Thermometer

Description

A laboratory thermometer is a glass tube containing a liquid (traditionally mercury, now often an alcohol or digital sensor) that expands or contracts with temperature. The expansion moves the liquid along a calibrated scale, allowing temperature to be read.

Key features:

• Range: typically -10 to 110 degrees C (or 0 to 360 degrees C for high-temperature work)
• Resolution: usually 1 degree C graduations; some have 0.5 or 0.2 degree C
• Reading precision: to the nearest half graduation (e.g., +/- 0.5 degrees C for 1 degree C graduations)
• Bulb: the reservoir at the bottom where the liquid collects; must be fully immersed

How to use

1. Select a thermometer with an appropriate range. Do not use a -10 to 50 degree C thermometer for a reaction that might reach 80 degrees C.
2. Immerse the bulb fully in the liquid being measured. If the bulb is partially in air, the reading is a mixture of air and liquid temperatures.
3. Wait for the reading to stabilise (typically 30-60 seconds). The thermometer needs time to reach thermal equilibrium with the liquid.
4. Read at eye level, looking directly at the scale, to avoid parallax error.
5. Record to the nearest half graduation (e.g., 24.5 degrees C if the reading falls halfway between 24 and 25).

Uses in IGCSE Chemistry

• Energetics experiments: Measuring temperature changes when acids react with alkalis, metals react with acids, or salts dissolve
• Rate experiments: Recording the temperature of the reaction mixture to ensure the control variable is maintained
• Distillation: Monitoring the temperature at the top of the flask to identify when a pure substance is distilling (constant temperature)
• Melting/boiling point determination: Establishing the temperature at which a substance changes state

Common errors

Not immersing the bulb fully: The bulb must be surrounded by the liquid. Partial immersion gives a reading that is influenced by the air temperature.

Reading before equilibrium: The reading keeps changing for the first 30-60 seconds. Wait until it is stable.

Parallax error: Looking from above or below the scale gives an incorrect reading. Read at eye level.

Using the thermometer as a stirrer: Thermometers are fragile. Stirring with them risks breakage and mercury contamination. Use a separate glass rod.

Wrong range: A thermometer that does not cover the expected temperature range will either fail to measure (liquid does not reach the scale) or break (liquid has nowhere to expand).