Water & Water Pollution (Cambridge (CIE) IGCSE Chemistry): Exam Questions

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.

i) Rain water is collected in reservoirs. How is it treated before entering the water supply?

 [2]

 ii) State two uses of water in the home.

[2]

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)

 name ..................................................

 H₂O (g) → H₂O (l)

 name ..................................................

 [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.

Separate: Chemistry Only

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]

Separate: Chemistry Only

Plants require three main elements for healthy growth which be provided by fertilisers.

Give the formulae of three ions used to make fertilisers which provide the three required elements.

Calcium nitrate is used as a fertiliser to help prevent blossom end rot in tomatoes which causes browning of the bottom of the fruit. It provides both calcium and nitrogen.

Write the formula for calcium nitrate.

Separate: Chemistry and Extended Only

Using your answer to part (b) calculate the percentage by mass of nitrogen in calcium nitrate.

(A_r: Ca = 40, N = 14, O = 16) 

Separate: Chemistry Only 

Explain why salts from fertilisers end up in bodies in groundwater.

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.

Fertilisers are used to promote plant growth.

Two fertilisers are ammonium phosphate, (NH₄)₃PO₄, and calcium dihydrogenphosphate,Ca(H₂PO₄)₂.

Describe a test to distinguish between these two fertilisers.

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

result ..................................................

Separate: Chemistry and Extended Only

Many fertilisers are manufactured from ammonia.

Describe how ammonia is made in the Haber process. Give the essential conditions and an equation for the process.

Separate: Chemistry Only

State the essential plant nutrient not supplied by ammonium phosphate.

Separate: Chemistry and Extended Only

The soluble compound, calcium dihydrogenphosphate is made by heating the insoluble mineral rock phosphate, Ca₃(PO₄)₂, with sulfuric acid.  

i) Why would rock phosphate not be effective as a fertiliser?

 [1]

 ii) The phosphate ion, PO₄^3–, from the rock phosphate is changed into the dihydrogenphosphate ion, H₂PO₄^–.

 PO₄^3– + 2H₂SO₄→ H₂PO₄^–+ 2HSO₄^– 

 What type of reagent is the phosphate ion? Give a reason for your choice.

 [2]

Separate: Chemistry Only

The Atacama desert in Chile has deposits of the salt sodium nitrate.

Very large amounts of this salt were exported to Europe for use as a fertiliser. After the introduction of the Haber process in 1913, this trade rapidly diminished.

 i) Explain why the introduction of the Haber process reduced the demand for sodium nitrate.

 [2]

 ii) Suggest why surface deposits of sodium nitrate only occur in areas with very low rainfall such as desert areas.

 [1]

 iii) The desert has smaller surface deposits of potassium nitrate. Suggest why potassium nitrate is a better fertiliser than the sodium salt.

 [1]

Extended Only

The equation for the decomposition of copper(II) nitrate is given below.

2Cu(NO₃)₂ → 2CuO + 4NO₂ + O₂

Copper(II) nitrate forms a series of hydrates with the formula Cu(NO₃)₂.xH₂O.

All these hydrates decompose to form copper(II) oxide.

1 mole of Cu(NO₃)₂.xH₂O forms 1 mole of CuO.  

i) What is meant by 1 mole of a substance?

 [1]

ii) 7.26 g of a hydrate, Cu(NO₃)₂.xH₂O, formed 2.4 g copper(II) oxide.

number of moles of CuO formed = ...................... number of moles of Cu(NO₃)₂.xH₂O in 7.26 g = ...................... mass of 1 mole of Cu(NO₃)₂.xH₂O = ...................... g mass of 1 mole of Cu(NO₃)₂ is 188 g the value of x in this hydrate = ......................

 [4]

Groundwater can contain fertilisers washed from fields. An example of a fertiliser that can be found is ammonium nitrate. 

i) Write a balanced equation, including state symbols, to show how ammonium nitrate can be made by a student via a neutralisation reaction. 

 [2]

 ii) Describe how the student could perform a chemical test for the presence of nitrate ions.

 [2]

Separate: Chemistry and Extended Only

Drinking water extracted from rivers in agricultural areas often contain nitrates. The first legal standard for nitrate in drinking water was set in 1980. The current UK drinking water standard is 50 mg / dm³ to safeguard against cases of methaemoglobinaemia or blue baby syndrome. 

Calculate the maximum mass in grams of nitrate that would be safe in a 240 cm³ glass of water. Give your answer to two significant figures.

Separate: Chemistry Only

An alternative fertiliser is ammonium phosphate.   

i) Give the formula for ammonium phosphate.

 [1]

 ii) Which ion that plants need is missing from this fertiliser?

 [1]

Separate: Chemistry and Extended Only

Phosphates can be found in reservoirs from which drinking water is obtained. A maximum of 0.025 mg / dm³ of phosphate ions is allowed by water companies. 

A sample of reservoir water has a concentration of 0.00014 mol / dm³.

Calculate the percentage of the maximum mass allowed of phosphate ions in the sample of reservoir water.

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.

Separate: Chemistry Only

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.

Separate: Chemistry and Extended Only

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]

Separate: Chemistry and Extended Only

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 = ..............

Separate: Chemistry Only

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]