(From CA Book)
A solid is: matter with a definite volume and shape. Marbles, pasta and grains of sand are solids, so their shapes and volumes do not change.
A liquid is matter that has definite volume, but no definite shape. At room temperature, water is a liquid; its shape can change but its volume does not.
A gas is matter that has no definite shape or volume. For example, if you take a balloon (the shiny kind) outside on a cold day, it looks smaller. The gas inside the balloon changes shape and volume.
Humidity is a measure of the amount of water vapor in the air.
evaporation
condensation
melting
freezing
change of state
molecules

Other vocabulary

particle