5.7 understand the changes that occur when a solid melts to form a liquid, and when a liquid evaporates or build to form a gas

Solid to liquid - when a solid is heated, its particles gain more energy, this makes the particles vibrate more which weakens the forces that hold a solid together, this makes the solid expand. At a certain temperature, known as 'boiling point' (different for different substances), the particles have enough energy to break free from their positions.

Liquid to gas - when a liquid is heated it will evaporate/boil. When this happens, the particles gain more energy, this energy makes the particles move faster which weakens and breaks the bonds holding the liquid together. At a certain temperature (different for different substances), the particles have enough energy to break their bonds.

NOTE: if you are unsure of these or need a little more info to understand, it may be able to head to post 1.2 of my chemistry blog where there is a little more info on interconversions