Using Appropriate Units
- The International System of Units (SI) is also called the metric system
- This is the international standard for measurement
- There are several SI base units that are used in science
SI Base Units Table
| Quantity | SI base unit | Symbol |
| length | metre | m |
| mass | kilogram | kg |
| time | second | s |
| temperature | Kelvin | K |
| amount of substance | mole | mol |
| current | Ampere | A |
| luminous intensity | candela | cd |
- Measurements of physical quantities can require very large and very small values, for example:
- The diameter of an atom is about 10–10 m or 0.0000000001 m
- One mole of a substance contains 6.02 × 1023 or 602 000 000 000 000 000 000 000 particles
- Powers of ten are numbers that can be achieved by multiplying 10 times itself
- These come under two categories of units:
- Multiples e.g. 102, 103
- Sub-multiples e.g. 10-1, 10-2
- Each power of ten is defined by a prefix, the most common ones used in biology are listed in the table below
Table of common prefixes in biology
| Prefix | Abbreviation | Power of ten |
| kilo- | k | 103 |
| centi- | c | 10–2 |
| milli- | m | 10–3 |
| micro- | μ | 10–6 |
| nano- | n | 10–9 |
- It essential that the correct scientific measurements are used when discussing biological experiments
- Ensure that the correct symbols are used in conjunction with the unit of measurement
- E.g. m3 for cubic metres
Units of Measurement Table
| Measurement | Base unit | Symbol | Units used |
| Length | Metre | m | 1000 m = 1 km 0.01 m = 1 cm 0.001 m = 1 mm 0.000001 m = 1 µm |
| Volume | Cubic metre | m3 | 109 m3 = 1 km3 0.000001 m3 = 1 cm3 10-9 m3 = 1 mm3 10-18 m3 = 1 µm3 |
| Volume | Cubic decimetre | dm3 | 0.001 dm3 = 1 cm3 |
| Area | Square metre | m2 | 10 000 m2 = 1 ha 0.0001 m2 = 1 cm2 |
| Mass | Kilogram | kg | 1000 kg = 1 tonne 0.001 kg = 1 g 0.000001 kg = 1 mg 10-9 kg = 1 µg |
| Time | Second | s | 60 s = 1 min 60 min = 1 hour |
| Pressure | pascal | Pa | 1000 Pa = 1 kPa |
| Energy | joule | J | 1000 J = 1 kJ |
| Temperature | degree Celcius | °C | |
| Amount of substance | mole | mol | 0.001 mol = 1 millimole |
- cm3 is the same as millilitre (ml)
- dm3 is the same as litre (l)
Exam Tip
Be careful when using the word "amount" in your answers. "Amount" has a very specific meaning in science - "mole". Instead refer to the mass, volume or concentration of a substance!
Significant figures
- Significant figures must be used when dealing with quantitative data
- Significant figures are the digits in a number that are reliable and absolutely necessary to indicate the quantity of that number
- There are some important rules to remember for significant figures
- All non-zero digits are significant
- Zeros between non-zero digits are significant
- 4107 (4.s.f.)
- 29.009 (5.s.f)
- Zeros that come before all non-zero digits are not significant
- 0.00079 (2.s.f.)
- 0.48 (2.s.f.)
- Zeros after non-zero digits within a number without decimals are not significant
- 57,000 (2.s.f)
- 640 (2.s.f)
- Zeros after non-zero digits within a number with decimals are significant
- 689.0023 (7.s.f)
- When rounding to a certain number of significant figures:
- Identify the significant figures within the number using the rules above
- Count from the first significant figure to the specified number
- Use the next number as the ‘rounder decider’
- If the decider is 5 or greater, increase the previous value by 1
Worked example
Write 1.0478 to 3 significant figures.
Answer:
Step 1: Identify the significant figures
They are all significant figures
Step 2: Count to the specified number (3rd s.f.)
1.0478
Step 3: Round up or down
1.05
Exam Tip
An exam question may sometimes specify how many significant figures the answer should be, make sure you keep an eye out for this!