Elements, compounds and mixtures Study Guide

The way a material behaves (its bulk properties) is directly linked to the type of chemical bonds it contains, the strength of these bonds, and how its atoms or ions are arranged. For example, ionic compounds like salt are solids with high melting points because they have strong electrostatic forces of attraction between oppositely charged ions in a giant lattice structure. A lot of energy is needed to overcome these forces. They conduct electricity only when molten or dissolved, as their ions become mobile. Simple molecules, like water or oxygen, have strong covalent bonds *within* each molecule but only weak intermolecular forces *between* molecules. This means they have low melting and boiling points, as little energy is needed to overcome these weak forces, making them gases or liquids at room temperature. They do not conduct electricity because they have no free charged particles. Giant covalent structures, such as diamond or silicon dioxide, consist of huge networks of atoms held together by numerous strong covalent bonds. Breaking these strong bonds requires a large amount of energy, resulting in very high melting and boiling points and making them very hard. They do not conduct electricity (except for graphite, which has delocalised electrons). Polymers are long chains of repeating units (monomers) held together by strong covalent bonds within the chain and weaker intermolecular forces between chains. Their properties vary widely depending on the strength of these intermolecular forces and how the chains are arranged, ranging from flexible plastics to rigid fibres. Metals consist of a lattice of positive metal ions surrounded by a 'sea' of delocalised electrons. The strong electrostatic attraction between the positive ions and the delocalised electrons (metallic bonding) leads to high melting points and makes metals strong. The delocalised electrons are also responsible for their excellent electrical and thermal conductivity.

When faced with a mixture, chemists need to choose the most suitable technique to separate and purify the desired substance. The choice of technique depends critically on the physical properties of the components in the mixture, such as solubility (whether a substance dissolves in a solvent), boiling points (the temperature at which a liquid turns into a gas), and particle size. For example, if you have a mixture of a soluble solid and an insoluble solid, you might dissolve the soluble solid, then filter out the insoluble one. If you have a solid dissolved in a liquid, evaporation or crystallisation could be used if the solid is the desired product and the solvent doesn't need to be recovered, or distillation if both the solvent and the pure solid are needed. Simple distillation involves heating the mixture, vaporising the liquid, and then condensing it back into a liquid, leaving the solid behind. Fractional distillation is used when separating liquids with different boiling points, as it allows for a more efficient separation. Filtration is ideal for separating insoluble solids from liquids, like sand from water. Chromatography, on the other hand, is used to separate dissolved solids that have different solubilities in a solvent and different attractions to a stationary phase, effectively separating components in a solution, often used for dyes or pigments.