Physical and chemical changes

Change is all around us: ice melts, milk turns into curd, iron rusts, a candle burns. Scientists divide changes into two kinds, physical and chemical. The question to ask is simple: has a new substance been formed?

Physical changes

In a physical change, the shape, size, colour or state of a substance changes, but no new substance is formed. The substance remains what it was. Most physical changes can be reversed.

• Ice melting into water, and water freezing into ice.
• Water boiling into steam.
• Sugar dissolving in water: evaporate the water and the sugar is back.
• Tearing paper, breaking glass, chopping vegetables: these cannot easily be undone, but the pieces are still paper, glass and vegetable.
• Stretching a rubber band, melting wax, a bulb glowing.

Chemical changes

In a chemical change, one or more new substances are formed, with properties different from the original. Such a change is also called a chemical reaction. Most chemical changes cannot be reversed by simple means.

• Burning of paper, wood or fuel: ash, smoke and gases are formed.
• Rusting of iron: a new brown substance, rust, is formed.
• Milk turning into curd.
• Cooking of food, baking of a cake.
• Ripening of fruit, digestion of food.
• Photosynthesis in plants.

Signs of a chemical change

One or more of these usually accompany a chemical change:

• heat or light is given out or taken in;
• a gas is produced (bubbles);
• the colour changes;
• a new smell appears;
• a sound is produced.

Add vinegar to baking soda: it fizzes as carbon dioxide gas escapes. That is a chemical change.

A candle shows both

When a candle burns, the wax near the flame melts: a physical change, since it hardens again. But the wax that burns turns into carbon dioxide and water vapour: a chemical change. Many everyday events are such a mixture.

Rusting and how to prevent it

Iron rusts when it is in contact with both oxygen and water (or moist air). Rust is iron oxide; it is flaky and slowly eats the metal away. Rusting is faster in the salty, humid air of coastal towns. It can be prevented by keeping air and moisture away from the iron:

• painting or oiling and greasing;
• galvanising, that is, coating iron with a layer of zinc, as on water pipes and roofing sheets;
• making stainless steel by mixing iron with chromium, nickel and carbon.

Crystallisation

Large, pure crystals of a substance can be obtained from its solution, as salt is obtained from sea water by evaporation, or sugar crystals from a hot, strong sugar solution left to cool. Crystallisation is a physical change.