As water freezes, it releases heat into the universe to balance the local loss of entropy encountered by ordering atoms into a rigid solid structure. Entropy, in contrast, is not a commodity that can be released by a reaction in the same way as heat. Also, water vapor would not be released on only one side of the equation. Water vapor would exist with in equilibrium with both solid and liquid, but would not appear as an additional product with the solid phase in this reaction. 

The energy required to cause a liquid-gas phase change is called the heat of vaporization, and is usually given in the units of kilojoules per mole. Enthalpy of vaporization and heat of vaporization are the same quantity.

When the heat of vaporization is given for a single gram of substance, with the units of Joules per gram, it is termed the specific heat of vaporization. This is the quantity given in the question: the energy required to vaporize exactly one gram of the sample.

Specific heat capacity refers to the energy needed to raise the temperature of one gram of a sample by one degree Celsius.

Freezing point is always a temperature, meaning it must correspond to a single point on the y-axis. During the freezing/melting process, heat is absorbed/released from the system without any change in temperature. This results in the temperature plateau at the freezing point.

While the substance is freezing during interval B, the freezing point is the temperature at which it freezes, X.

During intervals B and D, fusion and vaporization are taking place, respectively. Instead of raising the temperature of the substance, the energy that is added during these phase changes is used to overcome intermolecular forces. The energy breaks the attraction between particles, allowing them to separate and gain the properties of a higher energy phase (liquid or gas).

When the slope is not zero, the phase is steady and the added heat energy is used to increase the molecular kinetic energy of the particles, resulting in a temperature increase.

Use the given molar heat of fusion to calculate the amount of snow formed. Remember that heats of fusion and solidification are opposite processes, so the magnitudes of molar heats of fusion and solidification are the same and signs are opposite. This indicates that fusion (melting) is endothermic, while solidification is exothermic.

The given change in energy will be negative, since the question states that it is released from the system into the atmosphere.

Convert moles to grams, and grams to pounds.

We must use the given heats of fusion and vaporization to calculate the energy change involved in these two processes.

First, calculate the energy change when  of water solidifies. Convert the mass to moles and multiply by the heat released during freezing. This value will be equal in magnitude, but opposite in sign to the heat of fusion.

From this calculation we find that  of heat is released into the surroundings (a negative sign denotes an exothermic process). 

Next, find the energy change associated with the vaporization of of water, using the given heat of vaporization:

We find that  of energy is absorbed when this quantity of water is vaporized.  

Adding the two together we find a total of .

Since this is a positive number, that means that the energy is absorbed from the surroundings (endothermic).

Solving this problem means solving for three steps.

1. The heat needed to raise the temperature from –20^oC to 0^oC.

2. Melting the ice (changing phases).

3. Raising the water temperature from 0^oC to 50^oC.

Steps 1 and 3 are both solved by the equation .

Step 2 is solved using the enthalpy of fusion, and is multiplied by the number of grams being melted: .

Combining the steps, we get the following expression.

Vaporization and condensation refer to the transition from liquid to gas and from gas to liquid, respectively. Fusion and freezing, in contrast, refer to the transition from solid to liquid and from liquid to solid, respectively. Vaporization (boiling) and fusion (melting) each require an input of energy, making them endothermic processes with positive changes in enthalpy. Condensation and freezing result in a decrease in energy and an output of enthalpy, making them exothermic.

The given enthalpy of fusion tells us that are consumed for every mole of water. If two moles of water are present, then are consumed during melting.

Enthalpy of vaporization will be equal and opposite the enthalpy of condensation. Since condensation is exothermic, heat will be released and the change in enthalpy must be negative (not positive).

Heat of condensation:

Intermolecular forces play a key role in determining the energy required for phase changes. Strong intermolecular forces result in more resistance to changes that result in greater distance between molecules (greater entropy), as the forces cause the molecules to "stick" to one another. When a liquid is vaporized, the strength of the intermolecular force is overcome; similar heats of vaporization indicate similar intermolecular forces. A similar concept governs the transition from solid to liquid. If two compounds share similar enthalpies of fusion and vaporization, then they likely have similar intermolecular forces.

In the given table, ethanol enthalpy values are most similar to those of water, meaning it likely has similar intermolecular forces.

Plateaus in the graph represent phase changes: period B shows the transitions between solid and liquid, and period D shows the transitions between liquid and gas. When the substance transitions through period D, it undergoes either vaporization (C to E transition) or condensation (E to C transition). If the substances starts out at interval E and ends at interval C, then it has undergone condensation.

Condensation involves transition from a high energy gas to a lower energy liquid, and has a net decrease in heat energy and temperature. This heat release is known as an exothermic process.