Physical and Chemical Changes: How to Tell Them Apart

Changes all around us

Ice melts, paper burns, milk turns into curd, a balloon is inflated, iron gets rusty. All of these are changes, but they are not all the same kind. Scientists sort them into two groups: physical and chemical changes.

Physical changes

A physical change alters properties such as shape, size, colour or state (solid, liquid, gas), but no new substance is formed. Many physical changes can be reversed.

Examples:

• melting ice into water, and freezing it back,

• dissolving sugar in water (evaporating the water gets the sugar back),

• cutting paper into pieces,

• stretching a rubber band,

• boiling water into steam.

Chemical changes

A chemical change produces one or more new substances with different properties. It is usually difficult or impossible to reverse. A chemical change is also called a chemical reaction.

Examples:

• burning paper, wood or a candle wick,

• cooking food,

• milk turning into curd,

• rusting of iron,

• digestion of food.

Example: burning magnesium. A magnesium ribbon burns with a dazzling white light and leaves a white ash, magnesium oxide. When the ash is dissolved in water it forms magnesium hydroxide, which turns red litmus blue, showing it is a base. A completely new substance has formed.

Example: vinegar and baking soda. Mixing them produces bubbles of carbon dioxide gas. If the gas is passed through lime water, the lime water turns milky. This is the standard test for carbon dioxide.

Signs of a chemical change

A chemical change is usually accompanied by one or more of these:

• heat, light or sound is given out or absorbed,

• a gas is produced,

• the colour changes,

• the smell changes,

• a solid (precipitate) forms.

Is burning a candle physical or chemical?

Both happen at the same time. The wax melts (a physical change) and then the vapour of the wax burns (a chemical change) to produce carbon dioxide, water vapour, heat and light.

Rusting of iron

When iron is left exposed to both oxygen and water (moisture), it gets covered with a reddish-brown, flaky layer called rust (iron oxide):

iron + oxygen (from air) + water → rust

Rusting is a chemical change. It slowly eats away iron objects such as gates, bridges, ships and car bodies. Rusting is faster in coastal areas, where the air is humid and salty.

Preventing rust. Since rusting needs both air and water, keep them away from the iron:

• paint it or apply grease or oil,

• galvanise it: coat it with a layer of zinc,

• make stainless steel, an alloy of iron with carbon and metals such as chromium and nickel, which does not rust.

Crystallisation

Large, pure crystals of a substance can be obtained from its solution by crystallisation. Dissolve the impure substance in hot water, filter it, and let the solution cool slowly. Crystals form as it cools. Salt obtained by evaporating seawater is not as pure as salt obtained by crystallisation. Crystallisation is a physical change.

Summary