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The most commonly used measurement of temperature is the Celsius scale. The units of this scale are degrees Celsius (°C). This scale was designed to reference the freezing point (0°C) and the boiling point (100°C) of water.

There is no upper limit, but the lowest temperature possible is -273°C. At this temperature, almost all matter is solid and the vibrations of particles are incredibly small. Absolute zero is the coldest temperature possible. At absolute zero, all the particles in a substance stop moving. They have no energy left to lose, so the substance cannot get any colder.

Many scientists use the Kelvin scale instead.

Kelvin begins at absolute zero and so there are no minus numbers in the Kelvin scale, which makes calculations simpler. The unit is Kelvin (K). An increase in one degree Celsius is the same as one Kelvin.

Conversion between the scales:

• temperature in degrees Celsius (°C) = temperature in Kelvin - 273
• temperature in Kelvin = temperature in degrees Celsius (°C) + 273

To be able to measure temperature easily we require fixed points. Two common fixed points are the melting and boiling points of water. These are 0°C and 100°C respectively.

Thermometers measure temperature. The liquid (usually mercury or coloured alcohol) expands when heated which means that it rises up the glass to show a higher temperature.