📚 Concepts & Revision Notes — Experimental Chemistry

1. Measuring physical quantities

Almost every experiment starts with a measurement. There are five physical quantities you must be able to measure in the lab, each with an SI unit and its own apparatus.

Choosing the right volume apparatus

• Pipette — delivers one fixed volume very accurately, for example exactly 25.0 cm³. Used in titrations.
• Volumetric flask — holds one fixed, larger volume accurately (e.g. 250 cm³), used for making up standard solutions.
• Measuring cylinder — quick and flexible, but only reads to the nearest 0.5 cm³.
• Burette — delivers variable volumes and reads to the nearest 0.05 cm³, so it is the accurate choice when the volume is not fixed in advance.
• Gas syringe — measures the volume of a gas.

Reading the meniscus

The curved surface of a liquid in a narrow tube is called the meniscus. Always read it with your eye level with the liquid surface to avoid parallax error. Water curves downwards (concave), so you read the bottom of the curve. Mercury bulges upwards (convex), so you read the top.

2. Collecting and drying gases

How you collect a gas depends on two properties: how soluble it is in water, and whether it is denser or less dense than air.

Drying a gas Optional for G3/G2 ScChem

A collected gas is often damp. Pass it through (or over) a drying agent — but pick one that does not react with the gas itself:

Drying agent	Good for	Never use with
Concentrated sulfuric acid	most gases	ammonia (an alkaline gas — it reacts with the acid)
Quicklime (calcium oxide)	ammonia	carbon dioxide (an acidic gas — it reacts with the basic quicklime)
Fused calcium chloride	hydrogen, nitrogen, carbon dioxide	ammonia (it combines with the calcium chloride)

3. Choosing a separation technique

A mixture can be separated because each component keeps its own physical properties — particle size, solubility, density, boiling or melting point, magnetism. Every technique below simply exploits one property that the components do not share.

4. Separating solid–solid mixtures

Magnetic attraction

Only iron, cobalt and nickel are attracted to a magnet, so a magnet can pull one of these metals cleanly out of a mixture. Recycling plants use giant electromagnets to lift steel and iron out of mixed scrap.

Sieving

A sieve separates solids by particle size: small particles fall through the mesh, large ones stay behind. Bakers sieve flour to remove lumps, and archaeologists sieve soil so that tiny artefacts are caught while the fine earth passes through.

Using a suitable solvent

If one solid dissolves in a solvent and the other does not, add the solvent, stir, then filter. Classic example: salt mixed with sand. Water dissolves the salt but not the sand, so filtering removes the sand and evaporating or crystallising the salty filtrate gives back the salt.

Sublimation Optional for G3/G2 ScChem

Sublimation is when a solid changes directly into a gas without melting first (and the gas turns straight back into a solid when cooled). Only a few substances do this — iodine, naphthalene (mothballs) and dry ice (solid carbon dioxide) are the ones to remember.

To separate, warm the mixture gently: the subliming solid rises as a vapour, hits a cool surface (such as a cold flask held above), and re-forms as a solid there, called the sublimate. The other solid stays in the container. Try it in the sublimation lab →

5. Separating solid–liquid mixtures

Filtration — for insoluble solids

Pour the mixture through filter paper folded into a cone inside a funnel. The paper acts like an extremely fine sieve: liquid passes through its tiny pores while insoluble solid particles are too large and get trapped.

• The solid left on the paper is the residue.
• The liquid that passes through is the filtrate.

Try it in the filtration lab →

Evaporation to dryness — for dissolved solids

Heat the solution in an evaporating dish until all of the solvent has boiled away, leaving the solid behind. It is fast, but it has two limitations:

• Some solids decompose when heated strongly — sugar, for example, chars into a black mess instead of coming back as crystals.
• Everything dissolved comes out together, so the solid you get may actually be a mixture of several salts, not one pure substance.

Try it in the evaporation lab →

Crystallisation — the gentler option

A saturated solution is one that cannot dissolve any more solute at that temperature. Most solids are more soluble in hot solvent than in cold.

