Measuring Temperature with Thermometers
A thermometer measures temperature, and laboratory thermometers do this by using the expansion of a liquid in a narrow tube or by using an electrical sensor. In brief: to measure the temperature of a liquid, place the bulb in the middle of the liquid without touching the container, wait until the reading stops changing, and read the scale at eye level; water freezes at 0 °C and boils at 100 °C at normal atmospheric pressure.
Temperature measurements are needed for melting points, boiling points, reaction rates, calorimetry and gas laws. A reading is only reliable if the right instrument is chosen, used correctly and, when needed, calibrated. This article describes the types of thermometers, how to take a reading, how to calibrate a thermometer with fixed points, works through conversions and corrections and explains the history of the Celsius, Fahrenheit and Kelvin scales.
What types of thermometer are used in chemistry?
| Type | How it works | Typical range | Notes |
|---|---|---|---|
| Alcohol (dyed liquid) in glass | Liquid expands up a capillary | about −100 to 100 °C or more depending on the liquid | Safe, common in schools |
| Mercury in glass | Mercury expands up a capillary | −39 to about 350 °C | Accurate but toxic if broken; largely phased out |
| Digital thermistor probe | Electrical resistance changes with temperature | about −50 to 150 °C | Fast, easy to read |
| Thermocouple | Voltage from two joined metals | Up to over 1,000 °C | Furnaces, flames |
| Infrared thermometer | Measures emitted infrared radiation | Depends on model | Surface temperature without contact |
How do you use a laboratory thermometer correctly?
- Check that the liquid column is continuous and not broken into pieces.
- Hold or clamp the thermometer so the bulb is fully immersed in the liquid, in the middle of it, and not touching the glass bottom or the sides of the container.
- Stir gently with a stirring rod, not with the thermometer.
- Wait for 30 seconds or until the reading is steady.
- Read the scale at eye level, and estimate to half of the smallest division. If the divisions are 1 °C, record to 0.5 °C.
- Do not use a thermometer above its maximum temperature, because the liquid may burst the bulb.
How do you calibrate a thermometer?
Calibration checks readings against known fixed points. The ice point is a mixture of crushed ice and pure water, which is at 0 °C. The steam point is the temperature of pure water boiling at standard atmospheric pressure, 100 °C. If the thermometer reads differently at these points, a correction can be applied. Because the boiling point depends on pressure, the steam point is reliable only when the pressure is known.
What are the temperature scales?
| From | To | Formula |
|---|---|---|
| Celsius | Kelvin | K = °C + 273.15 |
| Kelvin | Celsius | °C = K − 273.15 |
| Celsius | Fahrenheit | °F = °C × 9/5 + 32 |
| Fahrenheit | Celsius | °C = (°F − 32) × 5/9 |
Worked example: conversion and calibration
- Celsius to kelvin. 37.0 °C + 273.15 = 310.15 K.
- Fahrenheit to Celsius. A recipe says 350 °F. (350 − 32) × 5/9 = 318 × 0.5556 = 176.7 °C, about 177 °C.
- Calibration data. A thermometer reads 1.5 °C in ice water and 99.0 °C in boiling water at sea level. The true interval of 100.0 °C is shown as 99.0 − 1.5 = 97.5 °C on the scale.
- Correction. The thermometer reads 50.0 °C in a sample. The true temperature is (50.0 − 1.5) × 100.0 ÷ 97.5 = 48.5 × 1.0256 = 49.7 °C.
- Temperature change. A solution warms from 21.5 °C to 27.0 °C. ΔT = 27.0 − 21.5 = 5.5 °C, which is also 5.5 K.
How does pressure change the boiling point of water?
The boiling point is the temperature at which the vapor pressure of the liquid equals the surrounding pressure. At lower pressures, as at altitude, water boils at a lower temperature.
| Altitude (m) | Approximate pressure (kPa) | Boiling point (°C) |
|---|---|---|
| 0 (sea level) | 101 | 100 |
| 1,000 | 90 | 97 |
| 2,000 | 79 | 93 |
| 3,000 | 70 | 90 |
| 5,000 | 54 | 83 |
Values are approximate and vary with the weather. This is why a calibration at the steam point must correct for the local pressure.
How did thermometers develop?
Galileo and others made early instruments called thermoscopes in the early 1600s, which showed changes but did not have a scale. In 1714 Daniel Gabriel Fahrenheit built mercury thermometers with a reproducible scale. Anders Celsius published his scale in 1742, with 100 degrees between the boiling and freezing points of water, and the scale was inverted soon after his death to its present form. William Thomson (Lord Kelvin) proposed an absolute scale in 1848. Digital sensors came into wide use in the late 20th century.
Which mistakes are common?
Mistake 1: Letting the bulb touch the container
The wall may be hotter or colder than the liquid. Keep the bulb in the liquid.
Mistake 2: Reading too soon
The thermometer needs time to reach the temperature of the sample.
Mistake 3: Using the thermometer as a stirrer
It can break. Use a stirring rod.
Mistake 4: Forgetting to convert to kelvin in gas calculations
Gas laws require absolute temperature.