10.1 Water & Water Pollution

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.

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.
 

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

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. 
 

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).

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.

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.

Separate: Chemistry Only

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.