Temperature

Syllabus tag: Kenya CBC | Grade 7 Mathematics | Strand 3.0 Measurements | Sub-Strand 3.6 Temperature (6 lessons)

Lesson objectives

By the end of this topic, you will be able to:

• Describe and compare temperature conditions using warm, hot, cold, hotter, warmer and colder
• Identify units of measuring temperature as degrees Celsius and Kelvin
• Convert temperature from degrees Celsius to Kelvin and vice versa
• Work out temperature in degrees Celsius and Kelvin in real-life situations

Temperature

Temperature measures how hot or cold something is. This topic names the units and converts between them.

a) Describing temperature

Everyday words compare rather than measure. Something is warm, hot or cold, and one thing is warmer, hotter or colder than another.

These words are relative. A day that feels hot in Nyahururu might feel mild in Mombasa.

Numbers remove the argument, which is why we measure.

b) The units

Degrees Celsius (°C) is the everyday unit. Water freezes at 0 °C and boils at 100 °C.

Kelvin (K) is the scientific unit. It starts at absolute zero, the coldest anything can be, which is −273 °C.

Kelvin has no negative values, because nothing can be colder than absolute zero.

Note that Kelvin is written without the degree sign. It is 300 K, not 300 °K.

c) Converting between them

To go from Celsius to Kelvin, add 273.

To go from Kelvin to Celsius, subtract 273.

The size of one degree is the same on both scales. Only the starting point differs, which is why the conversion is an addition rather than a multiplication.

d) Useful fixed points

Situation	°C	K
Absolute zero	−273	0
Water freezes	0	273
Room temperature	25	298
Body temperature	37	310
Water boils	100	373

Learning two or three of these gives you a check on any conversion.

e) Reading a thermometer

Check the scale before reading. Not every thermometer counts in ones.

Read at eye level, straight on. Reading from above or below gives a value that is too high or too low.

Below zero, the numbers count downward. A reading of −5 °C is colder than −2 °C.

f) Where this is used

A nurse checks a fever against 37 °C. A farmer watches for frost. A food seller keeps a fridge below 5 °C. A weather report gives daily highs and lows.

Words to know

• Degrees Celsius (°C) — the everyday temperature scale, where water freezes at 0°C and boils at 100°C.
• Kelvin (K) — the scientific temperature scale, starting at absolute zero, where water freezes at 273 K.
• Absolute zero — the coldest temperature theoretically possible, defined as 0 K.

:::checkpoint Check yourself

1. What is the freezing point of water in °C and in K?
2. Convert 40 °C into Kelvin.
3. Convert 250 K into degrees Celsius.
4. Why can Kelvin never be negative? :::

Bridge to practice

Try the exercises below — on describing, comparing and converting temperature between Celsius and Kelvin.