Water & Water Pollution (Cambridge O Level Chemistry)

This question is about water.

Describe a chemical test which shows the presence of water.

test ..................................................

colour change if water is present  ..................................................

How could you show that a sample of water is pure?

Describe how water is treated before it is supplied to homes and industry.

Water is needed for industry and in the home.

In many regions, drinking water is obtained by the distillation of sea-water. Explain how distillation separates the water from sea-water.

The diagram below shows part of the Water Cycle.

The rain drains into rivers and then into reservoirs. Describe how water is treated before it enters the water supply.

Waste water like this needs to be treated before being released into the environment. Waste water may require removal of organic matter and harmful chemicals. The three main steps are sedimentation, filtration and chlorination.

Explain what happens in sedimntation.

Explain why chlorination is required.

Table 5.1 shows some information about how the treated solid sludge was disposed of from one sewage treatment plant in 2000 and 2020.

Table 5.1

Calculate the percentage of treated solid sludge that was sent to landfill in 2000.

Give your answer to 3 significant figures.

The total mass of treated solid sludge increased from 2000 to 2020 and the proportions of how the sludge was disposed of changed.

Suggest one reason for the increase in the total mass of treated solid sludge and two reasons for the changes in the proportions of the disposal methods.

Use Table 5.1 to help you answer.

Water from reservoirs is treated to make it suitable for drinking. Even though the water has been treated it is not pure. One method to purify water is de-ionisation.

Describe another method which will produce pure water.

Explain why it is beneficial to drink water that is not deionized.

Chlorine is added to drinking water in order to make it safe to drink. The chlorine kills miro-organisms that can cause illnesses such as cholera and typhoid.

Suggest a concern with adding chlorine to drinking water.

When chlorine is added to water it undergoes a reaction which forms hydrogen chloride and chloric(I) acid. The chloric(I) acid is the compound that kills the bacteria. 

Cl₂ + H₂O → HCl + HClO

In this reaction chlorine is both oxidised and reduced. Explain why.

A scientist finds an unlabelled bottle on a shelf.

She thinks the bottle contains a solution of ammonium sulfate, (NH₄)₂SO₄.

Describe tests the scientist could do to show that the solution is ammonium sulfate.

Ammonium sulfate is often used as a fertiliser.

It is prepared by reacting ammonia (NH₃) with sulfuric acid (H₂SO₄).

A common test for water is to use anhydrous copper(II) sulfate. If water is present an exothermic reaction takes place and the solid turns from white to blue and becomes hydrated copper(II) sulfate. This reaction is reversible.

6.25 g of blue hydrated copper(II) sulphate, CuSO₄.xH₂O was gently heated in a crucible until the mass remaining was 4.00 g. This is the white anhydrous copper(II) sulphate. 

Use this information to calculate a value for x in CuSO₄.xH₂O.  

A_r's Cu = 64, S = 32, O = 16, H = 1 

x = ..............

Cobalt(II) chloride paper is used to test for water as well. Hydrated cobalt chloride has the formula CoCl₂∙6H₂O.