Full chapter
Experimental Chemistry
Select apparatus, separate mixtures and judge what measurements actually show.
O-Level 5086 / 5088 (2026) / SEC G3 K326 / K328 (2027)
Choose a measurement that answers the question
Apparatus choice depends on range, precision and what changes.
| Quantity or task | Apparatus | Reason |
|---|---|---|
| Time | Stopwatch | Measure an interval; define exactly when timing starts and ends. |
| Temperature | Thermometer or temperature probe | Measure the reaction mixture, not the hot container wall. |
| Mass | Balance | Measure mass directly; subtract the container mass. |
| Approximate liquid volume | Measuring cylinder | Quick measurement where modest precision is sufficient. |
| One accurate fixed volume | Volumetric pipette and filler | Delivers its calibrated volume, such as 25.0 cm3. |
| Accurate variable volume | Burette | Volume delivered = final minus initial reading. |
| Gas volume | Gas syringe | Collect gas directly and read the change in volume. |
Read a liquid meniscus at eye level to avoid parallax; for a clear aqueous solution read its bottom. Record units and precision consistently. A larger container is not automatically more accurate. A pipette supplies one fixed volume; a burette is appropriate when the required volume is found during a titration.
Worked example
Measure a rate rather than an endpoint
Compare how magnesium reacts with two acid concentrations.
- Use equal masses and surface areas of magnesium, equal acid volumes and the same initial temperature.
- Collect hydrogen in an airtight gas-syringe setup and record volume at regular times.
- Compare initial gradients or the time to reach the same selected volume; repeat to assess consistency.
Gas volume against time reveals rate. Final gas volume alone may be identical despite different rates.
Check your understandingA burette reads 1.20 cm3 initially and 23.65 cm3 finally. What volume was delivered?Think it through, then reveal the answer
Collect a gas using its properties
Solubility and density determine a suitable setup.
A gas syringe is convenient for measuring gas volume. The bung and connections must be airtight, the plunger must move freely, and the capacity must exceed the expected volume. Begin collection promptly: gas lost before sealing makes measured volume too small.
| Method | Suitable when | Example or limitation |
|---|---|---|
| Displacement of water | Gas is insoluble or only slightly soluble in water | Hydrogen or oxygen; unsuitable for very soluble ammonia. |
| Upright gas jar with gas entering near the bottom | Gas is denser than air and would dissolve in water | Carbon dioxide can displace air upwards. |
| Inverted gas jar with gas entering near the top | Gas is less dense than air | Ammonia displaces air downwards. |
| Gas syringe | Gas can be collected without significant leakage or reaction with apparatus | Useful for volume-time measurements; very soluble gases need an appropriate dry setup. |
Check your understandingWhy is collection over water unsuitable for ammonia?Think it through, then reveal the answer
Separate by solubility and particle size
Recover the intended substance, not just any solid.
Filtration separates an insoluble solid from a liquid. The residue remains on the filter paper; the filtrate passes through. Dissolved salt particles pass through with water, so filtering salt solution does not remove the salt. Wash a residue with a small amount of suitable solvent to remove soluble impurities, then dry it.
- Add water and stir
Salt dissolves; sand remains insoluble.
- Filter
Sand is the residue; salt solution is the filtrate.
- Wash and dry the sand
Washing removes remaining salt solution.
- Recover salt from the filtrate
Evaporate water for a heat-stable salt, or concentrate and crystallise.
For crystallisation, heat a solution gently to remove some solvent until it is nearly saturated, then let it cool. Crystals form as solubility decreases. Filter, wash with a little cold solvent and dry. Evaporation to dryness removes all solvent and suits a stable dissolved solid; strong heating may decompose some compounds. Crystallisation often leaves soluble impurities in the remaining solution.
Check your understandingA clear sugar solution passes through filter paper unchanged. Explain why.Think it through, then reveal the answer
Distillation and liquid layers
Use boiling-point differences or immiscibility.
Simple distillation recovers a solvent from a solution containing a non-volatile solute. Heat salt water: water vaporises, enters a condenser, cools and becomes liquid distillate. Salt remains in the flask. Place the thermometer bulb near the side-arm to measure the vapour entering the condenser. Cooling water enters the condenser at the lower end and exits at the upper end to keep the jacket full.
Fractional distillation separates miscible liquids with different boiling points, especially when those boiling points are close. A fractionating column provides repeated evaporation and condensation. The vapour reaching its top is richer in the more volatile, lower-boiling component. Collect fractions over suitable temperature ranges. Crude oil is separated into groups of hydrocarbons this way; a fraction is generally a mixture, not one pure compound.
