One of the most important concepts in chemistry is the idea that the properties of any substance – element, compound or mixture – can be explained by its chemical structure. It could be described as 'telescopic' or 'zoom' thinking, which means that you begin thinking about a visible item, like a graphite pencil, but then zoom into the molecular level to explain its properties.
Physical and Chemical Properties
The properties of substances can be categorised into physical properties and chemical properties. Physical properties include: colour; melting and boiling points, which determine whether a substance is solid liquid or gas at room temperature; solubility in water and other solvents; and electrical conductivity. Chemical properties include the way a substance reacts with other chemicals, like acids, alkalis, metals, oxygen etc.
By considering the physical and chemical properties of a substance, a lot can be deduced about the chemical structure of that substance. Looking at the outward appearance of a substance, its colour, its shape and hardness gives clues as to its structure. Also, the effect of heat is a useful guide – whether it melts easily, straight away, or gradually, for example. A substance's solubility in water and other solvents gives further clues about its molecular bonding.
Simple Molecular Substances
These substances are made up of small molecules, or groups of atoms held together by covalent bonds. The bonds within the molecules are strong, but attraction between the molecules are weak. This means that they have low boiling and melting points, so they are often gases or liquids at room temperature. They do not conduct electricity because they contain no ions. Examples include sugar, methane, ammonia and water.
Giant Molecular Substances
These substances also have covalent bonds, but they are continuous structures with many atoms linked together by strong bonds. Each piece is therefore just one very large molecule. This means that they are often very hard substances which do not dissolve in water or conduct electricity, and have very high melting points. Examples include diamond and silica.
Ionic Substances
These substances are made up of atoms or small molecules which have an electronic change. They are held together by the attraction between positive charges and negative charges. They are usually soluble in water, forming solutions that conduct electricity. They are solid at room temperature and have high melting points. Examples include sodium chloride, ammonium nitrate and sodium hydroxide.
Composite Materials
Many substances are mixtures of various kinds of materials, which have properties which are combinations of the properties of the individual components of the mixture. These composite properties often make the material more useful. For example, glass-reinforced polyester (GRP) uses glass fibres suspended in plastic, so it has the lightness of plastic strengthened by the presence of the glass.
Sources:
C.Johnson, Chemistry for GCSE, Heinemann, 1987
J.Clark, Molecular Structures, Chemguide.co.uk, accessed 23rd January 2011
Simon Davies - Simon Davies has lived a varied life so far After completing a BSc in Chemistry at the University of Bristol, UK, he worked as a ...
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