High School Chemistry : Help with Melting and Freezing
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Example Question #1 : Help With Melting And Freezing
During the process of freezing, what do you know about changes in the temperature and entropy of water?
The temperature decreases and the entropy decreases
The temperature remains constant and the entropy remains constant
The temperature remains constant and the entropy increases
The temperature remains constant and the entropy decreases
The temperature increases and the entropy decreases
The temperature remains constant and the entropy decreases
The temperature of a sample during the process of a phase change will remain constant. Even though heat may be added or removed during this process, it is utilized to directly impact the formation and breaking of intermolecular interactions within the sample. The result is a change in the internal properties of the sample (the phase), with no actual change in temperature.
When a substance freezes it transitions from a liquid to a solid. The result is a much more ordered structure, as opposed to the fluidity associated with the liquid phase. Increasing order corresponds to a decrease in entropy because entropy is a measure of disorder.
Example Question #1 : Phase Diagrams And Transitions
Which of the following is associated with freezing, for most substances?
Particles move more slowly and are closer to each other
Particles move more slowly and are further apart from each other
Particles move more quickly and are closer together to each other
Particles move more quickly and are farther apart from each other
Particles move more slowly and are closer to each other
When a substance freezes, the average kinetic energy of the particles decreases. This means that the particles will move more slowly. Most substances also contract when they cool to increase the organization of the molecules (crystallization). Thus, for most substances, particles will get closer together as they freeze. One notable exception is water, which expands as it freezes (the same mass of water that freezes into ice will float in liquid water due to an increase in volume).
Example Question #2 : Help With Melting And Freezing
The following is the phase diagram for 
What state change is occurring on from point A to point G?
Condensation
Sublimation
Melting
Freezing
Vaporization
Melting
The section point A is in is the solid portion of the phase diagram. At low temperatures and high pressures, substances form solids. Increasing the temperature while keeping pressure high, ice melts into water. Point G is in the liquid portion of the phase diagram.
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