Pure substances

Figure 5 shows the structure of a molecule of compound S.

Use Figure 5 to deduce the empirical formula of compound S.

The melting points of three samples of S are shown in Figure 6.

State whether each of these samples, A, B and C, is pure or impure and justify your answers using the information in Figure 6.

A scientist uses chromatography in an investigation of compound S.

In the conditions used, compound S has an R_f value of 0.22.

Calculate the distance the spot of compound S moves if the solvent front has moved by 2.4 cm.

distance =_........ cm

*(c) A solution of sodium chloride in water needs to be separated to obtain a sample of pure, dry sodium chloride and a sample of pure water.

Figure 7 shows the boiling points of sodium chloride and water.

Explain this difference in boiling points in terms of the structure and bonding of sodium chloride and water and how this difference is used to choose a method to separate sodium chloride solution into pure, dry sodium chloride and pure water.

Potable water is water that is suitable for drinking.

River water can be treated to make it potable. Chlorination, filtration and sedimentation are three of the processes involved in making the river water potable.

Which row of the table shows these three processes in the order in which they are carried out?

State the reason why chlorine is added during the water treatment.

Describe how sedimentation is carried out.

Figure 3 shows the results of an analysis of a sample of potable water.

Using this information, explain why this sample of potable water is not the same as pure water.

A student wanted to distil a sample of potable water. Figure 4 shows apparatus the student used.

Name the piece of equipment labelled X in Figure 4.

The student made an error when setting up the equipment in Figure 4. This error meant no water could be collected in the test tube.

Explain what the student needs to do so water can be collected.

If liquid water is cooled below 0 °C it turns into the solid, ice.

Give the name for the change of state from liquid to solid.

Here are five statements about ice and water.

Place ticks in boxes by the two statements that are correct.

Figure 1 shows a label from a bottle of drinking water.

Explain why this drinking water should not be described as pure water.

State the information from Figure 1 that shows that the drinking water is neutral.

Calculate the mass of calcium ions in 250 cm³ of this drinking water.

mass =_........ mg

State how you know that calcium is a metal from its position in the periodic table.

A sample of potable water contains impurities.

Why is this sample of water potable even though it contains impurities?

Waste water can be used to produce drinking water. The processes used include sedimentation, filtration and chlorination.

What is sedimentation?

State why the waste water is filtered.

State the reason for chlorination.

Some salts can be added to waste water to remove impurities. In an experiment, different masses of salt A were added to 1000cm³ samples of waste water. The experiment was repeated with salt B. The percentages of impurities removed from the waste water are shown in Figure 7.

mass of salt in mg per 1000cm³ water

Figure 7

It was concluded that the best way to purify 1000cm³ of the waste water is to add 100 mg of salt B.

Use the information about salt A and salt B in Figure 7 to evaluate this conclusion.

*(d) A sample of water was contaminated with a dissolved solid.

Devise a plan to separate pure water from this mixture, including a test to show that the water obtained is neutral.

You may use some or all of the apparatus shown in Figure 8 and any other laboratory apparatus.

A sample of potable water contains impurities.

Why is this sample of water potable even though it contains impurities?

Waste water can be used to produce drinking water. The processes used include sedimentation, filtration and chlorination.

What is sedimentation?

State why the waste water is filtered.

State the reason for chlorination.

Some salts can be added to waste water to remove impurities. In an experiment, different masses of salt A were added to 1000cm³ samples of waste water. The experiment was repeated with salt B. The percentages of impurities removed from the waste water are shown in Figure 3.

mass of salt in mg per 1000cm³ water

Figure 3

It was concluded that the best way to purify 1000cm³ of the waste water is to add 100 mg of salt B.

Use the information about salt A and salt B in Figure 3 to evaluate this conclusion.

Waste water may contain phosphate ions, PO₄^3-.

Aluminium ions react with phosphate ions to form aluminium phosphate.

Complete the ionic equation for the formation of aluminium phosphate in this reaction.

_........ + PO₄^3- _........

Mixtures of substances can be separated using different techniques.

Which of the following is a mixture of substances?

Figure 3 shows the apparatus that a student set up to obtain pure water from ink.

There are three mistakes in the way the apparatus has been set up.

One mistake is that the bulb of the thermometer is too low.

The bulb of the thermometer should be level with the side arm.

Give a reason why the bulb of the thermometer should be level with the side arm.

State one other mistake in Figure 3.

Paper chromatography is used to separate the substances in five different food colourings, P, Q, R, S and T.

Figure 4 shows the chromatogram at the end of the experiment.

The steps needed to carry out the chromatography experiment are listed below. They are not in the correct order.

1 leave the solvent to rise up the paper 2 put solvent in the beaker 3 draw a start line on the piece of paper 4 place the paper in the beaker 5 remove the paper when the solvent is near the top 6 put small spots of the food colourings on the start line

List the steps in the correct order.

The first two steps have been done for you.

Explain, using Figure 4, which food colouring contains the greatest number of coloured substances.

During chromatography of the food colourings, the solvent front moved 8.00 cm and the food colouring R moved 2.30 cm.

Calculate the R_f value for food colouring R. Give your answer to two significant figures.

R_f value =_........