Making Solutions: Concentration, Dilution & Serial Dilutions
Revise Making Solutions: Concentration, Dilution & Serial Dilutions for Biology 3 (IAL) WBI13 (AS Level) — revision notes and instant AI marking. Free to start.
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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
Naming the control — "distilled water, 0 mol dm⁻³" — is a mark on its own in almost every planning question, and it is the one students most often forget.
5. Where the Uncertainty Comes From
Every dilution is made from two measured volumes, so each carries the uncertainty of both.
5 cm³ stock in a syringe ± 0.05 cm³ → 1.0 %
20 cm³ water in a cylinder ± 0.5 cm³ → 2.5 %
────────────────────────────────────────────────
total uncertainty in the diluted concentration ≈ 3.5 %
What mark schemes look for
When asked how to make a dilution more reliable, the answer is finer apparatus for the volumes — a graduated pipette rather than a measuring cylinder — not "be more careful".
Practice Questions
Practice Question 1
Describe how you would make 20 cm³ of 0.4 mol dm⁻³ glucose solution from a 2.0 mol dm⁻³ stock.
Practice Question 2
Explain why a serial dilution is used to produce very low concentrations rather than diluting the stock directly.
Practice Question 3
A student makes a serial dilution but uses the same pipette for every transfer without rinsing. Explain the effect on their results.
Practice Question 4
A student dissolves 4.5 g of glucose (Mr = 180) in water and makes it up to 250 cm³. Calculate the concentration in mol dm⁻³.
What to Memorise
1 dm³ = 1000 cm³g dm⁻³ = mol dm⁻³ × MrC₁V₁ = C₂V₂Make up TO the final volumeSerial = equal ratios, not equal stepsFresh pipette every transferAlways include a 0 controlAt least five concentrations
Concepts Checklist
Exam Tips
What mark schemes look for
Write "make up to 25 cm³ with distilled water", not "add 20 cm³ of water". The first describes what you do; the second is only sometimes equivalent.
The trap
Volumes in cm³ and concentrations in dm⁻³. Convert before you calculate, not after.
Easy marks
The distilled-water control at 0 mol dm⁻³ is a free mark in planning questions, and the one most often left out.
Worth remembering
If a question asks why a serial dilution was used rather than a single dilution, the answer is always about being able to measure the volumes accurately.