Chemistry, Third edition

PHYSICAL AND CHEMICAL CHANGES

Physical and chemical changes

Physical changes

Theboilingof water to produce steam is an example of a physical change. The water

changes state, from a liquid to a gas, but it does not change into a different type of

matter. When salt is dissolved in water, it appears to disappear – but we know it is still

there because the water tastes ‘salty’. The salt has broken down into small particles

which are thoroughly mixed with the water molecules, but the chemical identity of

the salt has not changed. Dissolvingis another example of a physical change. The

substance which has dissolved (in this case, salt) is called the solute; the substance in

which it has dissolved (in this case, water) is called the solvent.

Chemical changes (chemical reactions)

Achemical reactioninvolves the conversion of one type of matter into another. New

substances are formed. After a chemical reaction there is no change in the total

mass from that present initially. This is the law of conservation of mass. For

example, rust is a compound formed from the chemical combination of iron, oxy-

gen and water. If a piece of iron is allowed to rust completely:

total mass of rust 

mass of iron mass of combined water mass of combined oxygen

In another example, the amount of money in your bank account may get smaller

between pay cheques, but the money hasn’t been destroyed – it has simply gone into

other accounts! The same principle applies to chemical reactions; atoms are not

destroyed but they rearrange in different ways. Charcoal, burning on your barbeque,

may appear to do a vanishing act:

charcoaloxygen (from air) carbon dioxide

However, if you added the mass of oxygen, from the air, used up in the reaction and

2.2

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BOX 2.3

Molecules

Examples of elements that normally exist as

diatomic molecules include: oxygen, hydrogen,

nitrogen, chlorine, bromine and iodine (O 2 , H 2 ,

N 2 , Cl 2 , Br 2 and I 2 ). Phosphorus exists as a

tetraatomicmolecule (P 4 ), whereas a

common type of sulfur molecule contains

eight atoms (S 8 ) (Fig. 2.2).

Molecules, made up of atoms, are also

incredibly small. A single water molecule,

consisting of two atoms of hydrogen joined to

one atom of oxygen, has a mass of 3 

10 ^23 g (Fig. 2.3).

Fig. 2.2Molecules of elements. Fig. 2.3Water molecule.

Physical and

chemical changes

Which of the following

changes do you think are

physical or chemical?

(i) the burning of coal

(ii)the freezing of water

(iii)the melting of solder

(iv)the dissolving of sugar

in tea

(v)the magnetization of

iron.

Exercise 2A