Chemistry, Third edition

16 2 · ELEMENTS, COMPOUNDS AND REACTIONS

●Light energyis emitted by the sun.

●Sound energyis produced by a person speaking.

●Nuclear energygenerates electrical power.

●Chemical energyis stored in a battery.

Forms of matter

Matter is generally observed in three physical states: solids,liquidsandgases. A

fourth state of matter, plasma, is formed at very high temperatures and may be

found in the atmospheres of stars.

Matter is composed of elements,compoundsandmixtures. The composition of

matter is summarized in Fig. 2.1.

1. Elements

Elements are basic forms of matter and cannot be broken down into simpler sub-

stances by chemical reactions.

Commonly known elements include copper, gold, oxygen, mercury and sulfur.

At present, there are 111 named elements (see the Periodic Table, shown on the

inside front cover, which is an arrangement of all known elements) of which 90 have

been found naturally; the others have been made by scientists.

2. Compounds

Compounds consist of two or more elements joined together in fixed proportions to

form a new substance.

For example, sodium and chlorine combine together to form sodium chloride,

or salt. Sugar is formed from a combination of the elements carbon, hydrogen and

oxygen. Chemical reactions, or chemical changes occur when new substancesare

formed: elements react together to form compounds, or compounds can be broken

down into their constituent elements by chemical changes.

Both elements and compounds are substances; the composition of a substance is

always the same, regardless of its source. Notice that the scientific meaning of the

word ‘substance’ is very precise – a substance is a single pure form of matter, and not

a mixture of different kinds of matter. Substances are relatively rare; however, most

naturally occurring materials on Earth are mixtures.

Elements

Pure substances

Compounds

Mixtures

Reactions

Separated

MATTER

(solid, liquids and gases)

Mixed in

var ying

proportions

Fig. 2.1Different forms of matter.