Check your understandingYou need pure water from seawater. Why choose distillation rather than evaporation to dryness?Think it through, then reveal the answer
Read the evidence in a chromatogram
Compare spots under the same conditions.
Place small sample spots on a pencil baseline above the solvent level. Ink would dissolve and add unwanted spots. As solvent rises through the paper, substances repeatedly distribute between the solvent and paper; different substances move different distances. Keep the vessel covered and mark the solvent front before it evaporates.
Compare an unknown with standards
Unknown U has two spots at the same heights as one spot in A and one in B.
Several spots indicate a mixture of separated components. A single spot suggests purity under these conditions, but two substances might move together. A spot matching a known sample supports identification; comparison is meaningful only with the same solvent and conditions.
Check your understandingU has two spots matching A and B. Can you conclude that U contains only A and B with certainty?Think it through, then reveal the answer
Use melting and boiling data
A physical measurement supports identity; a single match is not proof.
At a specified pressure, a pure substance melts sharply and boils at a characteristic temperature. Soluble impurities usually lower and broaden the melting range and can raise the boiling point. Compare measured data with reference values taken under comparable conditions; a mixture may have a changing boiling temperature.
Worked example
Interpret a melting range
Pure substance P melts at 82 degrees C. A sample melts over 76-80 degrees C.
- The lower, broader melting range is inconsistent with a pure sample of P.
- It is consistent with P containing impurities, although more evidence is needed to establish identity.
The sample is impure if it is P. A matching single temperature alone would not uniquely identify an unknown.
Worked example
Match two measurements to reference data
At the same pressure, pure P melts at 18 degrees C and boils at 126 degrees C; Q melts at 18 degrees C and boils at 154 degrees C. Unknown X melts sharply at 18 degrees C and boils steadily at 154 degrees C. A second sample, claimed to be Q, melts over 12-16 degrees C. Interpret both samples.
- The melting point alone cannot distinguish P and Q: both have the same value.
- The boiling point of X matches Q, not P. Within the supplied candidates, both measurements support pure Q.
- The second sample melts below the reference temperature over a range. If it is Q, the data indicate impurities.
X is consistent with pure Q. The second sample is impure Q if its claimed identity is correct; its melting range alone cannot establish identity.
Check your understandingA liquid is clear and colourless. Is it necessarily pure?Think it through, then reveal the answer
Quick revision
Revisit the essentials, then return to an explanation when you need it.
| Difference | Method |
|---|---|
| Insoluble solid in liquid | Filtration |
| Soluble solid wanted | Crystallisation or suitable evaporation |
| Solvent wanted | Simple distillation |
| Miscible liquids with different boiling points | Fractional distillation |
| Different movement with solvent and paper | Chromatography |
Choose measurements with suitable precision. Distinguish observation from inference. A clear liquid is not automatically pure.
Scope and references
Learning outcomes and sources
1. Experimental Chemistry (5086 / 5088 / K326 / K328). Use the outcome map to find the explanation for a particular syllabus requirement.
See the learning outcome map
1.1(a) Select measuring apparatus
- Time, temperature, mass and volume
- Burettes, pipettes, measuring cylinders and gas syringes
1.1(b) Select simple experimental setups
- Gas collection
- Rate measurements
Choose a measurement that answers the questionCollect a gas using its properties
1.2(a) Describe separation methods
- Solvent, filtration, crystallisation, evaporation
- Simple/fractional distillation
- Paper chromatography
Separate by solubility and particle sizeDistillation and liquid layersRead the evidence in a chromatogram
1.2(b) Choose methods from properties
- Solid-solid
- Solid-liquid
- Miscible liquid-liquid
Separate by solubility and particle sizeDistillation and liquid layers
1.2(c) Interpret chromatograms
- Comparison with known samples
- Rf not required
1.2(d) Infer identity and purity
- Melting and boiling data
- 2026 Pure Chemistry 6092
Official topic 1, pages 10-11. Original explanations mapped to the stated outcomes; 2026 and 2027 topic content agrees.
- 2027 Pure Chemistry K324
Official topic 1, pages 10-11. Original explanations mapped to the stated outcomes; 2026 and 2027 topic content agrees.
- 2026 Combined Chemistry 5086 / 5088
Official topic 1, pages 27. Original explanations mapped to the stated outcomes; 2026 and 2027 topic content agrees.
- 2027 Combined Chemistry K326 / K328
Official topic 1, pages 27. Original explanations mapped to the stated outcomes; 2026 and 2027 topic content agrees.
- Grail: 6092 Chemistry Complete Notes, Version 1
Background consultation: Chapter 1, purity and separation, pp. 8-10. Teaching additions and examples are original; syllabus scope and chemistry independently checked.