Sec 3 Chemistry Experimental Chemistry topical questions

A mixture contains sand (insoluble) and common salt dissolved in water.

1. (a)State the technique that separates the sand from the mixture.[1]
2. (b)Name the solid collected on the filter paper.[1]

A student needs 25.0 cm³ of hydrochloric acid for a titration.

1. (a)State the most appropriate piece of apparatus.[1]
2. (b)A measuring cylinder is not chosen. Give a reason.[1]

On a chromatogram, a dye travels 3.6 cm from the pencil line and the solvent front travels 9.0 cm.

1. (a)State the formula for the Rf value of the dye.[1]
2. (b)Calculate the Rf value.[2]

Samples W and X are solids. W melts sharply at exactly 44 °C. X begins softening at 40 °C and is fully liquid at 43 °C.

1. (a)Which sample is pure?[1]
2. (b)Explain how the melting behaviour shows this.[2]

A student investigates whether stirring makes sugar dissolve faster in water at room temperature.

1. (a)State the independent variable and the dependent variable.[2]
2. (b)State two conditions that must be controlled.[2]

Simple distillation is used to obtain pure water from salt solution.

1. (a)Explain why the thermometer bulb is placed level with the side-arm, not in the liquid.[2]
2. (b)Explain why cooling water enters at the bottom of the condenser.[2]

A black ink contains dyes P and Q. Chromatography of the ink and of two known dyes (1 and 2) gives these distances: solvent front 8.0 cm; ink spot A at 2.4 cm; ink spot B at 5.6 cm; dye 1 at 2.4 cm; dye 2 at 5.6 cm.

1. (a)Identify the dyes in the ink, giving a reason.[2]
2. (b)Calculate the Rf value of spot B.[2]
3. (c)State why the start line was drawn in pencil.[1]

Pure, dry crystals of a heat-stable salt are wanted from its aqueous solution.

1. (a)Describe how the solution is heated and how the point of crystallisation is judged.[3]
2. (b)Explain why the solution is not evaporated to complete dryness.[2]

A liquid mixture contains propanone (boiling point 56 °C) and ethanol (boiling point 78 °C). They are miscible.

1. (a)Name the technique used to separate them.[1]
2. (b)Describe what happens inside the fractionating column.[3]
3. (c)State the thermometer reading when the first fraction is collected.[1]

A factory claims a batch of a pure compound (accepted melting point 122 °C) is uncontaminated. A technician measures melting behaviour.

1. (a)Describe how the technician determines the melting point accurately.[2]
2. (b)The sample melts between 117 °C and 121 °C. State the conclusion and justify it.[3]

Ammonia gas is generated in a fume cupboard and must be collected dry.

1. (a)State the collection method and justify it with two properties of ammonia.[3]
2. (b)Name a suitable drying agent and explain why concentrated sulfuric acid is not used.[3]

A colourless mixture of two amino acids must be separated and identified on paper chromatography.

1. (a)Explain why a locating agent is needed and how it is used.[2]
2. (b)After the run, one amino acid is at 3.5 cm and the other at 5.25 cm; the solvent front is at 7.0 cm. Calculate both Rf values.[2]
3. (c)Explain why the two amino acids end up at different heights.[2]