Chemistry - During phase change in matter, why doesn't the temperature change?

Solution 1:

From Changes of Phase (or State):

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So, how could there be a change in heat during a state change without a change in temperature?

"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."

Solution 2:

For a first-order phase transition, you need to add the enthalpy of the phase transition. As an example, starting with ice below the melting point, you pump heat in, and raise the temperature. When you hit the melting temperature, the heat you put in goes towards the enthalpy of melting, and starts converting ice (sold) to water (liquid). Additional heat continues to melt more of the ice. Once all of the ice is converted (still at the melt temperature), than more heat starts increasing the temperature of the water.

A second-order phase transition does not have an enthalpy associated with it.

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Solution 3:

It is because the heat energy is used to overcome the inter-molecular force of attraction but when this inter-molecular force is broken it changes it state and the temperature starts increasing.