TL;DR Rate of reaction measures how fast reactants are used up or products are formed. Three standard O-Level practicals let you measure it: the disappearing cross (thiosulfate + HCl), the gas syringe method, and the mass loss method. Each method gives a different type of graph. The key skill is linking experimental data to factors like concentration, temperature, surface area, and catalysts using collision theory. Most marks are lost by plotting time instead of 1/time, forgetting units, or failing to control variables.
For marking priorities and examiner expectations, pair this walkthrough with the Paper 3 Practical Guide.
1 | What rate of reaction means
Rate of reaction is the change in the amount of reactant used up or product formed per unit time. In symbols:
Rate=time takenchange in amount of product or reactant
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Azmi·Senior Chemistry Specialist
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A faster reaction has a larger rate value. A slower reaction has a smaller rate value. The rate is not constant throughout a reaction - it is fastest at the start (when reactant concentrations are highest) and slows down as reactants are consumed.
Measure volumes of thiosulfate, acid, and water accurately
White tile or paper with a black cross drawn on it
Placed under the flask as the endpoint marker
Stopwatch
Times how long the cross takes to disappear
Thermometer
Checks that temperature is constant across trials
Method
Draw a bold black cross on a piece of white paper. Place it on the bench.
Using a measuring cylinder, add 25 cm\u00B3 of 0.15 mol/dm\u00B3 sodium thiosulfate solution to the conical flask.
Add 5 cm\u00B3 of 1 mol/dm\u00B3 dilute hydrochloric acid.
Immediately start the stopwatch and swirl the flask once.
Look down through the mouth of the flask at the cross. Stop the stopwatch the instant the cross is no longer visible.
Record the time t.
Rinse the flask thoroughly and repeat with different concentrations of sodium thiosulfate. Keep the total volume constant by adding distilled water (e.g. 20 cm\u00B3 thiosulfate + 5 cm\u00B3 water, 15 cm\u00B3 thiosulfate + 10 cm\u00B3 water, and so on). The acid volume stays at 5 cm\u00B3 each time.
Record the temperature before each trial to confirm it stays constant.
Data table
Volume of Na\u2082S\u2083O\u2083 / cm\u00B3
Volume of water / cm\u00B3
Time for cross to disappear, t / s
1/t / s\u207B\u00B9
25
0
20
5
15
10
10
15
5
20
Graph
Plot 1/t (y-axis) against volume of sodium thiosulfate (x-axis). Since the total volume is constant, the volume of thiosulfate is proportional to its concentration. You should see a straight line passing through (or close to) the origin, showing that rate is directly proportional to the concentration of thiosulfate.
Why 1/t and not t?
The time t is inversely related to rate: a shorter time means a faster reaction. Plotting t against concentration gives a curve, which is harder to interpret. Plotting 1/t converts this into a straight-line relationship, making it easier to identify direct proportionality.
3 | Experiment 2 - Gas syringe method
The reaction
Calcium carbonate (marble chips) reacts with dilute hydrochloric acid to produce carbon dioxide gas:
CaCO3(s)+2HCl(aq)→CaCl2(aq)+H2O(l)+CO2(g)
You can also use magnesium ribbon with dilute HCl (producing hydrogen gas) for the same type of experiment.
Apparatus
Item
Purpose
Conical flask with a side-arm or bung fitted with delivery tube
Contains the reactants and channels gas to the syringe
Gas syringe (100 cm\u00B3)
Measures the volume of gas produced over time
Stopwatch
Times the gas collection at regular intervals
Electronic balance
Weighs the marble chips to ensure equal mass in each trial
Measuring cylinder
Measures a fixed volume of acid
Method
Weigh out a fixed mass (e.g. 5.0 g) of marble chips. Use chips of roughly the same size.
Measure 50 cm\u00B3 of 1 mol/dm\u00B3 hydrochloric acid into the conical flask.
Reset the gas syringe to zero.
Add the marble chips to the acid, quickly fit the bung, and start the stopwatch.
Record the volume of gas in the syringe every ten seconds until the reaction is complete (the syringe reading stops increasing).
Repeat with a different variable - for example, use powdered marble instead of chips (surface area), or use 2 mol/dm\u00B3 acid (concentration), or warm the acid in a water bath before adding chips (temperature).
Data table
Time / s
Volume of CO\u2082 / cm\u00B3
0
0
10
20
30
40
50
60
Graph
Plot volume of gas (y-axis) against time (x-axis). The graph is a curve that rises steeply at first and then levels off as the reaction finishes. The gradient of the curve at any point is the rate of reaction at that instant. A steeper initial gradient means a faster rate.
To compare two experiments, plot both curves on the same axes. The curve with the steeper initial gradient has the faster rate. Both curves reach the same final volume if the same mass of marble chips is used (same limiting reagent), but the faster reaction reaches that volume sooner.
