Science required practicals
How do you do the rates of reaction required practical?
You change the concentration of one reactant and measure how fast the reaction goes, by two methods. In the first, magnesium reacts with hydrochloric acid and you collect the hydrogen, reading its volume every 10 seconds. In the second, sodium thiosulfate reacts with acid and turns cloudy; you time how long a cross under the flask takes to disappear. Higher concentrations react faster, because particles collide more often.
Last updated 23 September 2026 · Written and fact-checked by the GCSEwiz team
What does the rates of reaction practical investigate?
How changing the concentration of a reactant changes the rate of a reaction. AQA asks for two methods: one that measures the volume of gas given off, and one that watches a change in colour or cloudiness (turbidity).
It is a required practical on both GCSE Chemistry and Combined Science: Trilogy. AQA also wants it run as an investigation with a hypothesis, such as "the higher the concentration, the faster the reaction, because the particles collide more often".
The two reactions are magnesium + hydrochloric acid → magnesium chloride + hydrogen (Mg + 2HCl → MgCl₂ + H₂), and sodium thiosulfate + hydrochloric acid → sodium chloride + sulfur dioxide + sulfur + water (Na₂S₂O₃ + 2HCl → 2NaCl + SO₂ + S + H₂O). The sulfur is a fine solid, and it is what turns the mixture cloudy.
What equipment do you need?
For the gas method you need magnesium ribbon, hydrochloric acid at two or more concentrations and a way to collect the gas: an upturned measuring cylinder in a trough of water, or a gas syringe. For the cloudiness method you need sodium thiosulfate solution, hydrochloric acid and a cross on paper. Both need a stopclock.
- Magnesium ribbon cut into 3 cm strips
- Dilute hydrochloric acid at different concentrations, for example 1.0 and 1.5 mol/dm³
- A 100 cm³ conical flask with a bung and delivery tube
- A trough of water and a 100 cm³ measuring cylinder, or a gas syringe
- Sodium thiosulfate solution, and water to dilute it
- 10 cm³ and 100 cm³ measuring cylinders
- A black cross printed on paper
- A stopclock and eye protection
What is the method?
Run each method at several concentrations, keeping everything else the same. In the gas method you read the volume of gas every 10 seconds. In the cross method you time how long the mixture takes to hide the cross.
- Gas method: measure 50 cm³ of 1.0 mol/dm³ hydrochloric acid into the conical flask and fit the bung and delivery tube.
- Gas method: fill the measuring cylinder with water, turn it upside down in the trough and put the end of the delivery tube underneath it.
- Gas method: take out the bung, drop in a 3 cm strip of magnesium ribbon, put the bung back in as quickly as you can and start the stopclock.
- Gas method: record the volume of gas every 10 seconds until it stops changing, then repeat with 1.5 mol/dm³ acid.
- Cross method: put 10 cm³ of sodium thiosulfate solution and 40 cm³ of water into the conical flask, and stand it on the cross.
- Cross method: add 10 cm³ of dilute hydrochloric acid, swirl the flask and start the stopclock at the same moment.
- Cross method: look down through the flask from above and stop the clock when you can no longer see the cross.
- Cross method: repeat with 20, 30, 40 and 50 cm³ of thiosulfate, using less water each time so the total is always 50 cm³.
What are the variables?
In both methods you change the concentration of one reactant and measure how quickly the reaction goes. Temperature has to stay the same too, because it changes the rate as well.
| Variable | In this practical |
|---|---|
| Independent variable | The concentration of the acid (gas method) or of the sodium thiosulfate (cross method) |
| Dependent variable | The volume of gas collected over time, or the time taken for the cross to disappear |
| Control variable | The temperature of the solutions |
| Control variable | The volume of acid, and the total volume of thiosulfate and water (50 cm³) |
| Control variable | The amount of the other reactant: the same 3 cm of magnesium, or the same volume and concentration of acid |
| Control variable | The same flask and printed cross, judged by the same person |
How do you work out and present the results?
Gas method: plot the volume of gas (y-axis) against time (x-axis), with a curve for each concentration on the same axes. The steeper the curve, the faster the reaction. Both curves should level off at about the same volume, because each run used the same length of magnesium and there was more than enough acid to use it all up.
To find a mean rate, divide the volume of gas by the time. If 40 cm³ of hydrogen is collected in the first 20 s, the mean rate is 40 ÷ 20 = 2.0 cm³/s. On the Higher tier you may be asked for the rate at one moment: draw a tangent to the curve at that time and work out its gradient.
Cross method: the cross disappears once the same amount of sulfur has formed, so a shorter time means a faster reaction. That is why 1/time is used as a measure of rate. If the cross disappears after 50 s, 1/time = 1 ÷ 50 = 0.020 /s. If a more concentrated solution takes 25 s, 1/time = 1 ÷ 25 = 0.040 /s: twice the rate.
Record the times in a table with repeats and a mean for each concentration, leaving out any anomalous results. A graph of time against concentration gives a falling curve. A graph of 1/time against concentration rises, which shows the rate going up as the concentration goes up.
How do you make it accurate and safe?
Rub the magnesium with fine sandpaper first to remove its coating of oxide, which would slow the start of the reaction. Put the bung in quickly so no gas escapes before you start measuring. A gas syringe can replace the trough and measuring cylinder.
The weak point of the cross method is judging the exact moment the cross vanishes. Use the same printed cross, in the same light, with the same person watching every run. A light sensor and data logger can measure the cloudiness instead of your eyes. Repeat each concentration, work out a mean and keep the temperature the same throughout.
For safety, wear eye protection. The cross method gives off sulfur dioxide, which can trigger breathing problems, especially for people with asthma. Keep the room well ventilated, avoid breathing in the fumes, and once you have your time, get rid of the mixture the way your teacher tells you. In the gas method, hydrogen is flammable, so keep flames away.
What do exam questions ask about it?
Questions ask you to name the variables, read or draw the gas volume graph, calculate a mean rate or 1/time, and explain the effect of concentration using collision theory. Comparing the two methods comes up too: the gas method gives a whole curve from one run, while the cross method gives a single time that depends on judging a moment by eye. For example:
"Explain, in terms of particles, why the cross disappears more quickly when the sodium thiosulfate solution is more concentrated. (3 marks)"
A full answer links the steps: there are more particles in the same volume, so collisions happen more often, so the reaction is faster.
Common mistakes
The explanation deserves as much care as the method, as the first two show.
- Writing "more collisions" on its own. Say the collisions happen more often, or that there are more collisions per second: it is how frequent they are that sets the rate.
- Saying the particles move faster at a higher concentration. That is what a higher temperature does; a higher concentration puts more particles in the same volume.
- Changing the total volume when you dilute the thiosulfate. Keep it at 50 cm³, so only the concentration changes and the depth of liquid over the cross stays the same.
- Starting the clock late, or forgetting to swirl. Add the acid, swirl and start timing in one movement.
- Reading a longer time as a faster reaction. In the cross method, the shorter the time, the faster the rate.