Remember, temperature is a measure of the average kinetic energy of molecules. So, the potential energy of the molecules will increase anytime energy is being supplied to the system but the temperature is not increasing. Therefore the potential energy is increasing during segments 2 and 4.

When the kinetic energy is increasing (the temperature is also increasing) the substance is not going through a phase change. Therefore only the segments that are at an incline will have the substance in just one phase. The flat areas of the graph represent areas in which heat is being added, but there is no corresponding increase in temperature. Rather, this added heat energy is used to break the intermolecular forces between molecules/atoms and drive phase changes.

Finally, because liquids are higher in energy than solids, and lower in energy than gasses the middle slanted line must be the liquid phase. In this case it is labeled as segment 3.

When kinetic energy is increasing molecules are simply moving faster. However, when the potential energy is increasing molecules are changing phases. Therefore, when the potential energy is increasing is when the molecule is changing phases. Therefore there is a mix of molecules during segments 2 and 4.

Boiling is a phase change from liquids to gas. Therefore we are looking for a segment that is flat (because the potential energy is increasing) and that is between the liquid and gas phases. 

In this case, gas phase is the highest energy phase, and liquids is the next highest. Therefore the substance is boiling during segment 4.

Remember, temperature is a measure of the average kinetic energy. Therefore the kinetic energy will be the highest when the temperature is the highest.

The potential energy increases as the substance goes through phase changes. Therefore the potential energy is highest at the last possible instant of the most energy intensive phase change. In this case it is at the end of segment 4.

Because phase changes happen when the kinetic energy (temperature) remains constant melting will occur during a flat portion of the heating curve.

In this case it is the first flat portion because solid is the lowest energy phase represented in this diagram. Therefore the melting point is at .

This question is presenting us with a scenario in which heat is being added to a block of ice at its melting temperature. It's asking us to identify a false statement.

Recall that when anything goes through a phase change, the temperature remains the same until the phase change is complete. Furthermore, since temperature is a measure of the average kinetic energy of a system, this will not change either. Moreover, as energy is added to a system, the bonds between the components of that system will gradually weaken and then break as the entropy of that system increases.

Definition of triple point: where solid, liquid, and gas exist in equilibrium

the critical point is where the solid and gas cannot be distinguished