Chemistry: Properties of materials

Properties are the characteristics of a substance. We can split them into two main types. Physical properties are characteristics you can observe or measure without changing the chemical identity of the substance. Examples include colour, density, electrical conductivity, and melting point. For example, you can observe that water is a colourless liquid (a physical property) without turning it into something else. Chemical properties describe how a substance behaves in a chemical reaction. They tell you how it can change into a new substance. Examples include flammability (how easily it burns), toxicity, and reactivity with other chemicals like acids or oxygen. For instance, the fact that iron reacts with oxygen and water to form rust is a chemical property of iron.

When materials change, the change can be either reversible or irreversible. A reversible change is one that can be easily undone, returning the material to its original state. Most physical changes are reversible. For example, ice (solid water) melts to form liquid water. If you cool the liquid water, it will freeze back into ice. Dissolving sugar in water is also reversible; you can evaporate the water to get the sugar back. An irreversible change is one that cannot be easily undone. These are permanent changes that result in the formation of one or more new substances. Chemical changes (or chemical reactions) are typically irreversible. For example, when you burn a piece of wood, it turns into ash, smoke, and gases. You cannot easily turn the ash back into wood. Similarly, cooking an egg is an irreversible chemical change.

A mixture contains two or more substances that are not chemically bonded together. The different parts of a mixture have different physical properties, which we can use to separate them. Key methods include:

• Filtration: Used to separate an insoluble solid from a liquid (e.g., sand from water). The mixture is poured through filter paper; the liquid (filtrate) passes through, while the solid (residue) is trapped.
• Crystallisation/Evaporation: Used to separate a soluble solid (solute) from a liquid (solvent). Evaporation removes all the solvent to leave the solid. Crystallisation involves evaporating some solvent to make a saturated solution, then cooling it to form pure solid crystals.
• Simple Distillation: Used to separate a liquid solvent from a dissolved solid (e.g., pure water from salt water). The liquid is boiled, evaporates, and is then cooled in a condenser to turn back into a pure liquid.
• Fractional Distillation: Used to separate a mixture of liquids with different boiling points (e.g., ethanol and water). The mixture is heated, and the liquid with the lower boiling point evaporates first, rises up a fractionating column, cools, and condenses separately.
• Chromatography: Used to separate mixtures of soluble substances (like inks or food colourings). The separation depends on how soluble each substance is in the solvent and how strongly it sticks to the paper.

A pure substance consists of only one element or one compound. Pure substances have specific, fixed physical properties. One of the most important is that a pure solid has a sharp, fixed melting point, and a pure liquid has a sharp, fixed boiling point. For example, pure water always boils at 100°C and freezes at 0°C (at standard pressure). We can use this property to test for purity. If a substance is impure (i.e., it's a mixture), its melting and boiling points will be affected. Impurities have two main effects: 1. They lower the melting point and increase the boiling point. 2. They cause the substance to melt or boil over a range of temperatures, not at a single, sharp point. For example, impure water (like salt water) will start to freeze below 0°C and will boil at a temperature above 100°C. Therefore, checking the melting or boiling point of a sample and comparing it to the known value for the pure substance is a powerful way to determine its purity.