Question

The temperature of a substance does not change as the substance boils. Write two or three sentences to explain this at a molecular level.

Answer 1

"During a change in state the heat energy is used to change the bonding between the molecules. In the case of melting, added energy is used to break the bonds between the molecules. In the case of freezing, energy is subtracted as the molecules bond to one another. These energy exchanges are not changes in kinetic energy. They are changes in bonding energy between the molecules.

"If heat is coming into a substance during a phase change, then this energy is used to break the bonds between the molecules of the substance. The example we will use here is ice melting into water. Immediately after the molecular bonds in the ice are broken the molecules are moving (vibrating) at the same average speed as before, so their average kinetic energy remains the same, and, thus, their Kelvin temperature remains the same."

Changing state A substance must absorb heat energy so that it can melt or boil. The temperature of the substance does not change during melting, boiling or freezing, even though energy is still being transferred. A heating curve is a graph showing the temperature of a substance plotted against the amount of energy it has absorbed. You may also see a cooling curve, which is obtained when a substance cools down and changes state. gas 120 temperature 80 boiling 60 liquid 40 melting 20 20 solid -40 heat absorbed A heating curve for ice Notice that the temperature stays the same during a change of state, melting or boiling, even though heat energy is still being absorbed. The temperature also stays the same while a liquid freezes, even though heat energy is still being released to the surroundings. Specific latent heat The specific latent heat of a substance is a measure of how much heat energy is needed to melt or boil it. It is the energy needed to melt or boil 1 kg of the substance. Different substances have different specific latent heats. The specific latent heat of a given substance is different for boiling than it is for melting.