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Chemical and Physical Changes

Key Concepts

A chemical change involves the formation of atleast one new substance.
A chemical change is usually difficult to reverse.
A physical change does not involve the the formation of any new substances.
A physical change involves a change in the physical state or appearance of the substance.
A physical change is usually easy to reverse.

Physical Changes

No new substances are produced during a physical change.

Common physical changes include:

Physical Change Example

Melting a Solid
change of state Ice, solid water, can be heated to form liquid water.

Freezing a liquid
change of state Liquid water can be frozen to form solid water, ice.

Boiling a liquid
change of state Liquid water can be heated to form water vapour, a gas.

Condensing a gas
change of state Water vapour can be cooled to form liquid water.

Dissolving a solid Solid sodium chloride can be dissolved in liquid water to form sodium chloride solution.

Evaporating a solution Sodium chloride solution can be gently heated to remove the water as water vapour leaving the solid sodium chloride behind.

Grinding a solid Pieces of calcium carbonate can be ground down to form smaller pieces of calcium carbonate.

Changing the shape A piece of copper wire can be hammered out into a flat sheet of copper.

Chemical Changes

A chemical change results in the formation of one or more new substances.

There are a number of observations that we can make to detect a chemical change:

Observation Example

a gas is evolved when magnesium reacts with hydrochloric acid a new substance, hydrogen gas, is produced.

an odour is produced When hydrochloric acid is added to iron sulfide, the pungent odour of "rotten eggs" is produced due to the formation of the new substance hydrogen sulfide gas.

a new solid is formed When silver nitrate solution is added to sodium chloride solution, a new white solid of silver chloride is produced.

a colour change When metallic iron reacts with oxygen in the presence of water, a new red coloured solid called rust is formed.

a temperature change When sodium reacts with water the temperature of the water increases substantially due to the chemical reaction between the two reactants.

a solid disappears
(that is not due to a physical change) When solid sodium hydroxide is added to hydrochloric acid, the solid disappears due to a chemical reaction between the two reactants.