Chemical reactions, but bitesize.

Part 1The big idea: atoms rearrange

Sodium is a soft metal so reactive it bursts into flame in water. Chlorine is a poisonous green gas that was once used as a weapon. Yet when they react, they make sodium chloride — the salt on your chips. How can two dangerous things make something safe to eat?

The answer is what happens to the atoms. In a chemical reaction, the bonds in the starting substances break, the atoms rearrange, and brand-new substances form. The product has completely different properties from the things you started with — salt is nothing like sodium or chlorine.

We have names for the two sides of any reaction. The starting chemicals are the REACTANTS. The new chemicals that are made are the PRODUCTS. Reactants always go on the left; products always go on the right.

Part 2Why an arrow, not an equals sign

In maths, "equation" comes from aequare, meaning "to make equal", and you use an equals sign because both sides are the same. But in chemistry the two sides are NOT the same — the reactants have turned into different products. So we use an arrow (→) instead. The arrow means "turn into".

The same reaction can be written two ways. A word equation uses only the names. A symbol equation uses chemical formulae:

sodium + chlorine → sodium chloride

Na + Cl₂ → NaCl

Part 3What goes in a word equation (and what doesn't)

A word equation contains only the names of the reactants and products. Everything else gets left out:

· No descriptions — no colours, and no states (solid/liquid/gas).
· No catalysts — a catalyst speeds a reaction up but is not used up, so it isn't a reactant or product.
· No energy — heat, flames and sunlight are not chemicals, so they are never included.

For example: "Hydrogen peroxide, a colourless liquid, turns into water and oxygen, sped up by manganese oxide (a catalyst)." Strip out everything but the chemicals and you are left with:

hydrogen peroxide → water + oxygen

The colour and the catalyst are not written — only the reactant and the products.

Part 1Physical changes of state

Melt an ice cube, then put it back in the freezer — and you get ice again. After a real chemical reaction you cannot easily get the starting substances back. So is melting ice a reaction? No.

Melting and boiling are PHYSICAL changes. The substance changes its form, but its chemical formula does not change. Ice, water and steam are all H₂O:

melting: H₂O(s) → H₂O(l)

boiling: H₂O(l) → H₂O(g)

The same formula appears on both sides, so no new substance is MADE. And because nothing new was created, the change is reversible — cool it down and it freezes straight back to ice.

Those little brackets are state symbols. They tell you what physical state each chemical is in.

Part 2Mixtures and dissolving

Dissolve salt in water, then boil the water away — and the salt comes back. Dissolving makes a mixture: two or more substances mixed together but NOT chemically joined.

When salt dissolves, NaCl(s) becomes NaCl(aq). The formula doesn't change — it's still NaCl, just spread out in water. So the salt hasn't been destroyed and no new substance has formed. Evaporate the water and the salt comes back, completely unchanged. That makes dissolving a PHYSICAL change.

Part 3A 6-marker: iron and sulfur

This is the classic question that pulls it all together — practise writing it out fully.

Part 1The five signs

A firework bangs and flashes. Vinegar and baking soda fizz over the glass. Bread turns golden in the oven. There are clues that tell you a reaction is probably happening. There are five to remember:

· Temperature change — the mixture gets hotter or colder.
· Light or flame — light is given out, e.g. when something burns.
· Colour change — a new colour appears that wasn't there before.
· Effervescence — fizzing, because a gas is being produced.
· Precipitate — a solid forms in a liquid, turning it cloudy.

Part 2The health warning: signs are only clues

Here's the catch. These five signs are CLUES, not proof. The only sure sign that a reaction has happened is that a NEW substance has formed. Why? Because some of these signs also happen in physical changes:

A boiling kettle gives off bubbles and gets hot — but it's only water turning to steam (still H₂O), so it's a physical change, not a reaction. Dissolving can give a temperature change. Mixing coloured liquids gives a colour change. None of those are reactions.

Part 3A 6-marker: marble chip and acid

Part 1The fire triangle

Combustion comes from com- (up) + burere (to burn) — "burning up". It is a fuel reacting quickly with oxygen, giving out heat and light. A fuel is a substance that stores chemical energy, released by burning (wax, methane, petrol).

A fire needs three things, often drawn as the fire triangle:

· Fuel — a large store of chemical energy.
· Oxygen — usually from the air, which is about 20% oxygen.
· Heat — a source of heat to start AND keep the reaction going.

Remove any one side of the triangle and the fire goes out. That is exactly how every fire extinguisher works — by taking away the fuel, the oxygen, or the heat.

Part 2Combustion equations

When most fuels burn completely, the two products are always the same: carbon dioxide and water. So there is one general equation you can use for any fuel:

fuel + oxygen → carbon dioxide + water

To write the equation for a particular fuel, just swap "fuel" for its name.

Part 1Breaking one substance into two

Heat green copper carbonate strongly and it turns black, while a gas is given off. One substance has become two — just by heating it. That's thermal decomposition.

