CHEMISTRY COMMUNITY

why does the temperature remain constant during a phase change?

The temperature remains constant during a phase change because the energy is used to break bonds to change the phase rather than to change the temperature. Hope this helps!

Because all the energy are being used to either break a bond or form a bond, thus no energy is used for heating up the material

remember it is not technically breaking bonds, as in covalent bonds between Hydrogen and Oxygen molecules. It is breaking intermolecular forces between water molecules. Therefore, energy is needed to overcome and break the intermolecular forces which requires heat.

During the phase change, when solid melts into liquid, its temperature remains constant as the heat energy is stored as potential energy.

It is important to distinguish between breaking bonds and breaking the intermolecular attractions between molecules in a substance. During phase changes, the chemical bonds in the molecules of the substance aren't actually being broken. Rather, the energy added is being used to break the attractions between molecules in the substance. However the substance remains the same, so when water is evaporated it doesn't turn into H₂ and O, it stays H₂O.

The temperature stays the same because the energy going into the system is used to change the phase of the substance and not increase the temperature. Once the phase change is complete the temperature starts rising again.

Energy is used for the phase change, so there is no increase/decrease in temperature.

Additionally, more heat energy is required to break bonds when a substance is vaporizing (liquid to gas) as opposed to when a substance is melting (solid to liquid) because bonds have to be completely broken in the gas phase.