Understanding Chemical reactions: IGCSE Chemistry Explained

What actually counts as a chemical reaction?

You probably hear the phrase "chemical reaction" thrown around a lot in class, but do you know exactly where the line sits between a chemical change and a physical one? Getting this wrong in an exam is an easy way to lose marks, so let's sort it out.

A physical change is one where no new substance forms. You might change the shape, size, or state of something, but the actual molecules stay the same. Think of ice melting into water. It looks different, feels different, but it is still H₂O.

A chemical change is the opposite. Bonds break, new bonds form, and you end up with completely different substances. Burning magnesium ribbon in air is a classic example: you start with shiny magnesium metal and end up with a white powder of magnesium oxide. There is no simple way to reverse that.

Quick comparison table

Examiners love asking you to classify changes. A good rule of thumb: if you can get back to the starting material just by changing temperature or filtering, it is probably physical. If you would need a whole new reaction to reverse it, it is chemical.

Rate of reaction: why some reactions are fast and others take forever

Drop a piece of sodium into water and the reaction is over in seconds. Leave an iron gate outside and the rust takes years. Both are chemical reactions, but they happen at wildly different speeds. The IGCSE syllabus expects you to explain why, and it comes down to five key factors.

The five factors that affect rate

• Concentration - Higher concentration means more reactant particles packed into the same volume. More particles means more frequent collisions, which means a faster reaction. Picture a crowded dance floor versus an empty one: more people, more bumping into each other.
• Temperature - Raising the temperature gives particles more kinetic energy. They move faster, collide more often, and a greater proportion of those collisions have enough energy to break bonds (they exceed the activation energy). This is why food spoils faster on a hot day.
• Surface area - Grinding a solid into smaller pieces exposes more surface for reactant particles to hit. A whole sugar cube dissolves slowly; crushed sugar dissolves almost instantly. Same mass, totally different rate.
• Pressure (gases only) - Increasing pressure on gases squashes particles closer together, much like increasing concentration in a solution. More collisions per second, faster reaction.
• Catalysts - A catalyst speeds up a reaction without being used up itself. It works by providing an alternative reaction pathway with a lower activation energy. Enzymes are biological catalysts, and they come up constantly in both Chemistry and Biology IGCSE papers.

Collision theory in one paragraph

All five factors tie back to one idea: collision theory. For a reaction to happen, particles must collide with enough energy (the activation energy) and in the correct orientation. Anything that increases the frequency of successful collisions speeds the reaction up. Anything that decreases it slows things down. If an examiner asks you to "explain" a rate change, always link your answer back to collisions.

Worked example: interpreting a rate graph

Notice how the answer links surface area back to collision frequency. That is exactly the chain of reasoning examiners reward.

Practical methods you should know

Cambridge expects you to describe at least two practical approaches for measuring rate:

1. Loss in mass method - Place the reaction vessel on a balance and record the mass at regular intervals. As gas escapes, the mass drops. Plot mass lost against time. Works well for reactions that produce CO₂ (e.g. acid + carbonate).
2. Gas syringe method - Collect the gas in a syringe and record the volume at regular intervals. Plot volume against time. Gives a direct reading and avoids gas escaping into the room.
3. Timing a colour change or precipitate - For reactions like sodium thiosulfate and acid, you watch a cross drawn on paper disappear as the solution turns cloudy. Record the time for the cross to vanish. Shorter time means faster rate.

Reversible reactions and equilibrium

Not every chemical reaction is a one-way street. Some reactions can go forwards and backwards. These are called reversible reactions, and they are written with a special double arrow symbol (⇌) instead of a single arrow.

A good example is the thermal decomposition of ammonium chloride:

NH₄Cl ⇌ NH₃ + HCl

Heat ammonium chloride and it breaks apart into ammonia and hydrogen chloride gas. Cool those gases down and they recombine to form ammonium chloride again. The reaction works in both directions depending on conditions.

Dynamic equilibrium

When a reversible reaction happens in a closed system (nothing can enter or leave), it eventually reaches a point where the forward reaction and the backward reaction are happening at the same rate. This is called dynamic equilibrium.

The word "dynamic" is doing important work here. It does not mean the reactions have stopped. Both directions are still running, but because they are balanced, the concentrations of reactants and products stay constant. Think of it like two equally matched tug-of-war teams: the rope is not moving, but both sides are still pulling.

Redox: the electron shuffle

Redox reactions are one of those topics that sound intimidating but become straightforward once you see the pattern. The name itself is a mash-up of two processes: reduction and oxidation, and they always happen together.

Two ways to define oxidation and reduction

The electron definition is the one that works everywhere, and it is the one the IGCSE extended syllabus really pushes. A handy mnemonic: OIL RIG (Oxidation Is Loss, Reduction Is Gain).

Worked example: identifying oxidation and reduction

Oxidising and reducing agents

This trips people up every year. The reducing agent is the substance that gets oxidised (it gives away electrons, reducing something else in the process). The oxidising agent is the substance that gets reduced (it takes electrons, oxidising something else). Yes, it feels backwards, but the name describes what the substance does to the other reactant, not what happens to itself.

Common mistakes to dodge

• Confusing rate with yield. A catalyst makes a reaction faster but does not change how much product you get. If an exam question asks about rate, talk about speed and collisions, not amounts.
• Forgetting that mass is conserved. In a reaction on a balance, the total mass does not change unless gas escapes an open container. On paper, atoms in must equal atoms out.
• Saying equilibrium means "equal amounts." It means equal rates, not equal concentrations. You can have equilibrium with 90% products and 10% reactants.
• Mixing up oxidising and reducing agents. Remember: the reducing agent is itself oxidised. Use OIL RIG to keep the electron definitions straight.
• Leaving out collision theory. Any "explain" question about rate needs you to mention collisions, frequency, and activation energy. Just naming the factor ("higher temperature") without the mechanism is not enough for full marks on the IGCSE paper.

Quick self-check

1. Give two differences between a physical change and a chemical change.
2. Explain why increasing temperature speeds up a reaction. Use collision theory in your answer.
3. What does "dynamic" mean in the phrase "dynamic equilibrium"?
4. In the reaction 2Mg + O₂ -> 2MgO, which element is oxidised and which is reduced?
5. A student adds acid to large marble chips and to powdered marble. Both experiments use the same mass of marble and the same acid. Which produces gas faster, and why?

If you can answer all five without looking back, you are in solid shape for this section of your IGCSE Chemistry exam. If any tripped you up, reread that section and try again. Repetition is how these ideas stick.