Investigating The Rate of a Reaction
Section: 6. Chemical Reactions | Syllabus: Cambridge IGCSE Chemistry (0620)
Method 1: Change in Mass
Used when a gas escapes from the reaction flask. The flask sits on a balance, and mass is recorded at regular time intervals - mass decreases as gas is lost.
- Use a plug of cotton wool in the flask neck (not a sealed bung) so gas can escape while spray/spitting is contained
- Example: marble chips + acid → CO₂ escapes, mass decreases
Evaluation: very accurate (continuous readings, sensitive balance), but unsuitable if the gas produced is toxic, since it must be released rather than collected.
Method 2: Change in Gas Volume
Used when the reaction produces a gas. The volume of gas collected is recorded at regular time intervals.
| Setup | How it works |
|---|---|
| Gas syringe | gas pushes the plunger out; volume is read directly from the graduated barrel |
| Inverted measuring cylinder | gas is collected over water in an upturned, graduated cylinder |
Evaluation: a gas syringe gives quick, accurate, continuous readings, but the plunger can stick, and some gas may escape before the bung is sealed.
Worked Example: Identifying the Apparatus and State Symbols
- Question: CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + CO₂(g) + H₂O(l). (i) State the meaning of the symbol (aq) in this equation. (ii) Name the item of apparatus used to collect and measure the volume of carbon dioxide produced. [2]
- Answer (i): aqueous - the substance is dissolved in water
- Answer (ii): a (gas) syringe
Volume of gas collected is read from the syringe at regular time intervals and plotted against time
Method 3: Change in Turbidity (the "Disappearing Cross")
Used when a precipitate forms, making a solution cloudy.
- Draw a black cross on paper and place a conical flask on top of it
- Add the reactants and start a timer
- Look down through the flask; time how long until the cross is no longer visible
Classic example: Na₂S₂O₃(aq) + 2HCl(aq) → 2NaCl(aq) + SO₂(g) + S(s) + H₂O(l) - a pale yellow sulfur precipitate obscures the cross.
Evaluation: cheap and simple, but subjective (different observers judge "disappeared" differently) and gives only ONE time measurement per experiment, unlike the continuous readings from mass loss or gas volume.
Fair Testing: Control Variables
When investigating the effect of ONE factor, every other factor must be kept constant.
| Factor being tested | Must be kept constant |
|---|---|
| Concentration | temperature, mass and surface area of any solid, volume of solution |
| Temperature | concentration of all reactants, volumes, mass/surface area of any solid |
| Surface area | temperature, concentration, volume, and the total MASS of solid used (same mass, different sizes) |
| Catalyst | everything else - only the presence/absence (or amount) of catalyst changes |
When testing surface area, using the same mass of solid in different-sized pieces is essential for a fair comparison.
Common Mistakes
- ❌ Using a sealed bung (no gas escape route) with the mass-loss method
- ✓ use a loose cotton wool plug so gas can escape while spray is trapped, otherwise pressure builds up and no mass change is measurable
- ❌ Comparing surface area using different masses of solid
- ✓ always keep the mass of solid the same and only change the size/number of pieces, otherwise the comparison is not a fair test
- ❌ Judging the disappearing-cross endpoint with different observers each time
- ✓ use the same person throughout (or a light sensor/data logger) to keep the judgement consistent
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