Changes to materials

Back in Grade 6 you met irreversible changes — changes that make a brand-new material. This year you give them their proper name: chemical reactions.

In a chemical reaction, the atoms of the starting substances are rearranged to build one or more new substances. The starting substances are called the reactants; the new substances are the products.

Chemists summarise a reaction with a word equation. The reactants go on the left, the products go on the right, and an arrow shows the direction of change:

reactants → products

For example, when magnesium burns:

magnesium + oxygen → magnesium oxide

The arrow is read as "react to make" or "becomes". The "+" between two reactants means "and". As you saw last year, chemists can also use symbols and formulae to write a shorter symbol equation — but the word equation always tells the same story.

A word equation is a quick, clear way to record exactly what reacted and what was made.

There are millions of chemical reactions, but many follow the same pattern. Sorting them into families helps you predict what will happen.

• Combustion (burning) — a substance reacts quickly with oxygen, releasing heat and light. fuel + oxygen → oxides
• Oxidation — a substance reacts with oxygen, but slowly and without a flame. Rusting and the tarnishing of metals are oxidation.
• Thermal decomposition — a single compound is broken down into simpler substances by heating. compound → simpler substances
• Neutralisation — an acid reacts with an alkali. acid + alkali → salt + water
• Displacement — a more reactive metal pushes a less reactive metal out of its compound.

Notice that combustion and thermal decomposition are almost opposites: combustion joins substances together, while decomposition breaks one apart.

When you meet a new reaction, try to spot which family it belongs to. Once you recognise the pattern, you can often predict the products before you even do the experiment.

Chemical reactions do not only rearrange atoms — they also move energy in or out. There are two kinds.

• An exothermic reaction releases energy to the surroundings, usually as heat. The surroundings get warmer. Burning fuel, neutralisation and many metal reactions are exothermic.
• An endothermic reaction takes in energy from the surroundings. The surroundings get cooler. Thermal decomposition and some dissolving reactions are endothermic.

A simple test: touch the container. If it feels warm, energy has come out — exothermic. If it feels cold, energy has gone in — endothermic.

Some reactions are over in a flash; others take years. The rate of reaction is how fast the reactants are turned into products.

• Temperature — heating gives particles more energy, so they collide more often and harder. Hotter = faster.
• Concentration — more reactant particles in the same space means more collisions. More concentrated = faster.
• Surface area — breaking a solid into smaller pieces exposes more of it. Smaller pieces = faster.
• A catalyst — a substance that speeds up a reaction without being used up itself.

All four work the same way: anything that makes particles collide more often speeds the reaction up.

Here is one of the most important rules in all of chemistry: in any reaction, mass is conserved. The total mass of the products is exactly equal to the total mass of the reactants.

Why is this true? Because a reaction only rearranges atoms — it never creates or destroys them. Every atom in the reactants is still there in the products, just joined up in a different way. The same atoms must have the same total mass.

Sometimes a reaction looks as if mass has changed:

• Burning a candle seems to lose mass — but a gas (carbon dioxide) escapes into the air. Trap the gas and the mass is unchanged.
• Rusting iron seems to gain mass — but it has joined with oxygen from the air. Count that oxygen and the mass balances.

If you ever seem to lose or gain mass, look for a gas entering or leaving.

Understanding chemical reactions properly sets you up for everything to come:

• Symbol equations — you will balance equations using the conservation of mass you have just learned.
• Energy and fuels — exothermic reactions explain how fuels power cars, homes and bodies.
• Controlling reactions — knowing the rate factors lets you speed up useful reactions and slow down harmful ones.
• Respiration and photosynthesis — these biology reactions are exothermic and endothermic energy changes.

Whenever you see a change, ask three questions: what new substances formed, did energy go in or out, and how fast did it happen? Those questions turn every change around you into chemistry.