Sec 3 Combined Science (Chemistry) Experimental Chemistry & Purification notes

Chemistry is a practical subject, and the first practical skill is naming apparatus correctly and choosing it well. For time, use a stopwatch. For temperature, a laboratory thermometer. For mass, an electronic balance (reading to 0.01 g in school laboratories). For volume of liquid, the choice depends on precision: a 50 cm³ measuring cylinder reads to about 1 cm³, while a pipette (fixed volume) or burette (variable volume) reads to 0.1 cm³.

When a question says suggest suitable apparatus, it wants the instrument whose precision matches the measurement and a short justification. To measure "exactly 25.0 cm³", the two zeros matter: only a pipette or burette is good enough. To follow the volume of gas made every 30 seconds, a gas syringe is the standard choice because it shows the total gas collected at any moment.

Three collection set-ups cover every gas in the course. Collection over water: an inverted vessel full of water collects bubbles of gas that do not dissolve. Upward delivery (the vessel sits mouth-up): for gases denser than air, which sink to the bottom and push air out of the top. Downward delivery (mouth-down): for gases less dense than air, which rise into the vessel. When the volume must be measured as well, connect a gas syringe instead.

A separation works because the substances differ in some physical property — solubility, boiling point, or how far they travel in a solvent. The syllabus groups mixtures into three types, and each has a go-to plan:

• Solid–solid (e.g. sand and salt): add a suitable solvent that dissolves just one of them, then filter. The soluble solid is recovered from the filtrate by crystallisation.
• Solid–liquid (a dissolved solid in a liquid, e.g. salt water): keep the solid by evaporation or crystallisation; keep the liquid by simple distillation.
• Liquid–liquid, miscible (e.g. ethanol and water): separate by distillation — simple if the boiling points are far apart, fractional if they are close.

Two vocabulary marks come up every year: the residue is the solid trapped on the filter paper, and the filtrate is the liquid that passes through. And when crystals are wanted, never evaporate to dryness: heating to the point of crystallisation (small crystals form on a glass rod dipped in and lifted out) and then cooling slowly gives large, well-formed, purer crystals.

In simple distillation a solution is boiled; the vapour leaves the flask, is turned back to liquid in a water-cooled condenser, and drips into the collecting vessel as the distillate. Dissolved solids stay behind. Three set-up details earn marks: the thermometer bulb sits level with the side-arm (it must read the temperature of the vapour, not the liquid), cooling water enters at the lower end of the condenser jacket and leaves at the top (so the jacket stays full and flows against the hot vapour), and the collection vessel must not seal the condenser outlet shut.

Fractional distillation adds a column packed with glass beads between flask and condenser. Vapours condense and re-evaporate many times up the column, so the liquid with the lower boiling point enriched vapour arrives first. It is the method for two miscible liquids whose boiling points are close (ethanol, 78 °C, and water, 100 °C); simple distillation suffices when the difference is large.

• Simple distillation: one liquid wanted, dissolved solids rejected — e.g. pure water from salt water.
• Fractional distillation: two (or more) miscible liquids wanted separately — e.g. ethanol from a fermented mixture; crude oil into fractions.
• Either way the thermometer reading during steady distillation equals the boiling point of the substance arriving — evidence for what is coming over.

In paper chromatography a spot of the mixture is placed on a pencil start line (pencil, not pen — pen ink would dissolve and add its own spots). The paper dips into a shallow layer of solvent, which climbs the paper by capillary action and carries the substances up with it. The paper is the stationary phase; the solvent is the mobile phase. A substance that strongly attracts the paper travels slowly; one that prefers the solvent travels far.

Reading a chromatogram: each spot is one substance, so two spots means at least two substances. To identify what they are, run the mixture alongside known samples on the same paper: a spot that ends at the same height as a known sample suggests that substance is present. A sample showing a single spot that matches no known sample is one substance that is not any of the references — chromatography identifies only what you have standards for.

A pure substance changes state at one fixed temperature: it melts sharply and boils at a single temperature. Two things happen when a soluble impurity is mixed into a solid: the melting point starts lower than the pure value, and melting is spread over a range instead of happening sharply. For liquids, dissolved impurities usually raise the boiling point and spread boiling over a range too.

• Sharp melting at a temperature matching the data book → the sample is pure and identified.
• Melting over a range starting below the data-book value → impure; the wider the range, the more impurity.
• Boiling steadily at one temperature that matches a data-book boiling point → consistent with the pure liquid.
• Boiling over a range → a mixture (e.g. ethanol and water together boil between 78 °C and 100 °C).