Using units

Standard units

Scientists use SI units. The table shows some common units:

QuantityUnit nameUnit symbol
lengthmetrem
masskilogramkg
timeseconds
temperaturedegree Celsius°C
energyjouleJ

Prefixes for units

Scientists measure a wide range of quantities, some very large and some very small. Measurements for different orders of magnitude have specific prefixes:

Prefix namePrefix symbolFactorPower
teraT1,000,000,000,000× 1012
gigaG1,000,000,000× 109
megaM1,000,000× 106
kilok1,000× 103
centic0.01× 10-2
millim0.001× 10-3
microμ0.000,001× 10-6
nanon0.000,000,001× 10-9

Chemists also use the prefix deci (d, power 10-1) when discussing volume:

  • 1 dm = 10 cm
  • 1 dm3 = 10 × 10 × 10 = 1000 cm3

Converting units

To convert between a unit with difference prefixes, multiplication or division of the right factor of ten (10, 100, 1000) is used.

Always do a sense check when multiplying or dividing and question whether the answer expected will be larger or smaller.

Interconverting units
Question

Convert 5 cm into mm.

1 cm = 10 mm so the conversion is × 10

5 × 10 = 50 mm

This is what would be expected because millimetres are smaller than centimetres.

Time is dealt with differently if it is recorded in minutes, as this is not an SI unit:

  • 1 minute = 60 s
Question

Convert 2 minutes 40 s into s.

2 minutes = 2 × 60 = 120 s

2 minutes 40 s = 120 + 40

= 160 s