Because heat-sensitive solids survive gentle treatment, crystallisation is used when evaporation to dryness would destroy the product:

1. Heat the solution gently to evaporate some solvent and concentrate it until it is just saturated.
2. Let it cool slowly — as the temperature drops, the solvent can hold less solute, so the excess comes out as crystals.
3. Filter to collect the crystals.
4. Wash them with a little cold solvent and dry them between sheets of filter paper.

Try it in the crystallisation lab →

Simple distillation — when you want the liquid too

Evaporation and crystallisation throw the solvent away as vapour. If you want to keep both parts — say, drinking water from sea water — use simple distillation. The solution is boiled; the solvent vapour travels into a cooled condenser where it turns back into liquid, called the distillate, and drips into a receiver. The dissolved solute stays behind in the flask because its boiling point is far higher. Try it in the distillation lab →

Which one should I pick?

6. Separating liquid–liquid mixtures

Immiscible liquids — separating funnel Optional for G3/G2 ScChem

Immiscible liquids, like oil and water, do not mix — they settle into layers, with the denser liquid at the bottom. Pour the mixture into a separating funnel, let the layers settle, then open the tap: the bottom layer runs out into a beaker. Close the tap the moment the boundary reaches it, and the two liquids are apart. Try it in the separating-funnel lab →

Miscible liquids — fractional distillation

Miscible liquids (like ethanol and water) mix completely, so there is no layer to drain. Instead we use their different boiling points. A fractionating column packed with glass beads sits between the flask and the condenser. The beads give a huge surface where vapour repeatedly condenses and re-evaporates, so only the substance with the lowest boiling point makes it to the top first.

Watch the thermometer: it plateaus (stays steady) at the boiling point of whichever liquid is currently distilling over — for ethanol that is 78 °C. When the reading starts climbing again, the first liquid has finished and it is time to change receivers.

Industry runs on this idea: crude oil is split into petrol, kerosene and other fractions in giant columns; liquefied air is fractionally distilled to obtain nitrogen, oxygen and argon; and breweries use it to concentrate alcohol. Try it in the fractional distillation lab →

Chromatography — identifying what is in a mixture

Paper chromatography separates small amounts of dissolved substances. A spot of the mixture is placed on a start line drawn in pencil near the bottom of the paper (pencil, not pen — graphite is insoluble, so the line itself cannot travel and confuse the result). The paper stands in a shallow solvent. As the solvent soaks upwards, it carries the substances with it — the more soluble a substance is in that solvent, the further it travels. The finished paper, with its separated spots, is called a chromatogram.

Chromatography is used to check food additives are safe and permitted, to test athletes' samples for banned drugs, and in forensic work such as analysing DNA samples. Try it in the chromatography lab →

7. Testing for purity

A pure substance contains only one substance, so it has a sharp, fixed melting point and boiling point. A mixture melts and boils over a range of temperatures instead — and the more impurity there is, the bigger the shift.

• Dissolved impurities raise the boiling point.
• Dissolved impurities lower the melting point — which is exactly why salt is spread on icy roads: salty ice melts below 0 °C.

8. Try the labs

The best way to remember a technique is to run it. Each simulation lets you set up and perform the experiment yourself:

• 🧪 Filtration — separate an insoluble solid from a liquid.
• ♨️ Evaporation — boil a solution to dryness to recover the dissolved solid.
• 🔷 Crystallisation — stop at saturation and cool for pure crystals.
• 💧 Simple distillation — recover pure water from salt water.
• 🌡️ Fractional distillation — split ethanol from water at 78 °C.
• 🫙 Separating funnel — drain oil off water, layer by layer.
• 🔮 Sublimation — catch iodine vapour on a cold flask.
• 🎨 Paper chromatography — separate an ink and prove R_f never changes.

9. Syllabus at a glance (2023)

What topic 1 Experimental Chemistry asks you to be able to do. The three syllabuses word their outcomes differently — pick your own tab to see exactly what you are assessed on.

Wording is quoted verbatim from the 2023 syllabuses. G3 Pure = O-Level Chemistry · G3 ScChem = O-Level Science (Chemistry) · G2 ScChem = N-Level Science (Chemistry). Numbering differs between them: G2 ScChem has no Redox, Chemical Energetics or Rate of Reactions, so its later topics are numbered lower.