Chemistry, Third edition

MATTER AND ENERGY

3. Mixtures

Mixtures contain more than one substance (elements and/or compounds). The

composition of a mixture can be varied.

Air is a mixture of the elements oxygen and nitrogen, the compounds water

vapour and carbon dioxide, together with trace amounts of other gases. However,

the composition of a sample of air taken in an industrial, polluted, area would be

different from that of a sample taken on Mt Everest.

Elements: metals and non-metals

Elements can be roughly divided into metals and non-metals.

1. Metals

Metals usually have the following properties:

●They have a lustre (a ‘shiny’ appearance).

●They conduct heat and electricity.

●They can be drawn into wire (are ductile).

●They can be hammered into thin sheets (are malleable).

●They make a ‘clanging’ sound when you hit them (are sonorous).

●In chemical reactions metals react with acids, form basic oxidesand form

positively charged ions(you may not understand these terms at this point, but

will be able to refer back to these properties later on).

Examples of metals are gold, silver, copper and iron. Mercury is a liquid metal at

room temperature.

2. Non-metals

Non-metals usually have the following physical properties:

●They are poor conductors of heat and electricity.

●They cannot be hammered into sheets or drawn into wire, because they are

brittle.

17

BOX 2.1

Purity of substances

Purity is very important in chemistry. Pure

elements and compounds, for example a

pure sample of the element copper or

pure salt (the compound sodium

chloride), have reproducibleproperties. In

other words, different samples of salt

behave in exactly the same way under

identical conditions: they have the same

melting points, undergo the same

chemical reactions and so on. Mixtures,

because they can have varying

compositions, often do not behave in the

same way under identical conditions.

Different samples of soil, for example,

may have very different properties

although they are all referred to as ‘soil’.

Guaranteed pure reagents were first

advertised by the German chemical and

pharmaceutical firm of Merck in 1888.

During the First World War, when the

British found themselves unable to

obtain these pure chemicals, British

chemical manufacturers were

encouraged to produce chemicals to new

standards of purity. These chemicals

were classified AR (or Analytical

Reagent). Today, very pure chemicals

conform to a British standard and are

called ‘AnalaR’. Look at reagent bottles

in your laboratory and see if you can spot

AnalaR chemicals!