The temperature scales that we use today were designed for simplicity and easy reproduction. The Celsius temperature scale is used throughout the world and is a good example of how the effective communication of data between individuals and countries is dependent on an agreed system of units.

For historical and cultural reasons, there a few countries (notably the USA) in which the Fahrenheit temperature scale is still in use. The Kelvin (absolute) temperature scale is an adaptation of the Celsius scale with its zero at −273 °C.

- How was the range of the Celsius scale chosen to be from 0 °C to 100 °C?
- How the Kelvin scale is adapted from the Centigrade scale?
- important temperatures on the Celsius and Kelvin scales
- How to make calculations where data of both Celsius and Kelvin are present as input?
- How to solve numerical problems related to temperature – show with an example

## How was the range of the Celsius scale chosen to be from 0 °C to 100 °C?

On the Celsius scale (°C), sometimes called the centigrade scale, 0 °C is defined as the temperature at which pure water forms ice (at normal atmospheric pressure), and 100 °C is defined as the temperature at which pure water boils (at normal atmospheric pressure). It is important to realize that this temperature scale was devised for convenience – that is, these values were chosen, they were not discovered.

In particular, 0 °C is definitely not a zero of temperature, nor a zero of energy. It has no significance other than being the melting point of ice. (For example, 10 °C cannot be considered to be twice as hot’ as 5 °C.)

## How the Kelvin scale is adapted from the Centigrade scale?

After it was predicted that almost all molecular motion stops at −273 °C, it made sense to make this the true zero of temperature. This temperature is commonly called *absolute zero*. The Kelvin (absolute) temperature scale is an adaptation of the Celsius scale with its zero at −273 °C. (A more precise value is −273.15 °C.) On this scale, the unit is kelvin, K (not °K).

Changes in temperature of 1 °C and 1 K were chosen to be identical, which makes conversion from one scale to the other very straightforward:

T/K = θ/°C + 273

In the equation above, note that the use of the symbol T for temperature implies the Kelvin scale and the symbol θ implies the Celsius scale.

## important temperatures on the Celsius and Kelvin scales

Table 1 compares some important temperatures on the two scales.

Temperature | °C | K |
---|---|---|

Absolute zero | – 273 | 0 |

Melting point of water | 0 | 273 |

Body temperature | 37 | 310 |

Boiling point of water | 100 | 373 |

## How to make calculations where data of both Celsius and Kelvin are present as input?

When making calculations involving temperature changes, either degrees Celsius or kelvins may be used, but it is important to remember that when dealing with calculations involving just one temperature, kelvins must be used.

## How to solve numerical problems related to temperature – show with an example

- The normal freezing point of mercury is −39 °C. What is this temperature in kelvin?

T/K = θ/°C + 273

T = −39 + 273

T = 234 K