Chemistry: Properties of materials

All substances can be classified into three main groups. Elements are the simplest form of matter, made up of only one type of atom (e.g., oxygen, O₂; iron, Fe). Compounds are pure substances formed when two or more different elements are chemically bonded together in a fixed ratio. These bonds can only be broken by a chemical reaction (e.g., water, H₂O; carbon dioxide, CO₂). Mixtures consist of two or more elements or compounds that are not chemically bonded. Their components can be separated by physical means like filtering or evaporation (e.g., salt water, air).

Dissolving is the process where a substance breaks down and mixes completely with another substance to form a solution. The substance that dissolves is called the solute (e.g., sugar). The substance that does the dissolving is called the solvent (e.g., water). The resulting mixture is called a solution. In a solution, the solute particles are spread evenly throughout the solvent particles, which is why most solutions are transparent (you can see through them). This is a physical change, not a chemical reaction, and the total mass is conserved, meaning the mass of the solution equals the mass of the solute plus the mass of the solvent.

Solubility is a measure of how much solute can dissolve in a certain amount of solvent at a specific temperature. It is usually expressed as 'grams of solute per 100 grams of solvent' (g/100g solvent). When you keep adding solute to a solvent, you eventually reach a point where no more can dissolve. At this point, the solution is said to be saturated. Any solution that can still dissolve more solute is called an unsaturated solution. If you add more solute to a saturated solution, it will simply sink to the bottom without dissolving.

Several factors can change how much or how fast a substance dissolves. It's crucial to distinguish between factors affecting solubility (how much dissolves) and the rate of dissolving (how fast it dissolves). For most solid solutes, solubility increases as the temperature of the solvent increases. For gases, the opposite is true: their solubility decreases as temperature increases. The rate of dissolving is increased by: 1) Higher temperature (particles have more energy and move faster), 2) Stirring (brings fresh solvent into contact with the solute), and 3) Increased surface area (e.g., powder dissolves faster than a large crystal).

Paper chromatography is a technique used to separate and identify substances in a mixture, such as the different coloured dyes in an ink. It works because different substances have different solubilities in the solvent and different levels of attraction to the paper. In the experiment, a spot of the mixture is placed on a pencil line on chromatography paper (the stationary phase). The paper is then dipped into a solvent (the mobile phase), which soaks up the paper. As the solvent moves past the spot, it dissolves the mixture and carries it up the paper. Substances that are more soluble in the solvent and less attracted to the paper travel further, separating the mixture into its components.

These are common laboratory techniques used to separate mixtures based on differences in their physical properties. Filtration: Used to separate an insoluble solid from a liquid (e.g., sand from water). The mixture is poured through filter paper in a funnel. The liquid (filtrate) passes through the tiny pores in the paper, while the solid (residue) is trapped. Evaporation: Used to separate a soluble solid (solute) from a liquid (solvent) when you want to keep the solid. The solution is heated, causing the solvent to turn into a gas and escape, leaving the solid crystals behind. Simple Distillation: Used to separate a liquid solvent from a solution when you want to keep the liquid. The solution is heated to its boiling point, the solvent vaporises, passes into a condenser where it is cooled, and turns back into a pure liquid (distillate) which is collected separately.