Changes of state and the particle model

• •Required practical — regular solid: measure mass on a balance; measure length/width/height with a ruler (or diameter with callipers for a cylinder) and calculate the volume from the shape's formula; density = mass ÷ volume.
• •Required practical — irregular solid: measure mass on a balance; find the volume by displacement — lower the object into a eureka can filled to the spout with water, and the volume of water collected in a measuring cylinder equals the object's volume.

Density depends on how closely packed the particles of a material are. In solids and liquids, particles are closely packed with little space between them, so solids and liquids have similar densities (usually a liquid is slightly less dense than the same substance as a solid, though water is a well-known exception — ice is less dense than liquid water). In a gas, particles are spread much further apart and have much more energy to move around, so gases have a far lower density than solids or liquids of the same substance.

Mass is conserved during a change of state — if 20 g of liquid evaporates, the gas produced also has a mass of 20 g. No particles are created or destroyed, they simply rearrange and change how far apart they are.