Experimental design

O Level / IGCSE Chemistry · Experimental techniques and chemical analysis. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

Good experimental work starts with choosing the right apparatus. Time is measured with a stop-watch, temperature with a thermometer and mass with a balance. For the volume of a liquid, a volumetric pipette or a burette is much more accurate than a measuring cylinder. The volume of a gas is measured with a gas syringe.

You also need the words for solutions. A solute dissolves in a solvent to make a solution. A saturated solution holds the maximum amount of solute that can dissolve at that temperature.

In an acid–base titration, a pipette puts a fixed volume of one solution into a flask with a few drops of indicator. The other solution is added from a burette until the indicator just changes colour. This is the end-point.

Rules to remember

Volumetric pipette: one fixed volume (e.g. 25.0 cm³), very accurate. Burette: any volume, read to 0.05 cm³. Measuring cylinder: quick but less accurate
Gas syringe: measures the volume of a gas. Balance: mass. Stop-watch: time. Thermometer: temperature
Solvent: a substance that dissolves a solute. Solute: a substance that is dissolved in a solvent. Solution: solute(s) dissolved in a solvent
Saturated solution: contains the maximum concentration of solute dissolved in the solvent at a specified temperature
Residue: what remains after filtration, evaporation or distillation. Filtrate: the liquid or solution that passes through a filter
Methyl orange: red in acid, yellow in alkali. Thymolphthalein: colourless in acid, blue in alkali
End-point: the point where one drop from the burette makes the indicator just change colour

Common mistake

Students choose universal indicator for a titration. It changes colour gradually through many shades, so it gives no sharp end-point. Use methyl orange or thymolphthalein.

Worked example

The solubility of a salt is 40 g per 100 g of water at 25 °C. A student stirs 25 g of the salt into 50 g of water at 25 °C and then filters. What mass of residue is collected?

50 g of water is half of 100 g, so the maximum mass that dissolves = 40 ÷ 2 = 20 g.
25 g is more than 20 g, so the solution is saturated and some salt does not dissolve.
Mass of undissolved salt (the residue) = 25 − 20 = 5 g.

Answer: 5 g of residue; the filtrate is a saturated solution.

Practice questions

Try each one, then open the answer.

1. Which indicator is suitable for finding the end-point of a titration between hydrochloric acid and sodium hydroxide?

A
universal indicator
B
methyl orange
C
starch
D
limewater

Show answer

Answer: B. Methyl orange (or phenolphthalein) gives a sharp colour change at the end-point. Universal indicator changes gradually through many colours.

2. Sugar is stirred into water until it dissolves. Which term describes the water?

A
the solvent
B
the solution
C
the filtrate
D
the solute

Show answer

Answer: A. The solvent (water) dissolves the solute (sugar) to form a solution.

3. The solubility of a salt is 36 g per 100 g of water at 20 °C. A student stirs 50 g of the salt into 100 g of water at 20 °C. Which statement describes the result?

A
an unsaturated solution forms and all 50 g dissolves
B
a saturated solution forms with 36 g undissolved
C
an unsaturated solution forms with 14 g undissolved
D
a saturated solution forms and 14 g remains undissolved

Show answer

Answer: D. Only 36 g can dissolve in 100 g of water at 20 °C, so the solution becomes saturated (it holds the maximum amount of solute). The remaining 50 − 36 = 14 g stays undissolved.

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