HSC Year 11 Chemistry practice exams with worked answers

Matter is anything with mass that occupies space. Chemists first sort it by composition. A pure substance has fixed composition and fixed physical properties such as melting point. An element contains only one type of atom and cannot be broken into simpler substances by chemical means. A compound contains two or more elements chemically bonded in a fixed ratio, and it can only be broken down by chemical change.

A mixture contains two or more substances that are not chemically joined, so the proportions can vary and each component keeps its own properties.

Mixtures are either homogeneous or heterogeneous. A homogeneous mixture (a solution such as salt water, or an alloy such as brass) looks uniform throughout and has the same composition in every sample. A heterogeneous mixture (sand in water, granite, soil) has visibly different regions. Chemical properties describe how a substance reacts, for example whether it burns or reacts with acid. Physical properties, such as density, boiling point, solubility, magnetism and particle size, can be measured without changing the substance's identity.

Mixtures are separated by physical methods that exploit a difference in physical properties. Filtration separates an insoluble solid from a liquid by particle size. Evaporation recovers a dissolved solid from a solution because the solvent is volatile and the solute is not. Distillation separates liquids with different boiling points. Chromatography separates dissolved substances because they have different attractions to the stationary phase and different solubility in the moving solvent.

Sieving uses particle size, and magnetic separation uses the attraction of iron, nickel and cobalt to a magnet. No bonds are broken in any of these methods, so the components are recovered unchanged.

The percentage composition by mass of a component is (mass of component ÷ total mass of mixture) × 100. The percentages of all components should add to about 100%, and a total slightly below 100% suggests small losses during separation.

Properties link directly to uses. Copper is used in wiring because it is ductile and conducts electricity well, aluminium is used in aircraft because it has low density, and diamond is used on cutting tools because it is extremely hard.

These bulk properties come from the arrangement and bonding of particles at the nanoscale, which later sections explain: a model of the particles and the forces between them is used to explain why a material melts, conducts or dissolves as it does.