Chemical Change & Rate of Reaction
Cambridge IGCSE Chemistry
The Big Idea: Reactions happen when particles collide with enough energy. You can speed up or slow down any reaction by changing concentration, temperature, surface area, or adding a catalyst.
Quick Summary
Physical vs Chemical
Physical changes don't create new substances; chemical changes do and are usually hard to reverse.
Five Key Factors
Concentration, temperature, surface area, pressure, and catalysts all affect how fast reactions go.
Collision Theory
Reactions need particle collisions with enough energy (activation energy) to break bonds and form new substances.
Rate Measurement
Track how fast reactants disappear or products form using mass loss, gas volume, or colour change.
Graph Interpretation
Reaction rate is the gradient of a product-vs-time curve. Steeper at the start, then levels off as reactants run out.
Limiting Reactant
The reactant that runs out first determines how much product you can make.
1. Physical & Chemical Changes
What's the difference?
This is one of the first concepts examiners test because students often mix them up. Here's the clearest way to think about it:
Physical Change:
No new substances are created. The matter stays the same chemically, just looks or feels different. Easy to reverse.
Examples: Melting ice into water, dissolving salt in water, tearing paper, mixing oil and water.
Chemical Change:
New substances with different properties are formed. Bonds break and reform. Usually hard or impossible to reverse.
Examples: Burning paper to ash, rusting iron, cooking an egg, acid reacting with magnesium.
How to Tell the Difference (Exam Tip): Ask yourself: "If I did this in reverse, would I get the exact same substance back?" If yes, it's probably physical. If no (you've created something brand new), it's chemical.
Signs of a Chemical Reaction
Examiners love asking you to describe what happens. Watch for these:
🎨 Colour Change
One of the easiest signs. For example, copper is orange-brown. When it reacts with silver nitrate (colourless), you get silver (grey) and copper nitrate (blue). Two clear colour changes = chemical reaction happened.
🌡️ Temperature Change
Exothermic: Releases heat. Combustion, neutralisation, rusting. The container gets hot.
Endothermic: Absorbs heat. Photosynthesis, dissolving ammonium chloride. The container gets cold (sometimes used in cold packs).
💨 Effervescence (Fizzing)
Gas is produced. Usually hydrogen gas when a metal reacts with acid, or carbon dioxide when an acid reacts with a carbonate.
Other signs:
Light being produced (like burning magnesium burning super bright), smell changing, pH changing.
Practice Question 1
You mix copper powder with dilute hydrochloric acid in a test tube. The mixture fizzes vigorously and the test tube becomes hot. Is this a physical or chemical change? Explain your reasoning with two pieces of evidence.
4. Investigating the Rate of a Reaction