Why extra energy?


condenses	vaporises
		ENERGY
freezes	melts

Energy is required to separate particles that are attracted to one another. You will be most familiar with the idea that positives and negatives attract.

Potential energy is energy of position. The potential energy of the system is higher if the positive and negative particles are separated.

The particles in condensed phases are also attracted to one another. The diagrams give a snapshot of a very small sample of matter that is composed of blue particles.

The particles could be individual atoms, but more commonly they consist of several atoms joined by chemical bonds (as in H₂O).

The solid form has a regular arrangement of the particles where they are all stacked neatly.

While particles in a solid are in motion, the extent of their movement is very limited.

Heat energy supplied at the melting temperature overcomes the attractive forces between the particles so that they can move around one another to a larger extent and form a liquid.

The liquid phase has a more random arrangement of the particles, but the particles are the same.

Heat energy supplied at the boiling temperature increases the average energy the blue particles, so they can become separated from one another to form the gas.

The gas phase has the particles separated from one another, but the particles are the same.

The heat energy required to complete convert a liquid sample to gas is greater than that required to convert a solid sample to liquid because converting a liquid to a gas requires separating the particles to a greater extent.