14.2 Temperature Scales

2026 Syllabus Objectives

By the end of these notes, you should be able to:

  1. Understand that a physical property which varies with temperature can be used to measure temperature, and state examples — including the density of a liquid, volume of a gas at constant pressure, resistance of a metal, and e.m.f. of a thermocouple.
  2. Understand that the thermodynamic temperature scale does not depend on the properties of any particular substance.
  3. Convert temperatures between kelvin and degrees Celsius using T/K = θ/°C + 273.15.
  4. Understand that the lowest possible temperature is zero kelvin (0 K), known as absolute zero.

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1. Measuring Temperature Using Physical Properties

What is a thermometer?

A thermometer is any device used to measure temperature. Every thermometer works by using a physical property — that is, a measurable characteristic of a material — that changes in a predictable way as temperature changes. By measuring how much this property has changed, we can figure out the temperature.

Think of it like this: if you know that a rubber band gets longer every time it gets warmer, you could mark its length at two known temperatures, divide the space in between into equal steps, and use it as a thermometer. That is the basic idea behind all thermometers.

How is a thermometer calibrated?

Calibration means setting up a scale so that the thermometer gives accurate readings. A thermometer is usually calibrated at two fixed, known temperatures — most commonly:

  • 0 °C — the melting point of pure ice
  • 100 °C — the boiling point of pure water

The gap between these two points is then divided into 100 equal divisions to create the Celsius scale.

The Four Key Properties Used in Thermometers

Below are the four physical properties you need to know, each with a simple explanation of how it works as a thermometer.

(a) Density of a Liquid — The Liquid-in-Glass Thermometer
  • Density is how much mass is packed into a given volume. When a liquid gets hotter, it expands — it takes up more space, so its density decreases.
  • A liquid-in-glass thermometer (like the classic mercury thermometer) uses this idea.
  • A glass bulb at the bottom holds a large amount of liquid. As temperature rises, the liquid expands and is pushed up through a very thin glass tube called a capillary tube.
  • The higher the liquid rises, the higher the temperature.
  • The scale is marked along the side of the tube.

In simple terms: As temperature increases → the liquid expands → it rises up the tube → you read the higher temperature on the scale.

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