KS3 Chemistry topics

Particle model & states

Solids, liquids and gases as arrangements of particles.

Atoms, elements & compounds

The cast list of chemistry — what everything is made of.

Builds on: Particle model & states — “Atoms only make sense as the particles in the model.”

Separating mixtures

Filtering, evaporating, distilling — chosen by particle picture. Read the full explainer →

Builds on: Particle model & states — “Choosing a separation means picturing the particles.”

Melting & freezing

Heating gives particles enough energy for a solid's structure to give way into a liquid; freezing is the reverse. For a pure substance both happen at one fixed temperature.

Builds on: Particle model & states — “Melting is the particle arrangement giving way — no picture, no story.”

Boiling vs evaporating

Evaporation happens from the surface at any temperature; boiling happens throughout the liquid at one fixed temperature.

Builds on: Particle model & states — “Surface escape versus bulk bubbling is a particle-level distinction.” · Melting & freezing

Diffusion

Particles in liquids and gases move randomly and spread from crowded places to emptier ones — no stirring needed.

Builds on: Particle model & states — “Spreading smells only make sense once particles move on their own.”

Gas pressure

A gas pushes on its container because billions of tiny particles keep colliding with the walls.

Builds on: Particle model & states — “Pressure is particles drumming on the walls — you need the drummers.” · Diffusion

Dissolving & solutions

When a solute dissolves, its particles spread out between the solvent's particles. The sugar in tea has not vanished — it is hiding in the gaps.

Builds on: Particle model & states — “Sugar seems to vanish; the particle picture shows where it went.” · Diffusion

Pure substances & mixtures

In chemistry 'pure' means one substance only — one kind of particle throughout. Most everyday materials are mixtures.

Builds on: Particle model & states — “'One kind of particle or several' is the whole definition.” · Atoms, elements & compounds

Mass is kept in dissolving

When salt dissolves in water, the total mass stays exactly the same — the particles are still all there, just spread out.

Builds on: Dissolving & solutions — “You can't track dissolved mass until you can picture where it went.”

Chemical symbols

Every element has a one- or two-letter symbol — H, O, Na, Cl. The capital letter matters: Co is cobalt, CO is a compound of two elements.

Builds on: Atoms, elements & compounds — “Symbols are shorthand for elements — meet the elements first.”

Molecules

Atoms often travel joined in fixed groups called molecules — two oxygen atoms in O₂, one oxygen and two hydrogens in water.

Builds on: Atoms, elements & compounds — “A molecule is atoms holding hands; you need to know the hands.”

Compounds vs mixtures

In a compound the atoms are chemically joined in fixed proportions and hard to separate; in a mixture the substances just share the space and keep their own properties.

Builds on: Pure substances & mixtures — “Mixture was defined back with pure substances — this contrast leans on it.” · Atoms, elements & compounds — “Compound has to mean something before you can contrast it with mixture.”

Physical vs chemical change

A physical change (melting, dissolving) keeps the same substances and can usually be undone; a chemical change makes new substances.

Builds on: Atoms, elements & compounds — “'New substance or same substance' is the test — so 'substance' must be solid ground.” · Melting & freezing

A compound's new properties

When elements combine, the compound behaves nothing like its ingredients — table salt is nothing like sodium metal or chlorine gas.

Builds on: Molecules · Compounds vs mixtures — “The surprise only lands if you know the compound is genuinely new stuff.”

Properties of metals

Most metals share a checklist: shiny, strong, bendable, and good at conducting heat and electricity.

Builds on: Atoms, elements & compounds — “Metal or non-metal is a question you ask about an element.” · Describing properties precisely

Describing properties precisely

Using words like hard, strong, brittle, flexible and conductive to mean exactly one thing each — hard is not the same as strong.

Properties of non-metals

Non-metals are the mirror image: mostly dull, brittle when solid, poor conductors — and many are gases at room temperature.

Builds on: Atoms, elements & compounds — “Non-metals are elements too — the cast list comes first.” · Properties of metals

Signs of a chemical reaction

Clues that new substances are forming: an unexpected colour change, fizzing, a temperature change, or a new solid appearing.

Builds on: Physical vs chemical change — “Spotting a reaction means knowing what counts as genuinely new stuff.”

Combustion

Burning is a chemical reaction: a fuel combines with oxygen from the air, releasing energy and making new substances.

