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It’s easier to write a good lab report once you’ve seen one. Below is a complete example report for a classic school investigation: how temperature affects the rate of the reaction between sodium thiosulfate and hydrochloric acid. After each section, a short annotation (in italics, marked ✎) explains what makes it effective.
For the general structure and rules, see how to write a chemistry lab report.
Title
The effect of temperature on the rate of reaction between sodium thiosulfate and hydrochloric acid
✎ Specific: it names the independent variable (temperature), the dependent variable (rate) and the reaction.
Aim
To investigate how the temperature of sodium thiosulfate solution affects the rate of its reaction with hydrochloric acid.
Hypothesis
As the temperature increases, the rate of reaction will increase. Particles will have more kinetic energy, so they will collide more often, and a greater proportion of collisions will have energy equal to or greater than the activation energy.
✎ A testable prediction with a scientific reason based on collision theory, not just “it will go faster”.
Background
Sodium thiosulfate reacts with hydrochloric acid to produce a fine precipitate of sulfur, which makes the solution cloudy:
Na₂S₂O₃(aq) + 2HCl(aq) → 2NaCl(aq) + SO₂(g) + S(s) + H₂O(l)
The time taken for the solution to become cloudy enough to hide a cross drawn under the flask can be measured. A shorter time means a faster reaction, so 1/time can be used as a measure of the rate.
✎ Short, relevant, and it explains how “rate” will be measured.
Variables
| Type | Variable | How it’s controlled or measured |
|---|---|---|
| Independent | temperature of the thiosulfate solution | 20, 30, 40, 50 and 60 °C, using a water bath |
| Dependent | time for the cross to disappear | stopwatch, to the nearest second |
| Control | concentration of thiosulfate | 0.15 mol/dm³ throughout |
| Control | volume of thiosulfate | 50.0 cm³, measuring cylinder |
| Control | concentration and volume of acid | 5.0 cm³ of 2.0 mol/dm³ HCl |
| Control | cross and viewing position | same printed cross; same person viewing from above |
✎ Clearly separating independent, dependent and control variables shows the investigation is a fair test. See independent, dependent and control variables.
Risk assessment
| Hazard | Risk | Control |
|---|---|---|
| Sulfur dioxide gas (toxic, respiratory irritant) | inhaling fumes, especially for people with asthma | small volumes; good ventilation; dispose of mixture promptly into a beaker of sodium carbonate solution in a fume cupboard |
| 2.0 mol/dm³ hydrochloric acid (irritant) | eye or skin contact | eye protection; measuring cylinder; wipe up spills |
| Hot water bath | scalds | water no hotter than 60 °C; take care when handling flasks |
✎ Specific hazards (including a product, SO₂) with specific controls.
Equipment
- 100 cm³ conical flask
- 50 cm³ and 10 cm³ measuring cylinders
- Water bath (thermostatically controlled)
- Thermometer (±0.5 °C)
- Stopwatch (±0.01 s display)
- Paper with a printed black cross
- 0.15 mol/dm³ sodium thiosulfate solution; 2.0 mol/dm³ hydrochloric acid
Method
- 50.0 cm³ of 0.15 mol/dm³ sodium thiosulfate solution was measured with a measuring cylinder into a conical flask.
- The flask was placed in a water bath set to 20 °C until the solution reached 20 °C (checked with the thermometer).
- The flask was placed on the printed cross.
- 5.0 cm³ of 2.0 mol/dm³ hydrochloric acid was measured with a 10 cm³ measuring cylinder, added to the flask, and the stopwatch was started immediately. The flask was swirled once.
- The cross was viewed from directly above. The stopwatch was stopped when the cross could no longer be seen.
- The temperature of the mixture was measured again at the end, and the average of start and end temperatures was recorded.
- The mixture was poured into a beaker of sodium carbonate solution in the fume cupboard.
- Steps 1–7 were repeated at 30, 40, 50 and 60 °C.
- The whole experiment was repeated twice more, giving three results at each temperature.
✎ Detailed enough to repeat: volumes, concentrations, equipment and exactly when timing starts and stops.
