Pre assessment 4 pure 2025

A colourless substance is made in an experiment. Which are the possible methods to determine if this substance is pure?

• Measure the melting point of the substance and compare with reference value.

• Measure the boiling point of the substance and compare with reference value.

• Test the substance using paper chromatography and a locating agent.

Which of the following apparatus will be most suitable to measure an exact volume of 24.8 cm³ of hydrochloric acid?

• (Image of a pipette)

• (Image of a burette)

• (Image of a measuring cylinder)

• (Image of a syringe)

When testing for ammonia, chlorine, hydrogen and oxygen, what is the minimum number of items from the table above needed to identify these four gases?

• 2

• 3

• 4

• 5

Y is an alkaline gas that is very soluble in water. It is also less dense than air. Which is the best method to collect a sample of dry Y?

• A (Diagram A)

• B (Diagram B)

• C (Diagram C)

• D (Diagram D)

For two substances to be separable by paper chromatography, it is necessary that

• they are both liquids.

• they have different densities.

• they are both soluble in the same solvent.

• they have different colours.

The chromatogram of a sweet is shown below. Which of the following statements can be deduced from the chromatogram?

• A. The Rf value of the blue dye is greater than that of the red dye.

• B. The red dye is more soluble than the blue dye.

• C. The sweet is blue.

• D. The molecules of the blue dye are heavier than that of the red dye.

An unknown substance X, together with three known substances P, Q and R, were analysed by chromatography using two different solvents – water and ethanol. The diagram below shows the final chromatogram. Which one of the three substances P, Q and R is found in X?

• X is substance P.

• X is substance Q.

• X is substance R.

• X could be either substance P or Q.

What is the chemical formula of iron(III) oxide?

• FeO

• Fe₃O

• Fe₃O₂

• Fe₂O₃

During Valentine’s Day, a nurseryman’s catalogue claims that the brilliant colour of a new rose he is offering for sale is due to the fact that its petals contain the same colouring matter as the flowers of the scarlet geranium. Crystal, anxious to test the truth of the claim, dissolves the colouring matter out of the petals of each flower in a suitable solvent. Which of the following methods would you advise her to do?

• Mix the two solutions together and see if they are miscible.

• Perform crystallisation to determine if they give the same crystals.

• Check if the two solutions distil over at the same temperature.

• Place a few drops of each solution on separate pieces of filter paper and watch how the colouring matter spreads out in a suitable solvent.

Two liquids, X and Y, are placed in a separating funnel. Two layers are formed as shown in the following diagram. From this observation, we can deduce that liquid Y

• has a lower boiling point than X.

• is more soluble than X.

• is denser than X.

• could be water.