2. Temperature Scales and Conversions

Temperature is measured using standardised scales, each of which fixes two reference points — usually the freezing point and boiling point of water at standard atmospheric pressure — and divides the interval between them into a fixed number of equal parts called degrees.

2.1 The Celsius Scale

Devised by the Swedish astronomer Anders Celsius in 1742, this scale (symbol °C) fixes the freezing point of water at 0°C and the boiling point of water at 100°C under standard atmospheric pressure (1 atmosphere), with the interval divided into 100 equal parts. It is the scale used in everyday life in most countries, including India, and in scientific work worldwide alongside Kelvin.

2.2 The Fahrenheit Scale

Proposed by the German physicist Daniel Gabriel Fahrenheit in 1724, this scale (symbol °F) sets the freezing point of water at 32°F and the boiling point at 212°F, giving an interval of 180 degrees between the two reference points. Originally Fahrenheit calibrated his scale using a mixture of ice, water, and ammonium chloride (a frigorific mixture) to define 0°F, and used the approximate temperature of the human body for another reference point. It remains the everyday scale used in the United States.

2.3 The Kelvin (Absolute) Scale

Devised by Lord Kelvin (William Thomson), this is the SI unit of temperature (symbol K, note: no degree sign is used with Kelvin). It is called the absolute temperature scale because its zero point, 0 K (called absolute zero), represents the theoretical temperature at which molecular motion would be at its minimum possible value and a substance would possess minimum internal energy. Absolute zero corresponds to −273.15°C (commonly approximated as −273°C). The size of one kelvin is identical to the size of one degree Celsius, so the Kelvin scale is simply the Celsius scale shifted so that its zero coincides with absolute zero. On the Kelvin scale, water freezes at 273.15 K (≈273 K) and boils at 373.15 K (≈373 K). Absolute zero has never been physically achieved in a laboratory (the third law of thermodynamics states that it is unattainable, only approachable asymptotically), though temperatures within a fraction of a kelvin above it have been reached in cryogenics research.

2.4 Conversion Formulae

A useful fact for solving exam problems quickly: the Celsius and Fahrenheit scales read the same numerical value at −40° (that is, −40°C = −40°F), which can be verified directly from the conversion formula and is a handy checkpoint for whether a conversion has been performed correctly.

2.5 Reference Table: Corresponding Values