Water & Water Pollution (Cambridge (CIE) O Level Chemistry): Exam Questions

This question is about water.

Describe a chemical test which shows the presence of water.

Give the test and the colour change seen 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.

Rain water is collected in reservoirs.

How is it treated before entering the water supply?

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.

i) State the name of each of the following changes of state.

H₂O (l) → H₂O (g)

H₂O (g) → H₂O (l)

[2]

ii) Which one of the above changes of state is exothermic? Explain your choice.

[1]

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

i) Explain how acid rain is formed. 

[4]

ii) Fish live in water which is neutral (neither acidic nor alkaline). Acid rain decreases the pH of water in lakes and rivers. Both of the bases, calcium oxide and calcium carbonate, can neutralise this acid and increase the pH.

Explain why calcium carbonate is a better choice. 

[2]

Waste water from homes and industry 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 sedimentation.

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 natural sources contains substances that have entered it in different ways.

Nitrate and phosphate are two of these substances.

State the source of the nitrate and the source of the phosphate.

Sewage may also be present in water from natural sources.

Explain why sewage in drinking water is harmful to people.

Plastics are found in water from some natural sources.

State why plastics in water are potentially harmful.

A river runs past a housing estate.

Suggest one way in which sewage could enter the river.

Treated tap water is safe to drink, but it is not pure.

State two substances that may still be present in treated tap water.

Explain why the stages used in the treatment of the domestic water supply do not remove these substances.

A student says that tap water must be pure because it is safe to drink.

Explain why safe to drink and pure do not mean the same thing.

A village can take its drinking water from a borehole or from a river.

The table shows an analysis of both sources, together with the safe limit for each substance. Every value is given in mg / dm³.

Using the table, state which source exceeds a safe limit.

Identify the substance responsible.

Both sources contain lead compounds and calcium compounds.

Explain why a safe limit is set for lead compounds but not for calcium compounds.

Recommend which source the village should use.

Justify your recommendation using the table.

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 micro-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). 

i) Name the type of reaction that occurs between ammonia and sulfuric acid. 

[1]

 ii) Write a chemical equation for the reaction of ammonia with sulfuric acid. 

[1]

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) sulfate, CuSO₄∙xH₂O, was gently heated in a crucible until the mass remaining was 4.00 g. This is the white anhydrous copper(II) sulfate.

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

A_r: Cu = 64, S = 32, O = 16, H = 1

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

i) State the colour change when cobalt chloride paper is added to water. 

[1]

 ii) When hydrated cobalt chloride is heated, water is given off. Write the balanced symbol equation for this reaction.  

[2]

A water company tests the water in a river. The river water is treated at a works before it is supplied to homes.

The table shows some of the substances found in the river water. Each concentration is given in mg / dm³.

State why the dissolved oxygen in the river water is beneficial.

Calcium compounds and lead compounds are both metal compounds.

Explain why these two metal compounds have different effects on the people who drink the treated water.

The river also contains nitrate and phosphate.

Explain why high concentrations of nitrate and phosphate damage the aquatic life in the river.

At the treatment works the river water passes through a sedimentation tank and then a filtration tank. Chlorine is added to the water after it leaves the filtration tank.

Explain why chlorine must be added even though the water has already passed through both tanks.

A student is given three unlabelled samples of water, A, B and C. One of the samples is pure water.

The student measures the temperature at which each sample melts and the temperature at which each sample boils.

Identify the pure sample.

Justify your answer using both columns of the table.

Describe how dissolved substances change the melting temperature and the boiling temperature of water.

The student measures the pH of sample B and finds that it is exactly 7.

The student concludes that sample B must be pure water.

Explain why this conclusion is wrong.

A student is told to use distilled water, and not tap water, when preparing solutions in the laboratory.

To find out why, the student evaporates 100 cm³ of distilled water and 100 cm³ of tap water to dryness in two separate evaporating basins.

A white solid is left in the basin that held the tap water. The basin that held the distilled water is empty.

Explain what this result shows about the two types of water.

The student's teacher says that using tap water could make an experiment give a false result.

Suggest how a dissolved substance in tap water could lead a student to a wrong conclusion.

Before reaching the laboratory tap, the water has passed through a bed of carbon at the treatment works.

Explain why the student still cannot use this tap water in place of distilled water.

A student sorts four substances found in water from natural sources into two groups.

The student has made one error in the table.

Identify the error.

Explain why it is an error.

State why sewage is correctly placed in the potentially harmful group.

Phosphate is also correctly placed in the potentially harmful group.

Explain the harm that phosphate causes.

State the two sources from which phosphate enters water from natural sources.

A student measures the nitrate and phosphate concentrations in a river at a point upstream of a works that discharges treated waste water, and at a point downstream of it.

Both concentrations are higher downstream.

State the effect this will have on the river downstream.

Name the substance in the water that will decrease as a result.

The student writes the following conclusion.

The downstream water is more harmful, so it must contain more lead compounds.

Explain why this conclusion is not supported by the student's measurements.

Suggest one further measurement the student should have made at both points.

Explain why it would be useful.