Question

In one paragraph, explain the movement of energy during phase transitions.

Answer 1

All phase changes are accompanied by changes in the energy of a system. Changes from a more-ordered state to a less-ordered state (such as a liquid to a gas) are endothermic. Changes from a less-ordered state to a more-ordered state (such as a liquid to a solid) are always exothermic.

Step-by-step explanation:

Answer 2

Step 1 - Understanding a phase transition

During a phase transition, the intermolecular structure of a substance is modified. Let's take the transition from solid to liquid as an example.

In the solid state, the atoms or molecules are very close together in what is called a crystalline structure. When we heat a solid (let's say ice), the molecules start to move faster and faster, thus moving apart from each other.

During this whole process, no temperature raising is observed: all the energy is used to break intermolecular interactions. When they are finally weakened, the molecular are farther apart, which means the substance has reached the liquid state.

Step 2 - The movement of energy

During a phase transition, energy goes into the system (i.e., the substance) in order to allow the weaking of the molecular interactions. Energy can also go out the system, if a liquid turns solid, like freezing water, for example. It depends on the directionality of the phase transition.

So, in short:

solid --> liquid --> gas (energy flows to the system)

gas --> liquid --> solid (energy flows out of the system)