5.3.2Tools
Using Units, Symbols and Numerical Values in Biology — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 8 of 10
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5SI Units and Prefixes
Base units
- Base units Biology uses the metre (m) for length, kilogram (kg) for mass, second (s) for time, kelvin (K) for temperature and mole (mol) for amount.
Derived units
- Derived units Units such as cm³ for volume or g dm⁻³ for concentration are built from the base units.
Prefixes
- Prefixes Milli (10⁻³), micro (10⁻⁶), nano (10⁻⁹), kilo (10³) and mega (10⁶) change the size of a unit by a power of ten.
Temperature
- Temperature Biological work generally uses degrees Celsius (°C), though calculations on gases require kelvin (K).
| Prefix | Symbol | Factor |
|---|---|---|
| kilo | k | 10³ |
| milli | m | 10⁻³ |
| micro | µ | 10⁻⁶ |
| nano | n | 10⁻⁹ |
Part 2 of 5Conventions for Writing Data
Conventions for Writing Data
Negative indices
- Negative indices Compound units are written with negative indices, such as cm³ min⁻¹ or mol dm⁻³, rather than with a slash.
Table headings
- Table headings The unit is written once in the column heading, not after every value, for example 'Time / s'.
Significant figures
- Significant figures The number of significant figures in a calculated result should not exceed that of the least precise value used.
Decimal places
- Decimal places All the values in one column should have the same number of decimal places, matching the resolution of the instrument.
Symbols
- Symbols Symbols for quantities are in italics, such as t for time, while unit symbols are never pluralised, so 5 s, not 5 ss.
Part 3 of 5Converting Between Units
1
Write the value and the unit, and identify the unit you need to convert into.
2
Find the factor between the units, for example 1 mm = 1000 µm or 1 dm³ = 1000 cm³.
3
To change to a smaller unit multiply by the factor, and to a larger unit divide by it.
4
Check that the new value is sensible, since a smaller unit always gives a larger number.
5
For compound units convert each part separately, such as minutes to seconds in a rate.
Part 4 of 5Worked Example
Question
- A cell is 0.25 mm long.
- Give this length in µm.
- Then express a rate of 15 cm³ in 3 minutes in cm³ s⁻¹.
- 1 mm = 1000 µm, so 0.25 mm = 0.25 × 1000 = 250 µm.
- Rate in cm³ min⁻¹ = 15 ÷ 3 = 5 cm³ min⁻¹.
- Convert minutes to seconds5 ÷ 60 = 0.083 cm³ s⁻¹.
Answer The cell is 250 µm long, and the rate is 5 cm³ min⁻¹ (about 0.083 cm³ s⁻¹).
Part 5 of 5Exam Checklist & Tips
5.3.2 – Key Points for the Exam
- SI The International System of Units, the standard for scientific measurement.
- Prefix A symbol placed before a unit to show a multiple or fraction of ten.
- Significant figures The meaningful digits in a number, starting from the first non-zero digit.
- Derived unit A unit made by combining base units.
Common Mistakes
- Writing units as m/s instead of m s⁻¹ when the convention is negative indices.
- Repeating the unit after every value in a data table.
- Reporting an answer with more significant figures than the data justify.
- Dividing instead of multiplying when converting to a smaller unit.
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