Builds on: Signs of a chemical reaction — “Burning is your first named reaction — recognise reactions in general first.” · The atmosphere's recipe

Word equations

Recording a reaction as a sentence: reactants on the left, an arrow for the change, products on the right.

Builds on: Atoms, elements & compounds · Signs of a chemical reaction — “An equation records a reaction — have something to record.”

Everyday acids & alkalis

Acids and alkalis are already in the house — lemon juice and vinegar on one side, soap and oven cleaner on the other.

Using indicators

An indicator is a dye that changes colour to answer one question: is this an acid or an alkali?

Builds on: Dissolving & solutions · Everyday acids & alkalis — “Indicators answer 'acid or alkali?' — meet the contenders first.”

Hazard symbols

The warning pictograms on bottles — corrosive, toxic, flammable — and what each one tells you to do differently.

Structure of the Earth

The planet in cross-section: a thin rocky crust, a hot slowly-moving mantle, and a metal core.

Weathering of rocks

Rocks are slowly broken up where they stand — by ice in cracks, by weak acids in rain, and by plant roots.

The atmosphere's recipe

Air is a mixture with a stable recipe: about four-fifths nitrogen, one-fifth oxygen, and small traces including carbon dioxide.

Builds on: Pure substances & mixtures — “Air is your best everyday mixture — 'mixture' must mean something precise.”

The periodic table

The organising shelf for every element.

Builds on: Atoms, elements & compounds — “The table organises elements — know what one is first.”

Chemical reactions & equations

Atoms rearrange; word and symbol equations record it.

Builds on: Atoms, elements & compounds — “Reactions rearrange atoms; you need the cast list.” · Signs of a chemical reaction · Word equations — “Word equations are the sentence; symbol equations are the spelling.”

Acids, alkalis & pH

The pH scale and neutralisation.

Builds on: Chemical reactions & equations · Everyday acids & alkalis — “You need to have met acids and alkalis before scaling them.”

Reading heating curves

A graph of temperature against time flattens while a substance melts or boils — the energy is busy changing the state, not the temperature.

Builds on: Melting & freezing — “The flat step on the graph is melting — recognise the event before the graph.” · Boiling vs evaporating

Solubility & saturation

There is a limit to how much solute a solvent can hold; a solution at that limit is saturated. Warming the solvent usually raises the limit.

Builds on: Dissolving & solutions — “Saturation is dissolving meeting its limit.” · Pure substances & mixtures

Chromatography

Separating a mixture of dissolved substances — inks, dyes, plant pigments — by how far each one travels up wet paper.

Builds on: Dissolving & solutions — “Chromatography only separates what dissolves.” · Separating mixtures

Brownian motion — the evidence

Smoke grains in air jiggle endlessly because unseen air particles keep bumping into them — the particle model, caught on camera.

Builds on: Particle model & states — “The jiggling grains are meaningless until you know what's bumping them.” · Diffusion

Reading chemical formulae

A formula like H₂O is a recipe: the small subscript counts the atoms of the element just before it.

Builds on: Chemical symbols — “Formulae are built from symbols — read the letters before the words.” · Molecules — “H₂O describes a molecule; the idea comes before the notation.”

Naming compounds: -ide and -ate

Compound names carry information: -ide means two elements only, -ate means oxygen has joined the pair.

Builds on: Chemical symbols · Compounds vs mixtures — “You're naming compounds, so 'compound' must already be clear.”

Counting atoms in formulae

Work out exactly how many atoms of each element a formula contains — including formulae with brackets, like Ca(OH)₂.

Builds on: Reading chemical formulae — “Counting atoms starts from reading the formula correctly.”

Particle diagrams of substances

Drawing and reading circle diagrams that show whether a box of particles is an element, a compound or a mixture.

Builds on: Particle model & states — “The diagrams are particle pictures — drawn, not imagined.” · Compounds vs mixtures — “Mixed versus chemically joined is exactly what these diagrams show.”

Polymers: long-chain molecules

Plastics are polymers — molecules thousands of atoms long, built from one small unit repeated over and over.

Builds on: Molecules — “A polymer is one molecule repeated thousands of times — molecule comes first.”

Groups & periods

The table's columns are groups — families with similar behaviour; the rows are periods. An element's address hints at its character.

