Required Practical: Measuring Temperature Changes

This lesson covers the AQA GCSE Combined Science Trilogy (8464) Required Practical on investigating the variables that affect temperature changes in reacting solutions such as acid plus alkali. You need to be able to describe the method, identify variables, evaluate results, and suggest improvements.

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Purpose of the Practical

The aim is to investigate how the temperature changes when different reacting solutions are mixed. This allows you to determine whether a reaction is exothermic (temperature rises) or endothermic (temperature falls).

Typical reactions investigated:

Reaction	Type
Hydrochloric acid + sodium hydroxide (neutralisation)	Exothermic
Citric acid + sodium hydrogencarbonate	Endothermic
Dissolving ammonium nitrate in water	Endothermic
Adding a metal powder to a copper sulfate solution (displacement)	Exothermic

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Apparatus

graph TD
    A["Polystyrene Cup \n(acts as a calorimeter)"] --> B["Lid with hole \n(reduces heat loss)"]
    B --> C["Thermometer / \nTemperature Probe"]
    D["Measuring Cylinder \n(measure volumes)"] --> A
    E["Stirring Rod"] --> A

Equipment	Purpose
Polystyrene cup	Good insulator — reduces heat loss to surroundings (acts as a simple calorimeter)
Lid	Reduces heat loss by convection and evaporation
Thermometer / temperature probe	Measures the temperature before and after mixing
Measuring cylinder	Measures accurate volumes of solutions
Stirring rod	Ensures even temperature distribution throughout the mixture

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Method (Step-by-Step)

1. Measure a fixed volume of the first solution (e.g., 25 cm³ of 1 mol/dm³ HCl) using a measuring cylinder.
2. Pour the solution into the polystyrene cup.
3. Record the initial temperature of the solution ($T_1$T1​).
4. Measure a fixed volume of the second solution (e.g., 25 cm³ of 1 mol/dm³ NaOH).
5. Add the second solution to the cup, place the lid on, and stir gently.
6. Record the highest temperature reached ($T_2$T2​) for an exothermic reaction, or the lowest temperature reached for an endothermic reaction.
7. Calculate the temperature change:

$\Delta T = T_2 - T_1$ΔT=T2​−T1​

8. Repeat the experiment at least three times and calculate a mean temperature change.

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Variables

Variable	Detail
Independent variable	The variable you change — e.g., type of acid, concentration, volume, or type of metal
Dependent variable	The temperature change ($\Delta T$ΔT)
Control variables	Volume of solutions, starting temperature, concentration (if not the IV), use of lid, same equipment

Exam Tip: You must be able to identify all three types of variable for this practical. A common AQA question asks you to state the independent, dependent and control variables.

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Recording Results

Experiment	Initial Temp (°C)	Final Temp (°C)	Temperature Change (°C)
1	21.0	28.5	+7.5
2	21.0	28.0	+7.0
3	21.0	28.5	+7.5
Mean			+7.3

A positive temperature change indicates an exothermic reaction. A negative temperature change indicates an endothermic reaction.

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Worked Example

A student investigates the temperature change when 25 cm³ of 1.0 mol/dm³ HCl reacts with 25 cm³ of 1.0 mol/dm³ NaOH. The results are shown above.

Step 1: Calculate the mean temperature change:

$\Delta T_{\text{mean}} = \frac{7.5 + 7.0 + 7.5}{3} = \frac{22.0}{3} = 7.3 \text{ °C}$ΔTmean​=37.5+7.0+7.5​=322.0​=7.3 °C

Step 2: The temperature increased, so the reaction is exothermic — energy was transferred to the surroundings.

Step 3: Neutralisation ($\text{HCl} + \text{NaOH} \rightarrow \text{NaCl} + \text{H}_2\text{O}$HCl+NaOH→NaCl+H2​O) is always exothermic, which confirms the result.

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Evaluation

Sources of Error

Source of Error	Explanation	Improvement
Heat loss to surroundings	Even with a polystyrene cup, some heat escapes	Use a better-insulated calorimeter; ensure the lid is tight
Heat absorbed by the cup	The cup itself absorbs some heat	Use a thinner cup or apply a correction factor
Thermometer reading error	Human error in reading the scale	Use a digital temperature probe for greater accuracy
Incomplete mixing	Some reactants may not fully react	Stir thoroughly and allow time for the reaction to complete

How to Improve Reliability

• Repeat the experiment at least three times and calculate a mean.
• Use the same equipment each time.
• Ensure consistent technique (same stirring speed, same reading method).

How to Improve Accuracy

• Use a digital temperature probe instead of a glass thermometer.
• Use a vacuum flask calorimeter to reduce heat loss.
• Measure volumes with a burette rather than a measuring cylinder.

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Risk Assessment

Hazard	Risk	Control Measure
Hydrochloric acid	Irritant to skin and eyes	Wear safety goggles; wash skin immediately if splashed
Sodium hydroxide	Corrosive — can cause burns	Wear goggles and gloves; wash skin immediately if splashed
Ammonium nitrate	Irritant	Avoid skin contact; wash hands after use
Glassware	Cuts if broken	Handle carefully; report breakages

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Common Mistakes

Mistake	How to Avoid It
Not using a lid	Always use a lid to reduce heat loss — this affects your results
Reading the thermometer at the wrong time	Read the maximum (exothermic) or minimum (endothermic) temperature, not just any reading
Not controlling volume	Always use the same volumes of each solution
Forgetting to repeat	At least 3 repeats are needed for reliable results
Confusing temperature change sign	A positive $\Delta T$ΔT means exothermic; negative means endothermic

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Summary