat the temperature of the solid ice will increase. This also means that the kinetic energy of the molecules is increasing, and the Potential energy of the particles is constant and unchanging.
Point b: At point b the ice reaches its melting temperature of 0oC.  At this temperature the water molecules have gained so much kinetic energy that the forces keeping the water at solid distances are overcome. The molecular motion is so strong that the molecules are torn away from each other and enter the liquid phase. Ice that is melting gets no hotter. The added energy only causes the phase change.
Line c: Along line c there are two phases of matter present, solid ice and liquid water. The temperature and kinetic energy will not change, it will remain at 0oC during the phase change. The potential energy of the molecules will increase because the distance between the molecules is increasing. Between points b and d, any energy added to the molecules will not cause the molecules to move faster, it will only cause the molecules to move further apart.
Line e: Along line e there is only one phase of matter present, liquid water. Between points d and f the kinetic energy of the molecules, and therefore temperature of the water, will increase. The distance between the molecules, and therefore the potential energy, will remain constant.
Point f: At point f the water has reached it's boiling point. At this temperature the water molecules have gained so much kinetic energy that the forces keeping the water at liquid distances are overcome. The molecular motion is so strong that the molecules are torn away from each other and enter the gas phase.
Line g: Along line g there are two phases present, liquid and gas. The water is boiling. The temperature and kinetic energy will not change, it will remain at 100oC during the phase change. The potential energy of the molecules will increase because the distance between the molecules is increasing. Along line g any energy added to the molecules will not cause the molecules to move faster, it will only cause the molecules to move further apart, entering the gas phase. Water that boils gets no hotter. The added energy only causes the phase change.
The graph discussed here resulted from the addition of heat energy, starting with ice at -50oC. Removing heat energy from water vapor at 100oC would have the opposite effect. As heat energy is removed, the temperature and phase changes would be reversed as shown below.

The chart below shows the relationship between adding and removing heat energy from a substance. Note that the processes are opposite. The temperatures are the same for melting / freezing and Boiling/ condensing. The only difference is that in the first instance heat energy is being added and in the other heat energy is being removed.
 
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