Phase Changes of Water Graph

[Yasmina Zaarour 1G]

Hi! In lecture today, Dr. Lavelle had a graph of temperature vs heat supplied for water and the phases associated with each portion of the graph. My question is, are the slopes of the solid, liquid, and vapor portions of the graph supposed to be equal? That is, does it take the same amount of heat to raise the temperature of a solid to its melting point as it does to raise the temperature of a liquid to its boiling point?

[Neha Gupta 2A]

I don't know the exact numbers, but it takes much more energy to go from liquid > gas (boil) than to go from solid > liquid (melt). This is because gas particles are much more separated than both solids and liquids, so melting doesn't separate the particles as much as boiling does.

[Ria Nawathe 1C]

Like the previous answer stated, the enthalpy of vaporization is always greater than the enthalpy of fusion because a phase change from solid to liquid only weakens the intermolecular attractions enough so that the molecules can slide around each other, but liquid to gas requires the intermolecular attractions to be completely overcome so that the molecules can completely escape each other. Also, I do believe the slopes are different.

[Kiran Singh 3A]

No, I believe the slopes are not equal. The slope for the solid and vapor phases are steeper than the slope for the liquid because the liquid has a higher heat capacity, and a steeper slope means a lower heat capacity. Hope this helps.

[David Chibukhchian 2G]

I definitely agree with the other answers. I initially was wondering why there's this difference of steepness of the graph at various points, but I believe that the heat capacity is very significant to understanding the reason. I remember reading in the textbook that (for example) if a substance in liquid form has a higher heat capacity, its slope will be less steep than at other phases. We can think of this as meaning that it's harder to heat up that liquid, and thus the temperature ends up increasing more slowly compared to solid or gas states. That's what I gathered from today's lecture and the reading, I hope that helps!

[Inderpal Singh 2L]

No, although they look similar, the slopes should not be of equal value. Since liquid's heat capacity is higher, the slope of it is less steep compared to the solid and vapor phases.

[Xavier Herrera 3H]

The slope from solid to the freezing point is steeper than the slope from the freezing to boiling point because it takes more energy to change water from a liquid to gas than from a solid to liquid.

[Samudrala_Vaishnavi 3A]

The line just represents the heat's increase but not the direct or exact number. I would say the graph is just trying to represent a proportional relationship to the higher phase change and a higher temperature. The more heat or energy you have, the higher up the phase change (fusion to sublimation). For sublimation, however, the slope is steeper and there is a low heat capacity so less effort is needed to change phases so it happens faster.