Pure substances

Different tests can be used to identify chemicals.

A student measured the melting points of four substances.

Table 1 shows the results.

Which substance was pure?

Give one reason for your answer.

Anhydrous copper sulfate can be used to test for water.

The word equation for the reaction is:

anhydrous copper sulfate + water hydrated copper sulfate

Complete the sentence.

Choose answers from the box.

When water is added to anhydrous copper sulfate, the colour changes

The reaction between anhydrous copper sulfate and water is reversible.

How does the word equation show that the reaction is reversible?

The formula of anhydrous copper sulfate is CuSO₄

What is the total number of atoms in the formula CuSO₄?

Tick () one box.

Chlorine is a gas.

Describe the test for chlorine.

Give the result.

Some types of water contain dissolved substances.

A student investigated the mass of dissolved solids in distilled water and in sea water. Figure 5 shows the apparatus.

This is the method used.

1. Weigh an evaporating basin.

2. Add a sample of distilled water to the evaporating basin.

3. Weigh the evaporating basin and the water sample.

4. Heat the water sample.

5. Weigh the evaporating basin and contents.

6. Repeat steps 1 to 5 two more times.

7. Repeat steps 1 to 6 with sea water.

Give one safety precaution the student should take.

The method used did not give valid results. Which two improvements could the student make to give valid results? Tick (✓) two boxes.

A different student used a method which gave valid results. Table 2 shows the results.

The student concludes that distilled water is pure. Suggest one reason why.

Tap water is potable. One step in producing potable water is sterilisation.

Why is potable water sterilised?

Tick (✓) one box.

Chlorine is used to sterilise water. The reaction between chlorine and water is reversible. Complete the sentence.

Equilibrium is reached when the forward and reverse reactions

The test for chlorine uses damp litmus paper. Complete the sentence. Choose the answer from the box.

When damp litmus paper is put into chlorine gas

There is evidence that adding fluoride ions to drinking water reduces tooth decay.

Figure 6 shows the percentage of people showing tooth decay.

Compare the effects of adding fluoride ions to drinking water on tooth decay in people between the ages of 25 and 74. Use data from Figure 6.

Student A says that water companies should add fluoride ions to all drinking water. Student B disagrees. Suggest one reason why Student B may think that fluoride ions should not be added to all drinking water.

Some types of water contain dissolved substances.

A student investigated the mass of dissolved solids in distilled water and in sea water. Figure 1 shows the apparatus.

This is the method used.

1. Weigh an evaporating basin.

2. Add 20 cm³ of distilled water to the evaporating basin.

3. Weigh the evaporating basin and the water sample.

4. Heat the water sample for 2 minutes.

5. Weigh the evaporating basin and contents.

6. Repeat steps 1 to 5 two more times.

7. Repeat steps 1 to 6 with sea water.

The method used by the student did not give valid results. Describe one improvement the student could make to obtain valid results.

A different student used a method which gave valid results.

Table 2 shows the results.

Calculate the value X for the mass of dissolved solids in sea water in Test 2.

The student concludes that distilled water is pure. Describe a test to confirm that distilled water is pure.

Tap water is potable.

A stage in the production of potable water is sterilising. A gas is used to sterilise water. The equation for the reaction is:

What is meant by the symbol ?

The reaction is at equilibrium. The reaction is exothermic. What happens to the equilibrium position when the temperature is increased? Tick (✓) one box.

Describe a test to identify the gas used to sterilise water.

Another stage in the production of potable water is filtering. Explain why potable water contains dissolved solids after filtering.

The methods used to produce potable water depend upon available sources of water.

Suggest how copper sulfate can be used as a test for the presence of water.

The boiling point is used to check the purity of a sample of water.

In chemistry, what is meant by a ‘pure substance’?

The boiling point of a 250 g sample of water was 100.60 °C.

The boiling point of pure water in a data book is 100.00 °C.

Each 1% of impurity increases the boiling point of water by 0.12 °C.

Calculate the mass of the impurity in the sample of water.

Explain how distillation is used to obtain potable water from salty water.

Obtaining potable water from salty water is more expensive than obtaining potable water from ground water.

Explain why.

Refer to the processes used in both methods in your answer.

This question is about mixtures.

Which substance is a mixture? Tick (✓) one box.

Food colourings are often mixtures of dyes. What name is given to mixtures that are designed as useful products?

A student investigated a purple food colouring, Y, using chromatography. The student compares Y with dyes A, B and C.

Figure 8 shows the apparatus used.

Chromatography involves a stationary phase and a mobile phase.

Draw one line from each phase to what is used for that phase. Use Figure 8. Phase What is used

Figure 9 shows the student’s results.

What three conclusions can you make about the dyes in food colouring Y ?

In a different experiment a student recorded these results: Distance moved by dye G = 60 mm Distance moved by solvent = 80 mm Calculate the R_f value of dye G.

Water that is safe to drink contains dissolved substances.

What do we call water that is safe to drink?

Tick (✓) one box.

Describe a test for pure water.

Give the result of the test if the water is pure.

Describe a method to determine the mass of dissolved solids in a 100 cm³ sample of river water.

A sample of river water contains 125 mg per dm³ of dissolved solids.

Calculate the mass of dissolved solids in grams in 250 cm³ of this sample of river water.

Give your answer to 2 significant figures.

A water company allows a maximum of 500 mg per dm³ of sulfate ions in drinking water.

A sample of drinking water contains 44 mg per dm³ of sulfate ions.

Calculate the percentage (%) of the maximum allowed mass of sulfate ions in the sample of drinking water.

Water that is safe to drink contains dissolved substances.

What do we call water that is safe to drink? Tick (✓) one box.

Describe a test for pure water. Give the result of the test if the water is pure.

Describe a method to determine the mass of dissolved solids in a 100 cm³ sample of river water.

A sample of river water contains 125 mg per dm³ of dissolved solids. Calculate the mass of dissolved solids in grams in 250 cm³ of this sample of river water. Give your answer to 2 significant figures.

A water company allows a maximum of 500 mg per dm³ of sulfate ions in drinking water. A sample of drinking water contains 44 mg per dm³ of sulfate ions. Calculate the percentage (%) of the maximum allowed mass of sulfate ions in the sample of drinking water.