Facts At Your Fingertips: Matter and materials – Reactions and changes
Physical changes occur when substances change state. Chemical reactions occur when the atoms in molecules are rearranged to create different molecules.
Changes caused by physical reactions are usually easy to reverse, while chemical changes are not.
Chemical reactions
These reactions can be caused by various factors, such as heat or contact with other substances.
When food is cooked, heat causes the ingredients to react together chemically, altering their appearance, texture, temperature, and taste.
Physical changes
Some changes are physical, not chemical. When candle wax is heated, it melts and changes to a liquid.
However, its molecules do not change, just their physical state. The change can be reversed by remolding and cooling the candle wax.
Fast changes
Some changes can happen suddenly. When baking soda and vinegar are mixed, the two substances react, causing the liquid to erupt.
The speed of change can also be altered: if a potato is cut into small pieces and heated in boiling water, it will change (soften) faster than if it were a single, large piece because the heat has less far to travel to get to the center of the potato pieces.
Slow changes
Certain reactions happen over a long period of time—days, weeks, or even years.
Corrosion, such as rusting, occurs when metal objects are exposed for too long to the oxygen and moisture present in air, or other corrosive substances.
Catalysts
Some substances, called catalysts, change the rate at which a reaction occurs, although the catalyst itself is not changed in the reaction.
Most cars are equipped with a catalytic converter, which helps the polluting gases that fuel the car to react and create less harmful gases.
Many catalysts speed up reactions, but others, called inhibitors, slow down reactions. Preservatives, added to food in order to keep it fresh for a longer period, are examples of inhibitors.
Giving off heat
Some reactions release heat, light, or both.
These are called exothermic reactions and include the burning of wood or other fuels and reactions between acids and bases (see pages 44–45), resulting in the formation of a salt.
The burning of fuel is also known as a combustion reaction and can give off enough energy to power a car or a house.
Cool reactions
Endothermic reactions are the opposite of exothermic reactions.
They absorb heat, rather than releasing it, to change the molecules. Special instant ice packs, used to treat sports injuries, contain water and ammonium chloride.
When the pack is activated, the substances mix and react, causing an endothermic reaction which cools the affected area.
Man-made materials
Sometimes chemical reactions are used to create substances that do not occur naturally.
Man-made materials are often used for outdoor clothing.
These breathable, waterproof materials are made when certain molecules are combined in a process known as polymerization.
Firework displays are an example of exothermic reactions. They release heat and light.
When a spark is applied, the fuse burns down, causing the gunpowder and metal stars within the firework to explode in a spectacular mix of bangs, crackles, and light.