Builds on: The periodic table — “Groups and periods are the table's grammar — meet the table first.”

Group 1: the alkali metals

Soft metals that react eagerly with water — and more eagerly the further down the group you go.

Builds on: Properties of metals · Groups & periods — “Group 1 only means something once columns are families.” · Signs of a chemical reaction

Group 7: the halogens

A family of reactive non-metals — fluorine, chlorine, bromine, iodine — whose reactivity fades down the group.

Builds on: Properties of non-metals · Groups & periods — “Group 7 is a column story — you need the columns.”

Group 0: the noble gases

The unreactive family — helium, neon, argon — useful precisely because they refuse to join in.

Builds on: Groups & periods — “Group 0's calm only stands out inside the table's pattern.”

Conservation of mass

In a chemical reaction atoms are rearranged, never lost — so the mass before equals the mass after, once you count any gases. Read the full explainer →

Builds on: Particle diagrams of substances · Chemical reactions & equations — “Conservation is a claim about equations: both sides carry the same atoms.”

Thermal decomposition

Some compounds break apart into simpler substances when heated — one substance in, two or more out. Read the full explainer →

Builds on: Compounds vs mixtures · Chemical reactions & equations — “Breaking apart is still a reaction — the general idea comes first.”

Oxidation

Reactions where a substance combines with oxygen — from a metal slowly tarnishing to a fuel burning fast.

Builds on: Combustion — “Burning is oxidation at full volume; rusting whispers the same reaction.” · Chemical reactions & equations

Exothermic & endothermic

Some reactions push energy out and warm their surroundings; others draw energy in and cool them. The thermometer tells you which.

Builds on: Reading heating curves · Signs of a chemical reaction — “The temperature change you felt is the sign being explained.” · Energy stores & transfers (PHYS)

The reactivity series

Metals ranked by how eagerly they react — a league table that predicts winners before you mix anything. Read the full explainer →

Builds on: Properties of metals — “It's a league table of metals — you need to know the players.” · Signs of a chemical reaction

Displacement reactions

A more reactive metal takes the place of a less reactive one in a compound — a takeover you can watch in a test tube. Read the full explainer →

Builds on: Word equations · The reactivity series — “Displacement is the league table settling an argument — you need the rankings.”

Neutralisation

Mixing an acid and an alkali in the right amounts cancels both out, making a salt and water.

Builds on: Word equations · Acids, alkalis & pH — “Neutralisation is a journey along the pH scale — learn the map first.”

Measuring pH accurately

Moving from 'roughly green' to a number — matching universal indicator carefully or using a pH probe.

Builds on: Using indicators — “Careful measurement refines the colour-matching you already do.” · Acids, alkalis & pH — “A reading only means something on a scale you understand.”

Acids + metals

Many metals fizz in acid, making a salt and hydrogen gas — the more reactive the metal, the livelier the fizz.

Builds on: The reactivity series · Acids, alkalis & pH — “You need to know what an acid is before handing it a metal.”

The three rock families

Sedimentary rock is squeezed from layers of fragments, igneous rock is cooled magma, and metamorphic rock is either one changed by heat and pressure.

Builds on: Structure of the Earth — “Each rock family is born at a different depth — you need the floor plan.” · Weathering of rocks

The rock cycle

One diagram that links the three families: over deep time, any rock can be broken down, buried, squeezed, melted and remade as another.

Builds on: Weathering of rocks · The three rock families — “The cycle links the three families — meet them before you loop them.”

Concentration

Compare and calculate how crowded a solution is.

Builds on: Dissolving & solutions — “Concentration describes a solution — meet solutions first.” · Separating mixtures · Ratio & proportion problems (MATH) — “Concentration is a ratio: how much stuff in how much water.”

Limits of the particle model

The simple model treats particles as identical hard balls with nothing between them. Knowing where that picture breaks down is part of using it well.

Builds on: Particle model & states — “You have to know the model well before you're allowed to doubt it.” · Gas pressure · Brownian motion — the evidence

Writing simple formulae

Going from a compound's name to its formula for simple cases — magnesium oxide is MgO, sodium chloride is NaCl.

Builds on: Reading chemical formulae — “You can't write in a notation you can't yet read.” · Naming compounds: -ide and -ate

Ceramics & composites

Ceramics are hard, brittle and heatproof; composites combine two materials to get the best of both. A material's behaviour is chosen when its structure is.

