Chemical changes · GCSE Chemistry
Rates required practical
GCSE Chemistry revision on the rates required practical: marble chips and acid, sodium thiosulfate and HCl, measuring mass, gas volume or a disappearing cross, and fair tests.
Change one factor. Measure mass loss, gas volume, or time for a cross to disappear. Control everything else, repeat, and remember that 1/time is a measure of mean rate.
The important bits
What you need to know
- 1
Two common practicals: CaCO₃ + HCl, measuring mass loss on a balance or gas volume in a gas syringe; and Na₂S₂O₃ + HCl, timing how long a black cross under the flask takes to disappear as sulfur precipitates.
- 2
Independent variable examples: concentration of HCl, surface area of marble, temperature of thiosulfate. Dependent variable: mass, volume, or time to hide the cross.
- 3
Control variables: volume of solutions, mass of carbonate, temperature (unless that is what you change), swirling, and the same cross and flask for the turbidity method.
- 4
Rate can be compared as 1/time when the same amount of product is formed (same cross disappearance, or time to collect 20 cm³ of gas). A shorter time means a higher mean rate.
- 5
A gas syringe or an inverted measuring cylinder over water collects CO₂. A cotton wool plug on a flask on a balance lets CO₂ escape while stopping acid spray, so mass falls.
- 6
Repeat each concentration or temperature and calculate a mean time. Discard obvious anomalies (syringe stuck, chip stuck to the side, cross not the same).
- 7
Risk: acids are corrosive; hydrogen from metal-plus-acid practicals is flammable; sulfur dioxide can form if thiosulfate is overheated with acid — use a fume cupboard if the method produces SO₂.
- 8
Graphs: mass or volume against time for marble chips; 1/time against concentration for thiosulfate often gives a straight line through the origin if rate is proportional to concentration.
Quotations worth analysing
Short evidence. Real method.
“Na₂S₂O₃(aq) + 2HCl(aq) → 2NaCl(aq) + SO₂(g) + S(s) + H₂O(l)”
Solid sulfur makes the mixture cloudy. Time the same amount of sulfur each run by using the same cross. Mean rate ∝ 1/time.
“CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)”
Choose mass loss or gas volume. Same mass of CaCO₃ and same volume of acid if you are testing concentration or surface area fairly.
“mean rate ∝ 1 / time”
If the cross disappears in 20 s at one concentration and 40 s at another, the first mean rate is twice the second. Do not treat raw time as the rate.
Go deeper
Fair test first, collision theory second
If you change concentration and accidentally use warmer acid, you have mixed two factors. Measure volumes with a measuring cylinder or pipette, keep the total volume constant by diluting with water, and stand solutions in a water bath if temperature is the independent variable, using a thermometer. For chips versus powder, keep mass the same — powder will still have a larger surface area. Write the control variables in the plan; they are method marks. Then explain the result with collision theory: more concentrated acid, more frequent successful collisions with the carbonate surface.
Go deeper
Choose a measurement that matches the reaction
A gas syringe gives a volume–time graph you can draw a tangent on. A balance gives mass–time; the line falls. The cross method does not give a continuous graph of amount of sulfur, only a single time, so you plot 1/t against concentration or temperature. Students try to collect sulfur dioxide from thiosulfate in a syringe and then complain it dissolves. Stick to the turbidity method for that reaction. Cotton wool on a mass-loss flask is there so acid spray does not look like extra mass loss. Start the stopwatch when you mix, not when you remember to.
Go deeper
Uncertainty and anomalies belong in the evaluation
Timing a disappearing cross is subjective — different people judge “gone” differently — so one person should judge every run, or use a light sensor if the school has one. A gas syringe can stick. Chips vary in size even in the same bottle; using many small chips and a larger mass reduces that variation. Repeats and means are how you deal with this. Percentage uncertainty in time is larger when the reaction is very fast (small t), so very high concentrations may need a revised method (more dilute, or measure a larger gas volume). That is a better evaluation than “human error”.
See the idea in action
A student times the disappearing cross for sodium thiosulfate plus HCl. Times at 20 °C, 30 °C and 40 °C are 64 s, 32 s and 16 s (same concentrations). Mean rate as 1/t is 0.016 s⁻¹, 0.031 s⁻¹ and 0.063 s⁻¹. Rate roughly doubles every 10 °C. Explanation: higher temperature, particles move faster and a greater proportion of collisions exceed the activation energy. Controls: same volumes, same flask, same cross, same person judging the end-point. The student should repeat each temperature and mean the times before calculating 1/t.
Exam technique
Turn knowledge into marks
Name the independent and dependent variables, list controls, say how you measure (balance, syringe, or cross), then explain with successful collisions. Use 1/time if you only have an end-point time. Repeat and average.
Common mistakes
Do not give these marks away
- 01
Changing two variables at once, or using time as if it were rate without taking the reciprocal.
- 02
Forgetting cotton wool on a mass-loss flask, or starting the clock after the reaction has already fizzed.
- 03
Explaining a steeper mass–time graph with “more product” rather than a faster rate at the start.
In the disappearing-cross experiment, how should mean rate be compared?
AUse the time directly: a longer time means a faster rate
BUse 1/time: a shorter time to hide the cross means a higher mean rate
CWeigh the flask at the end
DCount the bubbles
Show the answer
Use 1/time: a shorter time to hide the cross means a higher mean rate. The same amount of sulfur is formed when the cross disappears, so rate is proportional to 1/time. Longer times mean slower reactions.
Quick questions
If this is the bit you searched
How do you measure rate of reaction in the GCSE required practical?
Follow mass loss or gas volume against time for marble chips and acid, or time how long a cross takes to disappear for thiosulfate and acid. Rate is change divided by time, or 1/time for a fixed end-point.
How do you keep the thiosulfate experiment a fair test?
Keep volumes, the flask, the cross and the observer the same. Change only concentration or only temperature. Repeat and calculate mean times.
Why put cotton wool in the neck of the flask on a balance?
Carbon dioxide can escape so mass falls, but acid spray is trapped. Spray loss would look like extra reaction if it landed off the balance pan.
What does a volume–time graph tell you?
The gradient at any point is the rate then. It is steepest at the start and becomes zero when the reaction stops. The final volume is the total gas produced.