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AS Level · Biology 3 (IAL) WBI13

Making Solutions: Concentration, Dilution & Serial Dilutions

Revise Making Solutions: Concentration, Dilution & Serial Dilutions for Biology 3 (IAL) WBI13 (AS Level) — revision notes, 2 practice questions and instant AI marking.

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Edexcel IAL Biology  •  Unit 3: Practical Skills in Biology I

Making Solutions: Concentration, Dilution & Serial Dilutions

🧪 Big idea: Half the Core Practicals depend on a row of solutions whose concentrations you chose. Getting those concentrations right — and being able to show the arithmetic — is a skill examined in its own right, not just a step on the way to the real experiment.

Summary — What This Topic Covers

  • Expressing concentration in mol dm⁻³ and g dm⁻³, and converting between them
  • Using C₁V₁ = C₂V₂ to make a solution of any concentration
  • Making a serial dilution and why the steps are equal ratios
  • Choosing a sensible range of concentrations for an investigation
  • Where the uncertainty in a dilution actually comes from

1. Expressing Concentration

Key Terms
mol dm⁻³ — moles of solute per cubic decimetre of solution.
g dm⁻³ — grams of solute per cubic decimetre.
1 dm³ = 1000 cm³ = 1 litre.
g dm⁻³ = mol dm⁻³ × M r mol dm⁻³ = g dm⁻³ ÷ M r 0.5 mol dm⁻³ glucose (Mr 180) = 0.5 × 180 = 90 g dm⁻³
Common mistake
Working in cm³ and forgetting the factor of 1000. If you dissolve 9 g in 100 cm³, that is 90 g dm⁻³, not 9.

2. Simple Dilution — C₁V₁ = C₂V₂

The formula
C₁V₁ = C₂V₂, where C₁ and V₁ are the concentration and volume of the stock you take, and C₂ and V₂ those of the diluted solution you end up with.

Worked example. Make 25 cm³ of 0.2 mol dm⁻³ sucrose from a 1.0 mol dm⁻³ stock.

C₁ = 1.0 V₁ = ? C₂ = 0.2 V₂ = 25 V₁ = (C₂ × V₂) ÷ C₁ = (0.2 × 25) ÷ 1.0 = 5 cm³ of stock then make up to 25 cm³ → add 20 cm³ of distilled water
The wording that costs marks
"Add 20 cm³ of water" and "make up to 25 cm³ with water" are the same here only because the volumes happen to add. Always write make up to the final volume — that is what you actually do, and it is what the mark scheme wants.

3. Serial Dilution

Key Term
A serial dilution is a repeated dilution by the same factor, each solution made from the one before. It produces a set of concentrations that fall in equal ratios — 1, 0.1, 0.01, 0.001 — rather than equal steps.
TENFOLD SERIAL DILUTION from 1.0 mol dm⁻³ tube 1: 1 cm³ stock + 9 cm³ water → 0.1 mol dm⁻³ tube 2: 1 cm³ tube 1 + 9 cm³ water → 0.01 mol dm⁻³ tube 3: 1 cm³ tube 2 + 9 cm³ water → 0.001 mol dm⁻³ each step dilutes by 1 in 10 — mix thoroughly before taking the next 1 cm³
  • Use a fresh pipette for each transfer, or you carry solute forward
  • Mix thoroughly before taking the next sample — an unmixed tube makes every later tube wrong
  • Errors compound: a mistake in tube 1 is carried into every tube after it
Why bother
Serial dilution is the only practical way to reach very low concentrations. Measuring 0.001 cm³ of stock directly is impossible with school apparatus; three tenfold steps get you there with volumes you can actually measure.

4. Choosing a Sensible Range

A common planning question asks you to choose the concentrations. Two rules make the answer easy to defend.

  • Use at least five different concentrations — fewer than five cannot show the shape of a curve
  • Space them to cover the range where the effect actually changes, including a 0 concentration control (distilled water)
Easy marks

5. Where the Uncertainty Comes From

Practice Questions

Practice Question 1
Practice Question 2
Practice Question 3
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