Experimental Techniques: IGCSE Chemistry Exam Guide

Experimental techniques is the first topic in the 0620 syllabus and one of the most practical. Questions appear on Papers 2 and 4 (choosing and describing separation methods) and Paper 6 (designing and evaluating experiments). The topic rewards precise vocabulary and clear procedural descriptions. The full content is under experimental techniques.

Separation methods: choosing and describing

The exam sets two types of question. The first asks you to choose the correct method for a given mixture. The second asks you to describe the method step by step.

How to choose

Mixture	What you want	Method
Sand in water	The sand or the water	Filtration
Salt dissolved in water	The salt	Evaporation / crystallisation
Salt dissolved in water	The pure water	Simple distillation
Ethanol and water	Both, separately	Fractional distillation
Oil and water	Both, separately	Separating funnel
Dyes in ink	Identify the components	Chromatography
Copper sulfate crystals from solution	Large, pure crystals	Crystallisation (slow evaporation)

How to describe (the marks)

When the question says “describe how you would…” it expects a step-by-step method. Each step is typically worth one mark. Include:

• The name of the equipment (filter funnel, evaporating basin, condenser, etc.).
• The specific action (pour the mixture through filter paper, heat gently, collect the distillate).
• How you know the process is complete (residue remains on filter paper, crystals start to form when a glass rod is dipped in, distillate stops collecting).

Distillation detail

Simple distillation is tested more often than fractional, and the marks come from knowing the apparatus:

1. Heat the solution in a flask.
2. The liquid with the lower boiling point evaporates first.
3. The vapour passes into the condenser, where cold water flows around the outside.
4. The vapour condenses back to liquid and is collected in a beaker at the end.

The condenser requires cold water flowing in at the bottom and out at the top (counter-current). This detail is tested on Paper 6 apparatus questions.

Fractional distillation

Used when two or more miscible liquids have different boiling points. The fractionating column provides a temperature gradient: the liquid with the lower boiling point reaches the top first and passes into the condenser.

The exam asks how fractional distillation separates crude oil into fractions. Each fraction contains hydrocarbons with similar boiling points. Shorter-chain hydrocarbons have lower boiling points and are collected at the top; longer-chain hydrocarbons have higher boiling points and are collected at the bottom.

Chromatography

Paper chromatography separates components of a mixture based on their different solubilities in the solvent.

The procedure

1. Draw a pencil baseline near the bottom of the chromatography paper (pencil, not ink, because ink would run).
2. Place a small spot of the mixture on the baseline.
3. Place the paper in the solvent so the solvent level is below the baseline.
4. Allow the solvent to rise up the paper by capillary action.
5. Remove the paper when the solvent front nears the top and mark the solvent front immediately.

Interpreting chromatograms

• Each spot represents a different substance.
• If a spot in the mixture lines up with a spot of a known substance, they are the same substance.
• The Rf value identifies a substance: Rf = distance moved by substance / distance moved by solvent front.
• A pure substance gives a single spot. An impure substance gives multiple spots.

Common errors

• Using pen instead of pencil for the baseline (the ink runs and contaminates the result).
• Not stating that the solvent level must be below the baseline (otherwise the spots dissolve directly into the solvent).
• Measuring Rf from the wrong point (always measure from the baseline, not from the bottom of the paper).

Purity and melting point

A pure substance has a sharp melting point that matches the data book value. An impure substance:

• Melts over a range of temperatures (not a sharp point).
• Melts at a temperature lower than the expected melting point.

The exam may give you a table of melting points for several substances and ask you to identify which is impure (the one with a range or a depressed value).

Measuring instruments and accuracy

Paper 6 frequently tests whether you can choose the right measuring instrument and read it accurately.

Measurement	Instrument	Precision
Volume of liquid (rough)	Measuring cylinder	1 cm3 or 0.5 cm3
Volume of liquid (precise)	Burette	0.05 cm3
Mass	Balance	0.01 g or 0.1 g
Temperature	Thermometer	0.5 C or 1 C
Time	Stopwatch	0.01 s

Read the meniscus at eye level for liquid measurements. The meniscus curves downward for water; read from the bottom of the curve.

More detail on Paper 6 specific techniques is in the Paper 6 guide.

Worked exam question

Q (Paper 4): A student has a mixture of sand and salt. Describe how the student could obtain a sample of pure, dry salt from this mixture. (4 marks)

Model answer:

1. Add water to the mixture and stir to dissolve the salt (1).
2. Filter the mixture through filter paper in a funnel to remove the insoluble sand (1).
3. Heat the filtrate gently in an evaporating basin to evaporate some of the water (1).
4. Allow the remaining solution to cool and crystallise, then pat the crystals dry with filter paper (1).

Each step earns one mark. Missing “add water to dissolve” at the start is the most common error, because students jump straight to filtering without dissolving the salt first.

Experimental techniques questions are among the most mark-accessible on the paper because they test procedure rather than deep understanding. If these marks are being lost, the issue is usually imprecise descriptions rather than missing knowledge. A trial lesson can sharpen the procedural language quickly.