Changes to materials

In Grade 6 you split changes into reversible and irreversible. This year we use sharper words: physical changes and chemical reactions.

A physical change alters how a material looks but makes no new substance. The same material is still there afterwards.

• Melting ice into water — still water, just a different state.
• Tearing paper, dissolving sugar, bending a wire — no new substance is made.
• Physical changes are usually easy to reverse.

A chemical reaction makes one or more brand-new substances. The atoms are rearranged into different combinations, so what you end up with is genuinely different from what you started with.

• Burning wood makes ash, smoke and gases — none of which is wood.
• A nail rusting, milk turning sour, an egg frying — all chemical reactions.
• Chemical reactions are usually very hard to reverse.

The single test: ask has a new substance been made? If yes, it is a chemical reaction. If no, it is only a physical change.

Since a chemical reaction makes a new substance, there are usually clear signs that one has taken place.

Look out for these signs:

• A colour change — for example, a shiny iron nail turning orange-brown as it rusts.
• Bubbles of a gas appearing, when no liquid is boiling.
• Heat or light given out — many reactions feel warm or glow.
• A new smell — toast burning, an egg going off.
• A solid forming in a clear liquid (called a precipitate).

One sign on its own is not always proof — boiling water bubbles but is only a physical change. But several signs together strongly suggest a new substance has been made.

Chemists describe a reaction with a word equation. To read one you need two new words:

• Reactants — the substances you start with.
• Products — the new substances you end with.

A word equation is written with the reactants on the left, the products on the right, and an arrow between them. The arrow means 'reacts to make' — it always points from reactants to products.

Some examples:

• magnesium + oxygen → magnesium oxide
• carbon + oxygen → carbon dioxide
• iron + sulfur → iron sulfide

When there are two reactants or two products, join them with a plus sign. The plus means 'and'; the arrow means 'reacts to make'.

One of the most important reactions of all is combustion — the proper science word for burning.

Combustion is a reaction in which a fuel reacts with oxygen, releasing heat and light. A fuel is any substance that burns to release energy — wood, candle wax, natural gas and petrol are all fuels.

Three things are needed for combustion, often called the fire triangle: a fuel, oxygen and heat to start it off. Remove any one and the fire goes out — which is exactly how firefighters work.

When a fuel that contains carbon burns fully in plenty of oxygen, the word equation is:

• fuel + oxygen → carbon dioxide + water

For example, when natural gas burns on a cooker:

• methane + oxygen → carbon dioxide + water

Combustion is always a chemical reaction — new substances are made — and it is irreversible. You cannot turn the carbon dioxide and water back into the wood or gas you started with.

Here is one of the most powerful rules in all of chemistry: conservation of mass.

In Grade 7 you learned that everything is made of atoms. In a chemical reaction those atoms are only rearranged into new combinations — no atoms are created and none are destroyed.

Because the atoms are all still there, the total mass stays the same. The total mass of the products is exactly equal to the total mass of the reactants.

This explains a puzzle. When wood burns it seems to lose mass — only light ash is left. But the missing mass has not vanished. It has escaped as gases (carbon dioxide and water vapour) you cannot see. If you weighed those gases too, the mass would balance perfectly.

The rule: mass in = mass out, every time.

Understanding chemical reactions opens the door to the rest of chemistry:

• Reactions of acids — acids reacting with metals and with alkalis are reactions you will write as word equations.
• Reactivity of metals — comparing how different metals react builds directly on combustion and reactions with oxygen.
• Balanced equations — conservation of mass is the rule behind balancing chemical equations in later years.
• Everyday science — cooking, rusting, respiration and burning fuels are all chemical reactions you now understand.

Whenever you see a change, ask: has a new substance been made, and could I write it as a word equation? That question turns the world into chemistry.