Elements, compounds and mixtures Study Guide

All substances exist in one of three common states of matter: solid, liquid, or gas, depending on the temperature and pressure. For chemistry at GCSE, we typically consider substances at standard atmospheric pressure. The state of a substance at a given temperature can be predicted by comparing that temperature to its melting point and boiling point. The melting point is the temperature at which a solid turns into a liquid, and the boiling point is the temperature at which a liquid turns into a gas. Above the melting point but below the boiling point, a substance will be a liquid. Below its melting point, it will be a solid. Above its boiling point, it will be a gas.

For example, consider water. Its melting point is 0°C and its boiling point is 100°C. At -10°C, which is below 0°C, water is a solid (ice). At 50°C, which is above 0°C but below 100°C, water is a liquid. At 120°C, which is above 100°C, water is a gas (steam). This principle applies to all pure substances, though their specific melting and boiling points vary widely. By having a table of melting and boiling points, you can determine the physical state of a substance at any specified temperature. This skill is crucial for understanding chemical reactions and material properties in various applications.

Chromatography is a powerful analytical technique used to separate and identify components in a mixture. It works on the principle that different substances have different affinities for a stationary phase and a mobile phase. In paper or thin-layer chromatography (TLC), the stationary phase is usually a piece of filter paper or a glass plate coated with a thin layer of adsorbent material, respectively. The mobile phase is a solvent that moves up the stationary phase by capillary action. As the solvent moves, it carries the components of a sample with it. Substances that are more soluble in the mobile phase and have less affinity for the stationary phase will travel further up the paper or plate. Conversely, substances with lower solubility in the mobile phase and stronger affinity for the stationary phase will travel shorter distances. A pure substance, by definition, contains only one chemical compound. Therefore, when subjected to chromatography, a pure substance should produce only a single spot on the chromatogram. If a sample produces more than one spot, it indicates the presence of multiple components, meaning the substance is impure. The 'Rf value' or retention factor can be calculated for each spot (distance travelled by spot / distance travelled by solvent front) and can be used to identify components by comparing them to known Rf values under identical conditions.