Measuring Temperature

Why this skill matters

Temperature measurement is central to energetics experiments (measuring temperature changes in neutralisation, dissolution, and combustion), rates experiments (investigating the effect of temperature on reaction rate), and melting/boiling point determination. Paper 5 and Paper 6 regularly ask you to plan experiments that involve temperature measurement or to evaluate temperature data.

Method and technique

Setting up

1. Select a thermometer with an appropriate range. For most IGCSE experiments, a -10 to 110 degrees C thermometer is standard. For energetics experiments measuring small temperature changes, a thermometer with finer graduations (0.5 degree C) gives better data.
2. Check that the thermometer is reading correctly at a known temperature (ice water at 0 degrees C or boiling water at 100 degrees C) if precision matters.
3. Ensure the bulb is fully immersed in the liquid being measured. If the bulb is partially exposed to air, the reading will be inaccurate.

Taking readings

1. Immerse the thermometer bulb in the liquid. Stir gently to ensure even temperature distribution.
2. Wait for the reading to stabilise (30-60 seconds for most situations).
3. Read the thermometer at eye level, looking straight at the scale, not from above or below.
4. Record the temperature to the appropriate number of decimal places (matching the resolution of the thermometer scale).

Recording temperature changes

For energetics experiments, record:

• The initial temperature of the reactants before mixing
• The temperature at regular intervals (every 30 seconds or every minute) after mixing
• The maximum or minimum temperature reached

The temperature change (delta T) is the difference between the initial and the highest (or lowest) temperature.

Common errors and how to avoid them

Reading before equilibrium: The most common error. The thermometer needs time to reach the temperature of the liquid. If the reading is still changing, wait.

Not stirring: Without stirring, the liquid may have hot spots and cold spots. The thermometer reads only the temperature at its bulb, which may not represent the whole solution. Stir gently with a glass rod (not the thermometer, which could break).

Wrong range: Using a thermometer that only goes to 50 degrees C for a reaction that reaches 80 degrees C is not useful. Check the expected temperature range before choosing.

Removing the thermometer to read it: Taking the thermometer out of the liquid changes its temperature. Read it while it is still immersed.

Marking points examiners look for

• Thermometer bulb fully immersed in the liquid
• Wait for the reading to stabilise before recording
• Read at eye level
• Stir the solution to ensure even temperature
• Record to the resolution of the thermometer (e.g., to nearest 0.5 degrees C)
• For energetics: record initial temperature before mixing and highest/lowest temperature after