Which of the following happens when a material undergoes a phase change?

a. the temperature changes.
b. heat flows into or out of the material.
c. the chemical composition changes.
d. the molecules break apart into atoms.

We start by describing that phase transition is most commonly used to describe the change between solid, liquid and gaseous states of matter, including plasma for particular cases. When the transitional state between two substances is reached, the temperature change becomes null until there is the exchange required for the union or disunity of the atoms. The thermal energy supplied or eliminated is equal to the binding energy between atoms.

There is no change in chemical composition unless a combustion state is reached. But that is not related to the phase transition. The molecules are not altered either because that would alter the composition and in the phase change the temperature in this state has no change.

Therefore the correct answer is B.