Overview

What you'll learn

Choose the right apparatus. Know which apparatus reads finely enough, and what to use to collect, dry or measure the rate of a gas.

Name the mixture. Spot the type: solid+solid, solid+liquid, or liquid+liquid. The separation method follows.

Use chromatography. Read chromatograms, calculate Rf values, and use Rf to identify unknown substances.

Judge purity. Use sharp melting and boiling points as a test of purity. Know why purity matters in food and drugs.

Tutor's Insight

"Most of you treat Topic 1 like a warm-up. Don't."
The apparatus questions in MCQ, the gas collection setups in structured papers, the separation sequence questions in data-based assessment — they all trace back here. Students who master this topic early build a scaffolding that holds up every topic that follows. Get the fundamentals locked in now.

1.1 Experimental Design

1.1 Experimental Design

Syllabus asks
  • Name and use appropriate apparatus to measure and collect substances, including recognising the smallest division of measuring instruments.
  • Describe, and explain where appropriate, how to collect a gas and measure the rate of a reaction using volume of gas produced or loss of mass.

1.1 Experimental Design

Measuring Apparatus

Apparatus What it measures Smallest division
Measuring cylinder Approximate volume of a liquid 1 cm³
Pipette One fixed, accurate volume (e.g. 25.0 cm³) fixed
Burette Any accurate volume up to 50 cm³ 0.10 cm³
Gas syringe Volume of a gas 1 cm³
Electronic balance Mass 0.01 g
Stopwatch Time 0.01 s
Thermometer Temperature 1 °C

Free Notes · O-Level Pure Chemistry

Read the full chapter

Gas collection, drying agents, separation techniques, chromatography, purity tests and 4 worked exam questions.

1.1 Experimental Design

Collecting Gases

Method 01
Displacement of water
For gases that don't dissolve much in water.
e.g. H₂, O₂, CO₂
Method 02
Upward delivery
For gases lighter than air. Hold the jar mouth-down.
e.g. H₂, NH₃
Method 03
Downward delivery
For gases heavier than air. Keep the jar upright.
e.g. Cl₂, HCl, SO₂
Mᵣ of air ≈ 29. If the gas's Mᵣ is less than 29, it rises. More than 29, it sinks.

1.1 Experimental Design

Drying Agents

WET GAS → DRYING AGENT → DRY GAS
Concentrated sulfuric acid
Dries acidic and neutral gases.
Quicklime (calcium oxide, CaO)
Dries basic and neutral gases.
Fused calcium chloride
Dries acidic, basic and neutral gases — except ammonia.
The drying agent must not react with the gas.

1.1 Experimental Design

Measuring Rate of Reaction

Method 01
Volume of gas vs time
Collect the gas in a gas syringe. Record the volume at fixed time intervals.
Method 02
Loss of mass vs time
Let the gas escape. Record the falling mass on a balance. Cotton wool stops acid spray escaping.
A steeper slope at the start means a faster reaction.

1.2 Methods of Purification & Analysis

1.2 Methods of Purification & Analysis

Syllabus asks
  • Describe and explain methods of purification, including filtration, crystallisation, simple distillation, fractional distillation, paper chromatography, and use of a separating funnel.
  • Suggest suitable methods of purification given information about the substances involved, and understand the significance of purity in food and drugs.

1.2 Purification & Analysis

Identifying the Mixture Type

Solid + Solid
Two solids
Magnetic separation · sublimation · or dissolve one solid, then filter and recover.
Solid + Liquid
A solid in a liquid
Filtration · evaporation to dryness · crystallisation.
Liquid + Liquid
Two liquids
Separating funnel if they don't mix · distillation or fractional distillation if they do.
Name the type of mixture first. The method follows from that.

1.2 Purification & Analysis

Separating Solid Mixtures

Method 01
Magnetic separation
A magnet lifts magnetic solids (iron, cobalt, nickel, steel) out of the mix.
Method 02
Sublimation
Separates a solid that sublimes (e.g. iodine, ammonium chloride). Heat gently. The vapour resolidifies on a cool surface.
Exam Habit

Sublime first — If your mixture has a solid that sublimes, do that step first. Otherwise you lose it.

1.2 Purification & Analysis

Separating Solid-Liquid Mixtures

Method 01
Filtration
Separates an insoluble solid from a liquid. Residue on the paper. Filtrate runs through.
Method 02
Evaporation to dryness
Boils off all the solvent. Only for solids that don't decompose on heating.
Method 03
Crystallisation
Heat to saturation, then cool. Crystals grow. For solids that do decompose on strong heating.
If the solute decomposes when heated, crystallise. If it doesn't, evaporate to dryness.

1.2 Purification & Analysis

Separating Liquid Mixtures

Don't Mix
Separating funnel
For immiscible liquids (e.g. oil + water). Let them settle. Run off the denser bottom layer.
Mix
Simple distillation
Recovers the solvent from a solution. Vapour condenses in the Liebig condenser.
Mix
Fractional distillation
Separates miscible liquids with different boiling points. The column lets the lower-bp liquid through first.
Immiscible liquids go to the separating funnel. Miscible liquids go to distillation.

1.2 Purification & Analysis

Chromatography

What it does
Chromatography

Separates and identifies soluble substances by how far each travels in a solvent.


Same Rf as a known sample (same solvent)? Likely the same substance.


Colourless substances need a locating agent to show up.


Draw the start line in pencil. Ink would dissolve and run.

Rf Value
Rf =
distance moved by substance distance moved by solvent front
Always less than 1. Quote it to 2 decimal places.

