Temperature - Measuring Hot and Cold
Temperature tells us how hot or cold something is. It affects weather forecasts, cooking, medicine, and industrial processes. Three temperature scales are used worldwide: Celsius, Fahrenheit, and Kelvin.
The German physicist Daniel Gabriel Fahrenheit created his scale in 1724, setting 0°F using the coldest temperature he could reliably reproduce in his lab – a mixture of ice, water, and salt (brine) – and originally setting the top of his scale close to human body temperature. Just eighteen years later, the Swedish astronomer Anders Celsius proposed a simpler scale based on the freezing and boiling points of pure water, though curiously his original 1742 version had the numbers reversed, with 0° as boiling and 100° as freezing, before being flipped to the version used today. The Kelvin scale takes its name from the Scots-Irish physicist William Thomson, better known as Lord Kelvin, who realised in the 1840s that there must be an absolute lowest possible temperature, below which nothing can get colder – the absolute zero that starts the Kelvin scale at 0 K.
The Three Temperature Scales
| Scale | Symbol | Freezing point of water | Boiling point of water | Used where |
|---|---|---|---|---|
| Celsius | °C | 0 °C | 100 °C | Most of the world |
| Fahrenheit | °F | 32 °F | 212 °F | USA, some Caribbean nations |
| Kelvin | K | 273.15 K | 373.15 K | Science and engineering |
Conversion Formulas
Celsius to Fahrenheit: F = (C × 9/5) + 32
Fahrenheit to Celsius: C = (F − 32) × 5/9
Celsius to Kelvin: K = C + 273.15
Kelvin to Celsius: C = K − 273.15
Common Reference Temperatures
| Situation | Celsius | Fahrenheit |
|---|---|---|
| Absolute zero (coldest possible) | −273.15 °C | −459.67 °F |
| Water freezes | 0 °C | 32 °F |
| Room temperature | ≈ 20 °C | ≈ 68 °F |
| Body temperature | 37 °C | 98.6 °F |
| Water boils | 100 °C | 212 °F |
Worked Examples
F = (25 × 9/5) + 32 = 45 + 32 = 77 °F.
C = (98.6 − 32) × 5/9 = 66.6 × 5/9 = 333/9 ≈ 37 °C.
C = 300 − 273.15 = 26.85 °C.
Change = 12 − (−5) = 12 + 5 = 17 °C drop.
Key Takeaways
- Celsius: water freezes at 0 °C, boils at 100 °C. Used in everyday life worldwide.
- Fahrenheit: water freezes at 32 °F, boils at 212 °F. Common in the USA.
- Kelvin: starts at absolute zero (0 K = −273.15 °C). Used in science.
- C to F: multiply by 9/5 then add 32. F to C: subtract 32 then multiply by 5/9.