4 | Experiment 3 - Mass loss method
The reaction
The same calcium carbonate + HCl reaction works here. Carbon dioxide escapes into the air, so the mass of the flask and its contents decreases over time.
Apparatus
Item
Purpose
Conical flask
Contains the reactants
Electronic balance (0.01 g)
Measures mass loss continuously
Cotton wool plug
Placed in the neck of the flask to prevent liquid spray from escaping while allowing gas to leave
Stopwatch
Times the mass readings at regular intervals
Measuring cylinder
Measures a fixed volume of acid
Method
Place the conical flask on the balance. Add 50 cm\u00B3 of 1 mol/dm\u00B3 HCl and a cotton wool plug. Record the initial mass.
Remove the flask from the balance briefly. Add 5.0 g of marble chips, replace the cotton wool plug, and place the flask back on the balance immediately. Start the stopwatch.
Record the mass every ten seconds until the reading is constant.
Calculate the mass loss at each time interval: mass loss = initial mass - current mass.
Data table
Time / s
Mass of flask and contents / g
Mass loss / g
0
0.00
10
20
30
40
50
60
Graph
Plot mass loss (y-axis) against time (x-axis). The shape is similar to the gas syringe graph - steep at first, then levelling off. A steeper curve means a faster rate.
Why cotton wool?
Cotton wool prevents liquid from splashing out of the flask (which would give an artificially large mass loss) while still allowing carbon dioxide gas to escape. Without the cotton wool, the gas cannot escape and no mass loss is recorded. Without any plug, acid spray may leave the flask and distort results.
5 | Factors affecting rate of reaction
Four factors appear regularly in the O-Level syllabus. For each factor, collision theory explains why the rate changes: particles must collide with sufficient energy (activation energy) and correct orientation for a reaction to occur.
Factor 1 - Concentration
What happens: Increasing the concentration of a reactant increases the rate.
Why (collision theory): More particles per unit volume means more frequent collisions, so there are more successful collisions per second.
Which experiment demonstrates it: Disappearing cross - vary the volume (and therefore concentration) of thiosulfate while keeping total volume constant. The 1/t vs concentration graph is a straight line through the origin.
Factor 2 - Temperature
What happens: Increasing the temperature increases the rate.
Why (collision theory): Particles move faster at higher temperatures, so they collide more frequently. More importantly, a greater proportion of particles have energy equal to or greater than the activation energy, leading to more successful collisions.
Which experiment demonstrates it: Disappearing cross or gas syringe. For the cross method, keep concentration constant but heat the thiosulfate solution in a water bath to different temperatures before adding the acid. You will see that the time for the cross to disappear decreases sharply as temperature rises.
Factor 3 - Surface area (particle size)
What happens: Smaller particles (greater surface area) react faster than larger particles.
Why (collision theory): Smaller particles expose more surface to the acid, so more collisions can occur at the same time.
Which experiment demonstrates it: Gas syringe or mass loss - compare the rate using large marble chips vs small marble chips vs powdered marble, keeping mass and acid concentration constant. Powdered marble gives the steepest initial curve.
Factor 4 - Catalyst
What happens: A catalyst increases the rate without being consumed in the reaction.
Why (collision theory): A catalyst provides an alternative reaction pathway with a lower activation energy. More particles now have enough energy to react, so the number of successful collisions per second increases.
Which experiment demonstrates it: Gas syringe - decompose hydrogen peroxide with and without manganese(IV) oxide (MnO\u2082) as a catalyst. With MnO\u2082, the oxygen gas is produced much faster and the curve is steeper.
6 | Worked example
A student investigates the effect of sodium thiosulfate concentration on rate of reaction using the disappearing cross method. Her results are shown below.
Volume of Na\u2082S\u2083O\u2083 / cm\u00B3
Time / s
1/t / s\u207B\u00B9
25
28
0.036
20
35
0.029
15
50
0.020
10
77
0.013
5
158
0.0063
Step 1 - Calculate 1/t.
For the first row: 1/t=1/28=0.036 s\u207B\u00B9 (to 2 significant figures).
For the last row: 1/t=1/158=0.0063 s\u207B\u00B9.
Step 2 - Plot the graph.
Plot 1/t on the y-axis and volume of Na\u2082S\u2082O\u2083 on the x-axis. The five points should lie close to a straight line that passes through (or near) the origin.
Step 3 - Describe the relationship.
The graph shows a straight line through the origin, indicating that the rate of reaction is directly proportional to the concentration of sodium thiosulfate. This is consistent with collision theory: doubling the concentration doubles the number of thiosulfate particles per unit volume, doubling the collision frequency and therefore the rate.
