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### Temperature during Phase Change

Posted: Wed Jan 10, 2018 1:59 pm
Why does temperature remain constant during a phase change even when more heat is added?

### Re: Temperature during Phase Change

Posted: Wed Jan 10, 2018 2:02 pm
If you look at vaporization for instance, water is not able to increase in temperature past 100 degrees celsius, because after 100 it must be in its gaseous state. So even though more heat is being added, the temperature of the water evaporating remains at 100 degrees celsius and the heat that is being added is released as the water vaporizes.

### Re: Temperature during Phase Change

Posted: Wed Jan 10, 2018 2:36 pm
The textbook says "At the melting point, the temperature remains constant, because all of the heat is being used to melt the sample."
So there is an energy barrier that must be overcome before a liquid could become a vapor.

### Re: Temperature during Phase Change

Posted: Wed Jan 10, 2018 2:43 pm
Yes! The energy goes into breaking the bonds so the temperature does not increase. For example when a solid melts into a liquid the heat is going into breaking the bonds of the solid!
Hope this helps!

### Re: Temperature during Phase Change

Posted: Sun Jan 14, 2018 1:25 pm
In changing the temperature of a substance, heat can play a role as long as we aren't at the points of phase change for that substance. For example, water at 50 C can increase or decrease in temperature since it remains in its liquid state; however, as it hits 100 C, it stops changing temperature, because now the water molecules have enough energy to vaporize, so any molecules left in the liquid state will be strictly no higher than 100 C. Once the molecules have vaporized (into steam), the temperature can begin to climb again since gaseous water molecules cannot drop below 100 C without changing back into their liquid phase.

### Re: Temperature during Phase Change

Posted: Sun Jan 14, 2018 3:38 pm
The heat supplied to a substance when the substance is at melting/boiling point will go towards breaking the inter-molecular/atomic bonds. Although there are many kind of bonds between molecules such as H Bonds, London Forces, Ionic bonds etc, all of them are bonds. Breaking of bonds is an endothermic process as heat is supplied to the system. When all the heat supplied goes to breaking the bonds, the temperature of the system does not increase.

### Re: Temperature during Phase Change

Posted: Sun Jan 14, 2018 5:57 pm
Also another way to think about it is that at 100 degrees Celsius, that is when boiling begins, not when all of it is 100% boiled. So more energy needs to be added to get it to be boiled 100%.