1.2 Purification & Analysis

Testing Purity

Pure substance
Sharp, fixed points
Melts and boils at one definite temperature. A single spot on a chromatogram.
Impure / Mixture
Melts over a range
Impurities lower the melting point and raise the boiling point.
Why it matters

Food and drugs must be pure. An impurity can be harmful, or it can change the dose a patient actually gets.

Practice

Exam-style questions

Question 01
The Question

Sand and salt

A mixture contains sand and common salt. Describe how you would obtain pure, dry samples of both.

Worked Answer
Dissolve → Filter → Evaporate
  1. Add water to the mixture and stir — the salt dissolves, the sand does not.
  2. Filter the mixture. The sand (residue) is collected on the filter paper; the salt solution (filtrate) passes through.
  3. Rinse the sand on the filter paper with distilled water, then dry it in an oven.
  4. Evaporate the salt solution to dryness over a water bath to obtain pure, dry salt crystals.
Question 02  ·  ACS(BR) 2024 Prelim
The Question
MCQ

Collecting carbon dioxide

Which apparatus can both collect and measure the volume of CO₂ produced?

  • 1  — Gas syringe
  • 2  — Measuring cylinder (over water)
  • 3  — Graduated test tube (inverted, over water)

A 1 only    B 2 only    C 1 and 3    D 1, 2 and 3

Worked Answer
C — 1 and 3
  1. A gas syringe (1) both collects the gas and measures its volume directly — correct.
  2. A measuring cylinder over water (2) collects the gas but its graduations start from the open end — you cannot read the volume accurately for CO₂ because CO₂ is slightly soluble in water, dissolving some of the gas.
  3. A graduated test tube inverted over water (3) collects and measures the volume — correct, though CO₂ solubility is a minor loss.
  4. Answer: C.
Question 03  ·  ACS(BR) 2024 Prelim
The Question
MCQ

Two mixtures with water

Ethanol is miscible with water. Hexane is immiscible with water. How is each best separated from water?

  • A  Ethanol: separating funnel    Hexane: fractional distillation
  • B  Ethanol: fractional distillation    Hexane: separating funnel
  • C  Ethanol: fractional distillation    Hexane: separating funnel
  • D  Ethanol: simple distillation    Hexane: separating funnel
Worked Answer
C
  1. Ethanol and water are miscible — use fractional distillation to separate them by their different boiling points (ethanol bp 78 °C, water bp 100 °C).
  2. Hexane and water are immiscible — they form two separate layers, so use a separating funnel.
  3. Answer: C.
Question 04  ·  AMKSS 2024 Prelim
The Question
Structured

Purifying barium chloride

A sample of barium chloride (soluble) is contaminated with barium sulfate (insoluble). Which sequence of steps best purifies the barium chloride?

  • A  Evaporate to dryness → Filter
  • B  Dissolve in water → Filter → Crystallise
  • C  Dissolve in water → Evaporate to dryness
  • D  Filter → Dissolve in water → Evaporate to dryness
Worked Answer
B
  1. Dissolve the mixture in water — barium chloride dissolves; barium sulfate does not.
  2. Filter — barium sulfate is removed as the residue; the barium chloride solution passes through as filtrate.
  3. Crystallise the filtrate by heating to saturation then cooling — this recovers pure barium chloride crystals. (Evaporation to dryness risks decomposing BaCl₂·2H₂O.)
  4. Answer: B.

Frequently Asked Questions

Experimental Chemistry — FAQ

What is Experimental Chemistry in the O-Level Chemistry syllabus (6092)?
Experimental Chemistry (Topic 1 of Syllabus 6092) has two subtopics: 1.1 Experimental Design — choosing and using apparatus, collecting gases, drying agents and measuring reaction rates; and 1.2 Methods of Purification & Analysis — separation techniques, chromatography, Rf values and testing purity. These skills underpin the practical and data-based questions across all three exam papers.
How do you choose the right apparatus to measure volume in Chemistry?
Match the apparatus to the precision you need. A measuring cylinder reads to 1 cm³ for approximate volumes, a pipette delivers one fixed accurate volume (e.g. 25.0 cm³), a burette measures any volume up to 50 cm³ to 0.10 cm³, and a gas syringe measures gas volume to 1 cm³. Always pick the instrument whose smallest division is fine enough for the reading.
What are the three ways to collect a gas?
Displacement of water for gases that don't dissolve much in water (H₂, O₂, CO₂); upward delivery for gases lighter than air, jar mouth-down (H₂, NH₃); and downward delivery for gases heavier than air, jar upright (Cl₂, HCl, SO₂). Compare the gas's Mᵣ with air (≈29): lighter rises, heavier sinks.
What is the Rf value in chromatography and how do you calculate it?
Rf = distance moved by the substance ÷ distance moved by the solvent front. It is always less than 1 and quoted to 2 decimal places. If an unknown has the same Rf as a known sample in the same solvent, it is likely the same substance.
How do you know if a substance is pure?
A pure substance melts and boils at one sharp, fixed temperature and shows a single spot on a chromatogram. An impurity lowers the melting point and raises the boiling point, so an impure substance melts and boils over a range. Purity matters in food and drugs because impurities can be harmful or change the dose.
When do you use crystallisation instead of evaporation to dryness?
Ask whether the solute decomposes when heated. If it does (e.g. sugar), crystallise: heat to saturation, then cool so crystals form. If it does not (e.g. common salt), you can evaporate to dryness.
What is the difference between simple and fractional distillation?
Simple distillation recovers a pure solvent from a solution — vapour condenses in a Liebig condenser. Fractional distillation separates two miscible liquids with different boiling points using a fractionating column, which lets the lower-boiling-point liquid through first (e.g. ethanol from water).

O-Level Pure Chemistry  ·  Syllabus 6092  ·  Topic 1 of 12  ·  © 2026 Overmugged. For personal study use only.