Builds on: Polymers: long-chain molecules · Describing properties precisely — “Comparing material families needs precise property words.”

Predicting with periodic trends

Using a group's pattern to predict how an element you have never met will behave — the table as a forecasting tool.

Builds on: Group 1: the alkali metals — “Group 1's staircase of reactivity is the trend you'll extend.” · Group 7: the halogens — “Group 7 runs the opposite way — predicting needs both directions.”

Mendeleev's gaps

Mendeleev left holes in his table for elements nobody had found — and described them in advance. The gaps were the theory's boldest test.

Builds on: The periodic table — “The gaps are holes in the table — you need the table.” · Predicting with periodic trends

Metals to non-metals across a period

Reading a row from left to right, elements shift from metal to non-metal, with the in-between cases along the staircase line.

Builds on: Properties of metals · Properties of non-metals · Groups & periods — “Reading across a period is reading the table's rows — learn its layout first.”

Balancing symbol equations

Adjusting the big numbers in a symbol equation until both sides carry exactly the same atoms — bookkeeping for matter.

Builds on: Counting atoms in formulae — “You balance by counting atoms, so counting must be automatic.” · Conservation of mass — “Conservation of mass is the law balancing enforces.” · Chemical reactions & equations — “Balancing is symbol equations done properly — the notation comes first.”

Catalysts

A catalyst speeds a reaction up without being used up — it is all still there at the end, ready to go again.

Builds on: Exothermic & endothermic · Chemical reactions & equations — “A catalyst speeds a reaction — reactions first.”

Comparing fuels fairly

Judging fuels by measuring the temperature rise they cause — with everything but the fuel kept the same.

Builds on: Combustion — “You're measuring burning, so know what burning is.” · Exothermic & endothermic — “Comparing fuels is comparing energy out — exothermic thinking, applied.” · Averages & range (MATH)

Rusting & its prevention

Iron rusts only when water and oxygen both reach it — so barriers, grease and sacrificial metals all work by shutting one of them out.

Builds on: Oxidation — “Rust is slow oxidation; stop the oxygen and you stop the rust.” · The reactivity series

Making a salt

The full practical: neutralise, filter off any extra solid, then evaporate gently until dry crystals appear.

Builds on: Separating mixtures — “Getting dry crystals out is a separation job — filter, then evaporate.” · Hazard symbols · Neutralisation — “Making a salt is neutralisation with a product worth keeping.”

Naming salts

The acid chooses the surname: hydrochloric acid makes chlorides, sulfuric acid makes sulfates, nitric acid makes nitrates.

Builds on: Naming compounds: -ide and -ate · Neutralisation — “You name what neutralisation makes — make it first.”

Neutralisation in daily life

Indigestion tablets, toothpaste and farmers' lime are all the same move: adding a mild alkali to cancel an unwanted acid.

Builds on: Neutralisation — “Indigestion tablets are neutralisation with the labels swapped for brand names.” · Measuring pH accurately

The carbon cycle

Carbon keeps moving — from air to plants, through food and fuels, and back to the air — with photosynthesis and combustion as two of the main valves.

Builds on: Photosynthesis — the word equation (BIOL) · Combustion · The atmosphere's recipe — “Carbon dioxide is a trace of the recipe — the recipe comes first.”

Greenhouse effect & climate

Carbon dioxide and other greenhouse gases hold warmth in the atmosphere; adding more of them tilts the planet's energy balance.

Builds on: The carbon cycle — “You can't weigh a disturbance to a cycle you can't trace.” · Weather vs climate (GEOG)

Fossil fuels are finite

Coal, oil and gas took millions of years to form and are being burned far faster — a store that empties in one direction.

Builds on: Combustion — “Fossil fuels matter because they burn — combustion is the point.” · The carbon cycle

Extracting metals from ores

Most metals are dug up locked inside compounds called ores; carbon can push the less reactive ones out — that is how iron is won. Read the full explainer →

Builds on: The reactivity series — “Carbon can only evict metals below it — the league table decides.” · Structure of the Earth

Recycling & finite resources

The Earth's stores of metals and fuels are limited; recycling trades a little effort now for ore and energy saved later.

Builds on: Extracting metals from ores — “You can't weigh recycling's case without knowing what extraction costs.”