Results
| Target temperature / °C | Mean actual temperature / °C | Time 1 / s | Time 2 / s | Time 3 / s | Mean time / s | Rate (1/time) / s⁻¹ |
|---|---|---|---|---|---|---|
| 20 | 20.5 | 121 | 118 | 124 | 121 | 0.00826 |
| 30 | 30.0 | 67 | 70 | 66 | 68 | 0.0147 |
| 40 | 39.5 | 38 | 41 | 37 | 39 | 0.0256 |
| 50 | 49.0 | 23 | 22 | 31 | 22.5 | 0.0444 |
| 60 | 58.5 | 14 | 13 | 15 | 14 | 0.0714 |
The result of 31 s at 50 °C was identified as anomalous (much higher than the other two repeats) and excluded from the mean. It’s likely the acid was added before the solution had fully reached temperature.
Observations: the solution turned from colourless to pale yellow and cloudy; a faint smell of sulfur dioxide was noticed.
✎ Units in headings; consistent precision; anomaly identified with a reason; qualitative observations included.
Analysis
A graph of rate (1/time) against temperature was plotted. The points lie on a smooth curve that gets steeper as temperature increases.
Pattern: as the temperature increased from about 20 °C to about 60 °C, the time for the cross to disappear decreased from 121 s to 14 s, and the rate increased from 0.00826 s⁻¹ to 0.0714 s⁻¹, an increase of about 8.6 times.
Every 10 °C rise roughly doubled the rate: from 20 to 30 °C the rate increased by a factor of 1.8; from 30 to 40 °C by 1.7; from 40 to 50 °C by 1.7; from 50 to 60 °C by 1.6.
Explanation: at higher temperatures, particles have more kinetic energy. They collide more frequently and, more importantly, a much larger fraction of collisions have at least the activation energy (as shown by the Maxwell–Boltzmann distribution), so more collisions lead to reaction.
✎ Describes the pattern with numbers, quantifies it, and explains it using the correct scientific model.
Evaluation
Uncertainty:
- Temperature: ±0.5 °C per reading. Because the temperature fell slightly during each run (typically by 1 °C at 60 °C), the actual reaction temperature is uncertain by about ±1 °C.
- Time: the stopwatch reads to 0.01 s, but judging when the cross disappears introduces a reaction-time and judgement uncertainty of about ±1–2 s. For the 60 °C run (14 s), that’s about ±10%, far larger than at 20 °C (121 s, about ±1.5%). See calculating percentage uncertainty.
Sources of error:
- Judging the end point (random): the moment the cross “disappears” is subjective. This caused the scatter in repeats. Improvement: use a light sensor and data logger to measure the cloudiness objectively.
- Temperature drop during the reaction (systematic, larger at higher temperatures): the mixture cooled once removed from the bath and when cold acid was added, so reactions happened slightly below the target temperature. Improvement: warm the acid to the same temperature too, and keep the flask in the water bath during the reaction.
- Measuring cylinders (random): about ±0.5 cm³ on 50 cm³ is 1%, and ±0.1 cm³ on 5.0 cm³ is 2%. Improvement: use a burette or pipette for the acid.
Reliability: repeats at each temperature were close (within about 3–6 s), apart from the one anomaly, which suggests the results are repeatable.
✎ Quantifies uncertainty, distinguishes random from systematic errors, states their effect, and gives specific improvements with reasons.
Conclusion
The rate of reaction between sodium thiosulfate and hydrochloric acid increased as temperature increased, roughly doubling for every 10 °C rise between 20 and 60 °C. This supports the hypothesis and is consistent with collision theory.
✎ Answers the aim directly, quotes the key finding, and links back to the hypothesis without claiming “proof”.
What made this report strong
- The title, aim and hypothesis are specific and linked.
- Variables are clearly identified and controlled.
- The method is detailed enough to repeat.
- Results are well presented, including repeats and a justified anomaly.
- Analysis uses numbers and correct theory.
- Evaluation is quantitative and specific.
- The conclusion answers the aim.
Key takeaways
- A strong report connects every section: aim → variables → method → results → analysis → conclusion.
- Tables need units in the headings and consistent precision; anomalies need a reason.
- Analysis should describe patterns with numbers and explain them with theory.
- Evaluation should quantify uncertainty and suggest specific improvements.
- Use this example alongside the guide to drawing good graphs.
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