Elicit a definition of matter based on the properties of mass and volume. Discuss how air is also classed as matter, and work out a lab which can be used to determine the mass and volume of a given amount of air. Use table salt as an example of matter, determine its formula and then discuss how table salt is classed as a pure substance. Determine that water is also a pure substance, but sea water is not. Explain the difference. Outline that chemistry will focus on the composition of substances and how they interact.