The words tell you exactly what it is: thermal (from thermos, "heat") and decomposition (from de- "apart" + componere "to put together") — breaking something down into simpler parts using heat.

The key feature: ONE reactant is broken down by heat into two or more products. The heat energy is what breaks the bonds inside the compound. A classic example is metal carbonates:

metal carbonate → metal oxide + carbon dioxide

Part 2Testing the gas: the limewater test

When a metal carbonate decomposes, the gas given off is carbon dioxide. You can prove it with the limewater test: bubble the gas through limewater and it turns cloudy white. No other gas in this topic does this, so cloudy limewater is the chemical test for CO₂.

Part 3A 6-marker: proving decomposition

Part 1Gaining oxygen

A bike chain left in the rain slowly turns orange and flaky. Magnesium ribbon burns with a blinding white light, leaving a white powder. Both reacted with the same gas from the air — oxygen.

Oxidation is a reaction in which a substance gains oxygen. Even though oxygen is only about 20% of the air, it is reactive enough to react with most metals. The general equation is:

metal + oxygen → metal oxide

The product always ends in -ide — magnesium oxide, iron oxide, and so on.

Part 2Rusting — and stopping it

Rusting is just slow oxidation of iron: iron + oxygen → iron oxide. It happens faster when water (and especially salt water) is present. Note that salt water speeds up rusting but is not used up, so it acts like a catalyst — it isn't written in the equation.

To stop iron rusting, you keep the oxygen and water away from it — that's why iron railings are painted, and bike chains are oiled. A barrier of paint or oil means oxygen can't reach the iron, so it can't oxidise.

Part 1Swapping places

In the Bronze Age, people built wood fires on green copper rocks and found flecks of pink copper metal in the ashes. The carbon in the wood had "stolen" the oxygen from the copper compound. How can one element push another out of a compound?

Displacement (from dis- "apart" + place) is when a MORE reactive element takes the place of a LESS reactive element in a compound. The general pattern is:

element 1 oxide + element 2 → element 2 oxide + element 1

But there's a vital condition: this only happens if element 2 is more reactive than element 1. A less reactive element cannot push out a more reactive one.

Part 2When displacement does NOT happen

Heat magnesium oxide with carbon and... nothing happens. Why? Because magnesium is more reactive than carbon, so carbon cannot push it out. A less reactive element can never displace a more reactive one.

That's also why you can prove iron has been displaced from iron oxide with a magnet: iron oxide is not magnetic, but the iron metal produced is. Move a magnet under the cooled ash and the specks of iron move — proof a displacement reaction has happened.

Part 3Telling the four reaction types apart

You've now met four reaction types. Use this checklist to classify any reaction you're given:

· Combustion — a fuel + oxygen → carbon dioxide + water.
· Thermal decomposition — ONE reactant broken by heat into two or more products.
· Oxidation — a substance reacts with oxygen → an oxide.
· Displacement — a more reactive element takes another's place in a compound.

Part 1The law: mass before = mass after

In a reaction, bonds break, atoms rearrange, and new bonds form. But no atoms are MADE or DESTROYED — they are only rearranged. Every atom you start with is still there at the end. That gives us the law of conservation of mass:

total mass of REACTANTS = total mass of PRODUCTS

So if 100 g of reactants react, you must get exactly 100 g of products. And if you know all the masses but one, you can work out the missing mass.

Part 2When mass APPEARS to change

Burn a log and the ash weighs far less. Leave an iron nail in damp air and the rusty nail weighs MORE. Yet not a single atom was made or destroyed. How can this be?

The answer is always a gas that isn't sitting on the balance:

· Appears to GAIN mass — a gas reactant (e.g. oxygen) joins from the air. When iron wool burns, oxygen atoms from the air join the iron, so the reading goes UP.
· Appears to LOSE mass — a gas product escapes. When acid reacts with a carbonate in an open beaker, CO₂ escapes into the air, so the reading goes DOWN.

In both cases mass is really conserved — the gas just isn't being weighed. Weigh everything (gas included, in a sealed container) and the masses are equal.

Part 3A 6-marker: magnesium vs marble

Part 1Why we balance — and the two rules

The equation Mg + O₂ → MgO has two oxygen atoms on the left, but only one on the right. An atom can't simply vanish — that would break conservation of mass. So a symbol equation must be balanced: the same number AND the same type of atom on each side.

There are two golden rules:

Rule 1 — you may NEVER change a chemical formula. MgO must stay MgO. The small subscript numbers describe the actual chemical; change them and you've written a different substance.

Rule 2 — you may ONLY add big numbers in front (called coefficients). Think of each formula as a "box" — you can add whole boxes, but you can't change what's inside one.

Part 2The seesaw method, step by step

To balance, count the atoms of each element on both sides, add a whole box to the lighter side, then recount. Repeat until both sides match. Here's the classic example, worked fully:

Part 3A 6-marker: spotting the error