Elements, Compounds and Mixtures

Explain why crystallisation is a good technique for separating sodium chloride from its aqueous solution.

State the name of the technique used to separate an insoluble solid from a liquid.

The diagram shows the apparatus used for simple distillation.

A mixture of water and ethanol is heated. Water has a boiling point of 100°C and ethanol has a boiling point of 78°C. If the thermometer reads 78°C while distillate is being collected, which liquid is distilling over? Calculate how many degrees lower this temperature is than the boiling point of water.

A sample of rock salt contains 16 g of sodium chloride mixed with 4 g of insoluble impurities. A student dissolves the sample in water, filters the solution, and then uses crystallisation to obtain sodium chloride crystals. What is the maximum mass of sodium chloride crystals the student can expect to obtain after crystallisation? Assume no losses occur during the process.

A student carries out chromatography to separate the dyes in two different food colourings, X and Y. The chromatogram below shows the results. Each spot represents a dye.

State which part of the chromatography paper you should draw the starting line on before adding the sample in a paper chromatography experiment.

The chromatogram below was obtained by separating a mixture of food dyes A and B. The solvent front travelled 6.0 cm from the pencil line. The diagram shows the position of the spots:

(a) State how the R_f value for dye B can be calculated. [1]

(b) Calculate the R_f value for dye B. Give your answer to two decimal places. [2]

In a paper chromatography experiment, the pencil line was drawn 1.0 cm from the bottom of the paper. The solvent front travelled 7.2 cm from the pencil line, and a blue dye moved 5.0 cm from the pencil line. Calculate the R_f value for the blue dye.

Explain why two different dyes can have the same colour but different R_f values in paper chromatography.