Core Practical: Investigating the Effect of Concentration on Rate

This lesson covers the Core Practical for investigating the effect of concentration on the rate of reaction between sodium thiosulfate and hydrochloric acid (the disappearing cross experiment) as required by the Edexcel GCSE Combined Science specification (1SC0). You must know the method, variables, how to process results and how to evaluate the experiment.

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Aim

To investigate how changing the concentration of sodium thiosulfate affects the rate of reaction with hydrochloric acid.

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Background

When sodium thiosulfate solution reacts with hydrochloric acid, a yellow precipitate of sulfur is produced, which makes the solution go cloudy.

Equation

sodium thiosulfate + hydrochloric acid → sodium chloride + water + sulfur dioxide + sulfur

Na₂S₂O₃(aq) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + SO₂(g) + S(s)

The sulfur produced is an insoluble yellow solid that forms a suspension, making the solution progressively more opaque. By timing how long it takes for the solution to become opaque enough to obscure a cross drawn on paper underneath the flask, we can measure the rate.

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Apparatus

Item	Purpose
Conical flask (250 cm³)	Reaction vessel
Measuring cylinders (×2)	To measure sodium thiosulfate solution and water
Measuring cylinder (small, 10 cm³)	To measure HCl
Stopwatch	To time the reaction
White paper with a bold black cross (X)	Placed under the flask
Sodium thiosulfate solution (e.g. 0.15 mol/dm³)	Reactant — concentration varied
Dilute hydrochloric acid (e.g. 1 mol/dm³)	Reactant — kept constant
Distilled water	To dilute sodium thiosulfate and keep total volume constant
Safety goggles	Eye protection

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Method

graph TD
    A["1. Place the conical flask<br/>on the paper with cross X"] --> B["2. Measure sodium thiosulfate<br/>and water into the flask<br/>(keep total volume constant)"]
    B --> C["3. Measure HCl<br/>in a separate<br/>measuring cylinder"]
    C --> D["4. Add HCl to flask<br/>and start the<br/>stopwatch immediately"]
    D --> E["5. Look down through<br/>the solution at the<br/>cross from above"]
    E --> F["6. Stop the stopwatch<br/>when the cross can<br/>no longer be seen"]
    F --> G["7. Record the time"]
    G --> H["8. Repeat with different<br/>concentrations of sodium<br/>thiosulfate (diluted with water)"]

    style A fill:#2980b9,color:#fff
    style B fill:#2980b9,color:#fff
    style C fill:#27ae60,color:#fff
    style D fill:#27ae60,color:#fff
    style E fill:#e67e22,color:#fff
    style F fill:#e67e22,color:#fff
    style G fill:#8e44ad,color:#fff
    style H fill:#8e44ad,color:#fff

Detailed Steps

1. Set up: Place the conical flask on the white paper so the cross is clearly visible through the bottom of the flask.

2. Measure solutions: Use a measuring cylinder to add the required volume of sodium thiosulfate solution to the flask. Add distilled water to make the total volume the same each time (e.g. total = 50 cm³).

3. Measure acid: In a separate measuring cylinder, measure 5 cm³ of dilute hydrochloric acid (1 mol/dm³).

4. Start the reaction: Pour the HCl into the flask and immediately start the stopwatch. Swirl once to mix.

5. Observe: Look down through the solution from directly above the flask at the cross beneath.

6. Stop timing: When the cross can no longer be seen through the cloudy solution, stop the stopwatch and record the time.

7. Wash and repeat: Rinse the flask thoroughly. Repeat using a different volume of sodium thiosulfate (and adjust the water to keep total volume constant).

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Varying the Concentration

To change the concentration of sodium thiosulfate while keeping the total volume the same, mix different proportions of sodium thiosulfate and water:

Experiment	Na₂S₂O₃ (cm³)	Water (cm³)	Total volume (cm³)	Relative concentration
1	50	0	50	Highest
2	40	10	50	High
3	30	20	50	Medium
4	20	30	50	Low
5	10	40	50	Lowest

Exam Tip: The total volume must remain constant — otherwise you change both concentration and the amount of reactant, making it an unfair test.

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Variables

Variable	Type	Detail
Concentration of Na₂S₂O₃	Independent (you change this)	Varied by dilution with water
Time for cross to disappear	Dependent (you measure this)	Recorded with a stopwatch
Volume of HCl	Control	Same volume and concentration each time
Total volume of solution	Control	Always the same (e.g. 50 cm³ Na₂S₂O₃ + water, plus 5 cm³ HCl)
Temperature	Control	Carried out at room temperature
Size and boldness of the cross	Control	Same cross used each time

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Expected Results

Concentration of Na₂S₂O₃	Time for cross to disappear (s)	1/Time (s⁻¹)
Highest	Shortest	Highest
↓	↓	↓
Lowest	Longest	Lowest

What the Data Shows

• As concentration increases, the time decreases — the reaction is faster.
• 1/time is proportional to rate: higher 1/time = faster rate.
• A graph of 1/time (y-axis) against concentration (x-axis) should give a straight line through the origin, confirming that rate is directly proportional to concentration.

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Processing Results

Calculating 1/Time

For each experiment, calculate:

$$\frac{1}{\text{time}} = \frac{1}{t}$$time1​=t1​

Worked Example

Na₂S₂O₃ volume (cm³)	Time (s)	1/Time (s⁻¹)
50	28	0.0357
40	35	0.0286
30	47	0.0213
20	71	0.0141
10	143	0.0070

Plotting 1/time against Na₂S₂O₃ volume gives a straight line through the origin.

Exam Tip: If asked to "plot a graph to show that rate is proportional to concentration", plot 1/time on the y-axis and concentration (or volume of Na₂S₂O₃) on the x-axis. A straight line through the origin proves proportionality.

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Evaluation

Strengths

• Simple experiment with minimal apparatus.
• Gives a clear trend showing the effect of concentration.

Limitations and Improvements

Limitation	Explanation	Improvement
Subjective endpoint	Different people may judge the disappearing point differently	Use a light sensor (data logger) to detect when a set amount of light is blocked — removes human judgement
Temperature not precisely controlled	Room temperature may fluctuate during the session	Use a thermostatically controlled water bath
Only one measurement per experiment	The disappearing cross gives a single time, not continuous data	Use a colorimeter or light sensor to track cloudiness over time
Sulfur dioxide produced	SO₂ is toxic and can irritate the airways	Work in a well-ventilated room or near a fume cupboard

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Risk Assessment

Hazard	Risk	Control Measure
Hydrochloric acid	Irritant to skin and eyes	Wear safety goggles; wash skin if splashed
Sodium thiosulfate	Low hazard but avoid skin contact	Wash hands after use
Sulfur dioxide gas	Toxic/irritant to airways	Use in a well-ventilated room; avoid inhaling fumes
Glassware	Cuts if broken	Handle carefully; report breakages

Exam Tip: Always mention that SO₂ is produced and state the precaution (well-ventilated area or fume cupboard). This is a common safety question.

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Summary