Question 1 (2 marks)
Knowledge
Why does increasing concentration usually increase reaction rate?
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More reactant particles per unit volume (1), leading to more frequent successful collisions (1).
Required Practical 11: Rates of Reaction | AQA GCSE Science | The Science Department
About this practical and its aim
Aim
Investigate how concentration affects reaction rate using both gas production and a colour/turbidity change.
Introduction
Here there are two experiments. Firstly, you measure how long it takes for a solution to become so cloudy that you can no longer see the cross underneath the conical flask (turbidity), secondly, you will take regular reading s of gas as it is evolved from a reaction. Both will show how variables affect the rate of a reaction, however, the end point only is given in the turbidity experiment whereas when collecting gas, we can plot a graph and look at how the gradients change over time.
The science behind the practical
Increasing concentration usually increases collision frequency. Reaction rate can be measured from volume of gas produced per time or from a fixed visible endpoint.
Key things to remember
Use gas syringe for volume readings; use a disappearing cross for turbidity; control other variables.
Equipment and apparatus
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Conical flasks
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measuring cylinders
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gas syringe and bung/tubing
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timer
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appropriate acid and solid reactants
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sodium thiosulfate solution and dilute acid
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paper cross
Method: How to carry out the experiment
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Method 1: Disappearing cross (initial rate method)
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Pour 40cm3 or room temperature Sodium thiosulphate solution into the conical flask using the 100cm3 measuring cylinder.
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Place the conical flask onto the cross provided
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Add 5cm3 of hydrochloric acid to the 10cm3 measuring cylinder
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Simultaneously, pour in the acid and start the timer
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Record the time when you can no longer see the cross when looking down through the flask
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Repeat the experiment using sodium thiosulphate solution from the water bath to observe the effect of raising the temperature on the rate of reaction.
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Ensure that you do not mix up the measuring cylinders or the reaction will begin early.
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All waste must go into the waste bucket, not down the sinks
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You can repeat this experiment with a different concentration of acid or acid at a different temperature to show what effects rates.
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Method 2: Gas syringe CO2 collection (continuous monitoring method)
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Using a weighing boat, weigh out 2g of calcium carbonate chips
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Place these into the bottom of the conical flask (with side arm).
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Connect the arm to the securely setup gas syringe
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Carefully measure 50cm3 of 0.2 moldm-3 hydrochloric acid.
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Simultaneously, pour the acid into the conical flask, insert the bung and start the timer.
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Record the volume on the gas syringe every 5 seconds.
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Repeat using 0.5 moldm-3 hydrochloric acid.
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Empty waste into sieve/bucket provided
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You can repeat this experiment with a different concentration of acid or calcium carbonate chips with a different surface area to show what effects rates.
Variables and fair testing
| Type of variable | What is measured or controlled? |
|---|---|
| Independent variable | Concentration of a reactant |
| Dependent variable | Gas volume per unit time or time for a cross to disappear |
| Control variables | Temperature; reactant mass/surface area; total volume; apparatus; observation endpoint |
Safety and managing risks
Follow your teacher's instructions and your school's risk assessment. This page is not a substitute for a formal risk assessment.
| Hazard | Risk | Precaution |
|---|---|---|
Acids and reacting solids |
Irritation or splashes |
Wear assessed eye protection, use suitable dilute acids. |
Sulfur dioxide from thiosulfate/acid reaction |
Respiratory irritation |
Follow concentration, scale and ventilation advice in the local risk assessment. |
Results, calculations and analysis
Recording, presenting and interpreting results
For gas: plot volume (cm³) against time (s), compare initial slopes. For turbidity: record time to hide the cross, calculate 1/time.
Calculations
Mean rate = gas volume ÷ time; relative rate for cross method = 1 ÷ time (s⁻¹).
Conclusions, accuracy and evaluation
Drawing a conclusion
Higher concentration generally produces faster gas production or a shorter time to the endpoint.
Evaluating the investigation
Use repeat runs, check gas-tightness, and standardise observer position and cross visibility.
More to learn
Two rate methods
Gas syringe gives readings throughout the reaction. The disappearing-cross method gives a single visible endpoint time.
Common exam mistake
For a volume–time graph, rate is the gradient, not the final gas volume alone.
GCSE examination practice
Answer each question in full before opening the model answer. Check your work against the marking points and look for scientific key words.
Question 2 (2 marks)
Calculation
A reaction forms 60 cm³ gas in 40 s. Calculate mean gas production rate.
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60 ÷ 40 = 1.5 cm³/s (2).
Question 3 (1 marks)
Analysis
On a volume–time graph, what does a steeper initial gradient indicate?
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A higher initial rate of reaction (1).
Question 4 (2 marks)
Evaluation
Suggest one improvement to make the investigation more reliable and explain why it helps.
Show model answer / marking points
Repeat measurements and calculate a mean (1), reducing the influence of random variation/anomalies (1). Alternative valid improvement specific to the experiment with explanation can gain both marks.
This page was updated on: 17th June 2026