Two kinds of change

A physical change alters only the state or appearance — no new substance forms and it is usually easy to reverse (e.g. melting ice, dissolving salt, evaporating water).

A chemical change (a chemical reaction) makes one or more new substances. The atoms are rearranged; it is usually hard to reverse and is often accompanied by an energy change.

Signs of a chemical reaction: a colour change, a gas (bubbles/effervescence), a precipitate forming, or a temperature change.

Physical or chemical?

What "rate" means

The rate (speed) of a reaction is how quickly reactants are used up or products are made. You can follow it by measuring how a quantity changes over time:

• Volume of gas made (gas syringe or upturned measuring cylinder) — cm³ per second;
• Loss in mass as a gas escapes (balance) — g per second;
• Time for a precipitate to hide a cross (the "disappearing cross", e.g. sodium thiosulfate + acid).

Reading a graph: a steeper curve = faster rate. The line levels off when a reactant runs out and the reaction stops. The gradient (slope) of a tangent gives the rate at that instant.

Reading a rate graph

Plot the volume of gas against time. The reaction is fastest at the start (steepest), slows as reactants are used up, then flattens when it finishes. The tangent's gradient gives the instantaneous rate.

Key: the gradient of the tangent at any point = the rate at that instant; both curves plateau at the same height because the amount of reactant is the same.

Reading the curve

Your turn — mean rate

Five things that speed reactions up

Cambridge expects you to describe the effect on rate of changing each of these:

• Concentration of a solution ↑ → faster.
• Pressure of a gas ↑ → faster (squeezes the particles closer).
• Surface area of a solid ↑ (smaller pieces / powder) → faster.
• Temperature ↑ → faster.
• Catalyst added (including enzymes) → faster; it is not used up.

Will it speed up or slow down?

Tap whether each change makes the reaction faster or slower.

Collision theory Supplement

Reactions happen when particles collide with enough energy. The minimum energy needed for a collision to react is the activation energy, E_a. Faster rate = more frequent successful collisions.

Explaining each factor with collision theory: higher concentration / pressure = more particles per unit volume → more frequent collisions. Larger surface area = more particles exposed → more collisions. Higher temperature = particles move faster (more frequent collisions) and have more kinetic energy (more collisions exceed E_a).

Why temperature is special Supplement

A common slip is to say heating only makes particles "move faster". For full marks you must give both effects:

• Particles move faster → collisions are more frequent;
• Particles have more kinetic energy → a greater proportion of collisions exceed the activation energy (this is usually the bigger effect).

Watch out: concentration, pressure and surface area change only the frequency of collisions — they do not change the energy of the particles. Temperature changes both.

Explaining a faster rate Supplement

Catalysts

A catalyst speeds up a reaction but is not used up — it can be recovered chemically unchanged at the end. Supplement A catalyst works by providing a different pathway with a lower activation energy, so more collisions succeed.

• Catalysts are often transition elements or their compounds (e.g. iron in the Haber process, vanadium(V) oxide in the Contact process).
• Enzymes are biological catalysts (proteins) used in fermentation, brewing and biological washing powders.

Misconception killer: a catalyst does not get used up and does not change the amount of product made — it only changes how fast you get there. It is not a reactant.

Reactions that go both ways

Some reactions are reversible: the products can react to re-form the reactants. We show this with the special ⇌ symbol instead of →.

The classic example is hydrated copper(II) sulfate. Heating drives off water; adding water reverses it (with a colour change you must know):

A similar reversible example: hydrated cobalt(II) chloride (pink) ⇌ anhydrous cobalt(II) chloride (blue).

Spot the reversible reaction

Dynamic equilibrium Supplement

In a closed system a reversible reaction reaches equilibrium when the forward and backward reactions happen at the same rate. The amounts of reactants and products then stay constant.

Misconception: equilibrium does not mean the reaction has stopped, and it does not mean equal amounts of reactants and products — just equal forward and backward rates with constant amounts.

Shifting the equilibrium Supplement

Change a condition and the position of equilibrium shifts to oppose the change:

• Temperature ↑ → shifts in the endothermic direction (away from the heat).
• Pressure ↑ → shifts towards the side with fewer gas molecules.
• Concentration ↑ of a substance → shifts away from that substance.
• A catalyst does not move the position — it just reaches equilibrium faster.

Haber process: N₂(g) + 3H₂(g) ⇌ 2NH₃(g), forward reaction exothermic. Conditions: ~450 °C, ~200 atm, iron catalyst — a compromise giving a reasonable yield at a reasonable rate.

Contact process: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), forward reaction exothermic. Conditions: ~450 °C, ~2 atm, vanadium(V) oxide (V₂O₅) catalyst — making sulfur trioxide on the way to sulfuric acid.

Predict the shift Supplement

Oxidation and reduction (oxygen)

The simplest definitions use oxygen:

• Oxidation = gain of oxygen.
• Reduction = loss of oxygen.

A redox reaction has oxidation and reduction happening together. Example: CuO + H₂ → Cu + H₂O — copper oxide is reduced (loses oxygen), hydrogen is oxidised (gains oxygen).

Identifying redox: if something gains oxygen while something else loses it, it is a redox reaction.

Redox as electron transfer Supplement

More powerfully, redox is defined by electron transfer. Remember OIL RIG:

Oxidation number (state): a change in oxidation number also signals redox — an increase = oxidation, a decrease = reduction. Use Roman numerals, e.g. iron(II) and iron(III).

Oxidising and reducing agents Supplement

• An oxidising agent oxidises something else — so it is itself reduced (it gains electrons / loses oxygen / oxidation number falls).
• A reducing agent reduces something else — so it is itself oxidised.

Two colour-change tests you must know:

• Acidified potassium manganate(VII) (an oxidising agent): purple → colourless when it oxidises a reducing agent.
• Potassium iodide (a reducing agent): colourless → brown as iodide ions are oxidised to iodine.

Misconception killer: the oxidising agent is the one that gets reduced, and the reducing agent is the one that gets oxidised. The names describe what they do to the other substance.

Oxidation or reduction? Supplement

Oxidised or reduced?

Tap a species, then tap the box for what happens to it in its reaction.

Name the oxidising agent Supplement

The big ideas to know

You've covered all of Cambridge IGCSE Topic 6 — Chemical reactions. Press Finish to see your score.