Methods of Separating & Purifying Substances Practice Questions

Which statement describes a pure substance in chemistry?

Put a cross ( ☒ ) in the box next to your answer.

☐ A a substance that is safe to drink

☐ B a substance that contains no harmful chemicals

☐ C a single element or compound, not mixed with any other substance

☐ D a substance that has not been made in a factory

Explain why a chemist would not describe the spring water as pure.

Water used in chemical analysis must not contain any dissolved salts.

State why spring water should not be used in chemical analysis.

Identify the sample that is pure salicylic acid. Use the data in the table to justify your answer.

The student says that sample A contains more impurity than sample C.

Use the data in the table to explain whether the student is correct.

In this experiment, which is the stationary phase?

Put a cross ( ☒ ) in the box next to your answer.

☐ A the water

☐ B the chromatography paper

☐ C the ink

☐ D the beaker

State why the start line is drawn in pencil and not in pen.

State why the level of the water in the beaker must be below the start line.

Calculate the Rf value of spot A.

Identify the two dyes present in ink X.

State how the chromatogram shows that ink X is not a pure substance.

Name the technique used to separate the three liquids from each other.

Describe how a pure sample of methanol is obtained from this mixture. Use the data in the table in your answer.

A sample of garden fertiliser is a mixture of potassium nitrate and small pieces of insoluble grit. Potassium nitrate is soluble in water.

Describe how a student could obtain dry crystals of potassium nitrate from this mixture.

A student set up the apparatus shown in the figure to obtain pure water from salty water. The flask contains salty water, which is being heated.

The apparatus has faults.

State two faults in this apparatus and explain the problem each one causes.

The treated water still contains some dissolved substances.

Explain why this water can still be described as potable.

State the purpose of the filtration stage and the purpose of the chlorination stage.

In some hot countries, potable water is made from sea water by distillation.

Explain why sea water can be made potable by distillation but not simply by filtering it.

A chemist is given a mixture containing sand, ethanol, water and sodium chloride. The sodium chloride is dissolved in the liquid.

Substance	Properties
sand	solid, insoluble in water and in ethanol
sodium chloride	solid, soluble in water, melting point 801 °C
ethanol	liquid, boiling point 78 °C
water	liquid, boiling point 100 °C

Devise a method the chemist could use to obtain, from this mixture, a dry sample of sand, a pure sample of ethanol and dry crystals of sodium chloride.

Use the properties in the table to justify each stage of your method.

One of the dyes in the colouring has an Rf value of 0.65 in this solvent.

Calculate the distance moved by the spot made by this dye.

Explain, in terms of the mobile phase and the stationary phase, why the two dyes in the green colouring moved different distances up the paper.