Double Award - Science for GCSE

Chapter 13

Mixtures, elements and compounds

1.	Materials A material is a form of matter which can be used in some way. Some materials, such as crude oil, sand and water are found in the world around us. We say that they are naturally-occurring materials or raw materials. These materials can be made into useful materials, like petrol and cement which we call man-made materials. There are five important types of useful materials: – metals (e.g. copper and steel) – plastics (e.g. polythene and PVC) – ceramics or pottery (e.g. concrete, bricks and china) – glasses (e.g. bottle glass and window glass) – fabrics (e.g. paper, wood and wool) In choosing materials for different uses we need to ask two questions. – Does it have the right properties for the job? – Is the cost reasonable? Different materials have very different properties. Some materials can cause hazards (e.g. petrol is very flammable, chlorine is poisonous). Hazard symbols are used to label materials and chemicals that might cause problems.
2.	Separating materials Most naturally-occurring materials are mixtures. For example, sea water is a mixture of two substances, water and salt. It is often necessary to separate mixtures. The method used depends on a difference in properties of the substances being separated.
3.	Methods of separation Filtration is used to separate an insoluble solid from a liquid (e.g. grains from coffee). Centrifuging is used to separate really tiny particles of a solid or liquid from a less dense liquid (e.g. a milk centrifuge spins rapidly, forcing denser milk below the cream). Evaporation is used to separate a dissolved solid (solute) from a solution by evaporating off some of the solvent and then allowing the solid to crystallise. (E.g. salt can be separated from sea water by heating the sea water so that most of the water evaporates and then allowing the solution to cool and crystals of salt to form.) Distillation is used to separate a solvent from a solution (e.g. to obtain pure water from sea water). Fractional distillation can be used to separate miscible liquids (i.e. liquids which mix completely). Fractional distillation is used to separate the different substances in crude oil. The different substances have different boiling points so they boil off at different temperatures and are condensed separately. Chromatography can be used to separate very similar substances such as dyes in ink, different sugars in urine and drugs in the blood.
4.	Elements and compounds Elements, such as iron, copper, carbon and oxygen, are the simplest substances. They cannot be broken down into simpler substances by chemical reactions. Elements are often divided into metals (such as iron and copper) and non-metals (such as carbon and oxygen). The easiest way to check whether an element is a metal or a non-metal is to see if it conducts electricity. Compounds are substances containing two or more elements combined together (e.g. carbon and oxygen are combined together in carbon dioxide). Mixtures contain two or more substances which are not combined together. They may be mixtures of elements, mixtures of compounds, or mixtures of elements and compounds. For example, pure dry air is a mixture of elements (nitrogen, oxygen and argon) and a compound (carbon dioxide). Most materials are mixtures.
5.	Chemical reactions When elements combine to form compounds, we say they have reacted and the process is called a chemical reaction. The substances which react are called reactants and the new substance which forms is called the product. For example, A synthesis reaction occurs when simpler substances combine to form a more complex substance. A decomposition reaction occurs when compounds are split into simpler substances. Decomposition is the reverse of synthesis. For example, The Law of Conservation of Mass is one of the most important scientific ideas. It summarises the fact that matter and materials cannot be created or destroyed. It says: ‘When substances react, the total mass of the products equals the total mass of the reactants.’