Subjective endpoint (the exact moment the cross disappears varies between observers)
Disappearing cross
Time may be recorded too early or too late, giving inconsistent 1/t values
Use a light sensor and datalogger to measure turbidity objectively
Gas escaping before the bung is fitted
Gas syringe
The initial volume reading is too low, and the initial gradient is underestimated
Add the acid to the marble chips already in the flask via a thistle funnel through the bung, or fit the bung as quickly as possible
Temperature not constant during the experiment
All three
Rate changes unpredictably if room temperature drifts or if the reaction itself is exothermic
Carry out all trials in a water bath at a controlled temperature
Balance sensitivity and drafts
Mass loss
Small mass changes may be missed or distorted by air currents near the balance
Use a balance reading to 0.01 g and a draft shield
Cotton wool absorbing some gas
Mass loss
Measured mass loss is slightly less than actual, giving an underestimate of rate
Accept this as a minor systematic error; it affects all readings equally
8 | Common exam mistakes
Mistake 1 - Plotting time instead of 1/time
If the question asks you to plot a graph showing how rate depends on concentration, you must plot 1/t on the y-axis, not t. Plotting t gives a curve, not the straight line needed to show direct proportionality.
Mistake 2 - Confusing rate with time
A shorter time means a faster rate, not a slower one. Students often write "the time increased so the rate increased" - this is backwards. Always check: if the reaction finishes sooner, the rate is higher.
Mistake 3 - Forgetting units
1/t has units of s\u207B\u00B9. Volume of gas per second has units of cm\u00B3/s. Mass loss per second has units of g/s. Table headings and graph axes must include units.
Mistake 4 - Not controlling variables
If you change concentration and temperature at the same time, you cannot attribute the change in rate to either variable alone. The controlled variables must be explicitly stated in your method. Common ones to fix: volume of acid, mass of solid, temperature, and particle size.
Mistake 5 - Writing vague error statements
"Human error" is not an acceptable source of error. Be specific: "reaction time in starting and stopping the stopwatch introduces an uncertainty of approximately 0.5 s, which is significant when total time is short (e.g. 28 s)."
Mistake 6 - Mixing up the two graph types
The gas syringe and mass loss methods plot amount (volume or mass) against time. The disappearing cross method plots 1/t against concentration. These are different graph types with different shapes and different meanings. Do not mix them up.
9 | Paper 3 practical tips - planning questions
If a Paper 3 planning question asks you to design a rate of reaction experiment, use this structure (and see the O-Level Chemistry planning question bank for worked model responses across acid-base, calorimetry, rate, and separation prompts):
State the aim. "To investigate how [independent variable] affects the rate of reaction between [reactants]."
Identify variables. Independent variable (what you change), dependent variable (what you measure), controlled variables (what you keep constant). List at least three controlled variables.
Choose a measurement method. Justify your choice. For example: "I will use the gas syringe method because the reaction produces a gas (CO\u2082), and the volume of gas collected can be measured accurately at regular time intervals."
Write a step-by-step procedure. Include specific quantities (e.g. "50 cm\u00B3 of 1 mol/dm\u00B3 HCl", "5.0 g of marble chips of size 2--4 mm"). State how many trials you will run and what you will change between them.
Describe how to process results. "Plot volume of CO\u2082 against time for each trial on the same axes. Compare the initial gradients of the curves."
Include a safety precaution. For acid reactions: "Wear safety goggles to protect eyes from acid splashes."
State an expected outcome. "If concentration is doubled, the initial gradient should approximately double, consistent with rate being proportional to concentration."
For detailed guidance on graph conventions that apply to chemistry and physics practicals alike, see the Practical Graph Skills guide.
10 | Frequently asked questions
What is the simplest way to measure rate of reaction at O-Level?
The disappearing cross method is the simplest setup - it requires only a flask, a stopwatch, a measuring cylinder, and a piece of paper with a cross. However, it has a subjective endpoint. The gas syringe method is more objective but requires more apparatus.
Can I use the gas syringe method for any reaction?
Only for reactions that produce a gas. Common O-Level examples include marble chips + HCl (CO\u2082), magnesium + HCl (H\u2082), and catalytic decomposition of hydrogen peroxide (O\u2082).
Why does the graph level off in the gas syringe and mass loss methods?
The graph levels off when all of the limiting reagent has been used up. No more product can be formed, so the volume of gas (or mass loss) stops changing. The final value depends on the amount of limiting reagent, not on the rate.
How do I calculate the rate at a specific time from a curved graph?
Draw a tangent to the curve at the time of interest. Calculate the gradient of the tangent: gradient = rise / run. The gradient equals the rate at that instant.
Does a catalyst change the total amount of product formed?
No. A catalyst increases the rate but does not change the equilibrium position or the total yield. On a gas syringe graph, both curves (with and without catalyst) reach the same final volume, but the catalysed reaction reaches it sooner.
What is the difference between rate and time?
Rate is the amount of product formed (or reactant used) per unit time. Time is how long the reaction takes. They are inversely related: a faster rate means a shorter time, and vice versa. When quoting rate values, always include units (e.g. cm\u00B3/s, g/s, or s\u207B\u00B9 for the 1/t method).
