Properties of materials

Not all metals behave the same way. Drop a piece of potassium into water and it fizzes and bursts into flame; drop in a piece of gold and nothing happens at all. We say potassium is more reactive than gold.

The reactivity of a metal is how strongly and quickly it reacts with other substances. Chemists have tested every common metal and placed them in order — a list called the reactivity series.

A metal high in the series, like potassium, reacts fast and easily. A metal low down, like gold, hardly reacts at all — which is why gold jewellery stays shiny for thousands of years. The reactivity series lets you predict how a metal will behave before you test it.

The reactivity series shows up clearly in three everyday reactions.

With oxygen. Many metals react with oxygen in the air to form a metal oxide. Reactive metals burn brightly — magnesium gives a dazzling white flame. Less reactive metals just tarnish slowly, and gold does not react with oxygen at all.

magnesium + oxygen → magnesium oxide

With water. Reactive metals react with water to give a metal hydroxide and hydrogen gas. Sodium fizzes across the surface; calcium bubbles steadily. Iron reacts only very slowly (this is part of rusting), and copper and gold do not react with water at all.

With dilute acid. Many metals react with a dilute acid to make a salt and hydrogen gas. The more reactive the metal, the faster it fizzes:

metal + acid → salt + hydrogen

Magnesium fizzes vigorously in acid; zinc and iron react more gently; copper, silver and gold do not react with dilute acid at all. In every case, the pattern follows the reactivity series — the higher the metal, the more vigorous the reaction.

You have met acids and alkalis before — this year you use the pH scale to measure them precisely.

The pH scale runs from 0 to 14:

• pH below 7 — the substance is acidic. Lemon juice and vinegar are acids.
• pH exactly 7 — the substance is neutral. Pure water is neutral.
• pH above 7 — the substance is alkaline. Soap and oven cleaner are alkalis.

The further from 7, the stronger the acid or alkali. A pH of 1 is a strong acid; a pH of 13 is a strong alkali. You can measure pH with universal indicator, which turns a range of colours, or with a pH meter for an exact number.

What happens when you mix an acid and an alkali? They react together and cancel each other out. This reaction is called neutralisation.

The word equation for neutralisation is always the same:

acid + alkali → salt + water

A salt is the compound formed when the acid is neutralised. Common table salt (sodium chloride) is just one example — there are many different salts. The type of salt depends on which acid and which alkali you used.

Neutralisation is genuinely useful in everyday life:

• Indigestion remedies contain a mild alkali to neutralise excess acid in the stomach.
• Farmers add lime (an alkali) to neutralise soil that is too acidic for crops.
• Toothpaste is slightly alkaline to neutralise the acids that cause tooth decay.

So neutralisation is not just a lab reaction — it solves real problems.

In Grade 6 you learned that engineers choose materials by their properties. Now you can add a powerful new property to the list: reactivity.

When a material will be exposed to air, water or chemicals, engineers must think about how much it will react — because a reaction can mean corrosion, the slow damage of a metal by its surroundings. Rusting of iron is the most familiar example.

• Aircraft and drink cans use aluminium. It is reactive, but it quickly forms a thin oxide layer that protects the metal underneath from further attack.
• Underground pipes and bridges use steel that is coated, painted or galvanised (covered in zinc) to keep water and oxygen away.
• Jewellery and electrical contacts use gold. It is very unreactive, so it stays bright and conducts reliably for decades.
• Cooking pans use stainless steel, a special mixture that resists corrosion far better than ordinary iron.

So choosing a material now means weighing up all its properties together — strength, density, conductivity and reactivity — to find the best fit for the job.

Understanding reactivity and acids prepares you for a lot of chemistry to come:

• Chemical reactions — you will write word equations for the reactions you have met here.
• Displacement reactions — a more reactive metal can push a less reactive one out of its compound.
• Extracting metals — how a metal is obtained from its ore depends on its place in the reactivity series.
• Rates of reaction — you will measure how fast reactions like metal-and-acid actually go.

Whenever you meet a new metal or solution, ask: how reactive is this, and what is its pH? Those two questions unlock how it will behave in almost any situation.