IGCSE Chemistry: Practical Techniques — Titrations, Chromatography and Separation — Practice Questions

These are original questions written for Marlbridge, in the style and at the standard of the examination. They are not reproduced past-paper questions — examination boards hold copyright in their own papers. Use these alongside the official past papers available free from your board.

Related: Practical Techniques study guide

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Questions

1. Name the most appropriate piece of apparatus for measuring: (a) a precise, variable volume of liquid added gradually; (b) one exact, fixed volume of liquid. [2]

2. Define the terms solute and solvent. [2]

3. Describe how to carry out an acid–base titration to find the volume of acid needed to exactly neutralise a fixed volume of alkali, including how the end-point is identified. [4]

4. A mixture of four soluble coloured dyes is analysed by paper chromatography.

(a) Describe how the chromatogram is set up and run. [2] (b) State how you would use the chromatogram to decide whether a food colouring is a pure single dye or a mixture of dyes. [1]

5. In a chromatography experiment, a substance travels 6.0 cm and the solvent front travels 8.0 cm from the same baseline.

(a) Calculate the Rf value of the substance. [2] (b) State one reason why the Rf value calculated could not be greater than 1. [1]

6. A student has a mixture of salt dissolved in water and wants to obtain pure, dry salt crystals. Name the technique used, and describe briefly how it works. [2]

7. Explain the difference between simple distillation and fractional distillation, and state one situation where each would be used. [4]

8. A solid substance is found to melt gradually between 118°C and 124°C, whereas the pure substance is known to melt sharply at 125°C. State what this suggests about the sample, and explain your reasoning. [2]

9. Suggest a suitable method to separate a mixture of sand and salt, given that sand is insoluble in water and salt is soluble. [3]

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Further questions

10. A mixture of colourless amino acids is separated by paper chromatography.

(a) Explain how the spots are made visible, since the substances have no colour of their own. [1]

(b) Describe how the identity of an unknown spot can be determined using the same chromatogram. [2]

11. Give the correct apparatus for measuring (a) an approximate volume of liquid, and (b) the volume of a gas produced in a reaction. [2]

Define (c) residue and (d) filtrate. [2]

12. A pure liquid has a known boiling point of 78°C. An impure sample of the same liquid is found to boil over a range starting at 80°C. State the effect of impurity on boiling point, and contrast this with the effect of impurity on melting point. [2]

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Answers

1. (a) Burette [1]. (b) Volumetric pipette [1].

2. Solute — a substance that is dissolved in a solvent [1]. Solvent — a substance that dissolves a solute [1].

3. Measure a fixed volume of the alkali into a conical flask using a volumetric pipette [1], and add a few drops of a suitable indicator [1]. Fill a burette with the acid and add it gradually, swirling the flask [1]. The end-point is reached when the indicator’s colour change becomes permanent with the addition of a single further drop [1].

4. (a) Spot the mixture near the base of a strip of chromatography paper, above the level of the solvent [1]; stand the paper in a suitable solvent and allow the solvent to rise, carrying the components different distances [1]. (b) A pure dye produces exactly one spot; a mixture separates into two or more spots [1].

5. (a) Rf = 6.0 / 8.0 [1] = 0.75 [1]. (b) The substance can never travel further than the solvent itself [1].

6. Crystallisation [1]. The solution is gently heated to evaporate some of the water until saturated, then left to cool, so that solid crystals form from solution as it becomes too concentrated to stay dissolved, and are then filtered off and dried [1].

7. Simple distillation separates a solvent from a dissolved solid, or one volatile liquid from a mixture where only one component is volatile [1] — used, for example, to obtain pure water from salt solution [1]. Fractional distillation separates two or more miscible liquids with different, known boiling points [1] — used, for example, to separate ethanol from water [1].

8. The sample is impure [1]. A pure substance melts sharply at a single, exact temperature; melting over a range, and below the true melting point, is evidence of impurity [1].

9. Add the mixture to water and stir, so the salt dissolves [1]; filter to remove the insoluble sand as the residue [1]; evaporate (or crystallise) the filtrate to recover the dissolved salt [1].

10. (a) A locating agent is sprayed onto (or applied to) the paper afterwards to reveal the spots [1]. (b) Compare the position (or Rf value) of the unknown spot against known reference substances run on the same chromatogram, under the same conditions — same solvent, same paper [1]; a matching position/Rf suggests the same substance [1].

11. (a) Measuring cylinder [1]. (b) Gas syringe [1]. (c) Residue — a substance that remains after evaporation, distillation, filtration or a similar process [1]. (d) Filtrate — a liquid or solution that has passed through a filter [1].

12. Impurity raises the boiling point, and causes it to boil over a range rather than sharply [1] — the opposite effect to melting point, where impurity lowers the melting point and causes melting over a range [1].

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Where marks are usually lost

• Confusing simple distillation (one volatile component) with fractional distillation (two or more miscible liquids of different boiling point).
• Applying the Rf formula upside down — it is always substance distance over solvent distance.
• Describing purity only as “a different melting point” rather than the key idea that impurity produces a range.
• Adding the indicator to a titration meant for salt preparation and then crystallising with it still present.
• Saying colourless substances “can’t be chromatographed” — a locating agent reveals their spots after the run.
• Assuming impurity always lowers a transition temperature — it lowers melting point but raises boiling point.