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GCSE Questions and Answers

Hard Water

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6 consecutive GCSE Chemistry papers: 2002-7

2 Water is the most important solvent known to man.a) Give one chemical test for water and the result you would expect to observe if water was present.

Test: _______________________________ [1]

Observations: ______________________________________________________________ [2]

2002, Paper 2

2 Water is the most important solvent known to man.a) Give one chemical test for water and the result you would expect to observe if water was present.

Test: (anhydrous) cobalt chloride (paper) [1] or anhydrous copper sulphate [1]Observations: Blue [1] to pink [1] or white [1] to blue [1]

Hydrated copper (II) sulphate

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CuSO4 + 5H2O CuSO4.5H2O

White BlueAnhydrous Hydrated--------------------------------------------------

CoCl2 + 6H2O CoCl2.6H2O

Blue PinkAnhydrous Hydrated---------------------------------------------------

Na2CO3 + 10H2O Na2CO3.10H2O

White WhiteAnhydrous Hydrated

I know that the above processes are physical changes that can be reversed using heat

From “Learning Outcomes”

b) Public water supplies go through two main processes to convert reservoir water to drinking water.

(i) Write the name of each process in the blank boxes in the diagram.

Reservoir

Water →

Physical

Process

Chemical

Process Drinking

→ Water→

b) Public water supplies go through two main processes to convert reservoir water to drinking water.

(i) Write the name of each process in the blank boxes in the diagram.

Reservoir

Water →

Physical

Process

Chemical

Process Drinking

→ WaterFiltration

[1]

→ Chlorination

[1]

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(ii) What is the purpose of chemical process?

__________________________ [1]

(ii) What is the purpose of chemical process?

Sterilise / kills germs or bacteria [1]

c) A student tested four samples of water (each 5cm3) from different areas of Northern Ireland by shaking with 3 drops of soap solution. Another 5cm3 of each sample of water was boiled and the soap solution test was repeated. The observations were recorded in the table.

Sample Observation with soap solution

Observation for boiled sample with soap solution

A No lather Lather

B Lather Lather

C Lather Lather

D No lather No lather

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From our “Learning Outcomes

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CaCl2 MgCl2

CaSO4 MgSO4

Ca(HCO3) 2 Mg(HCO3) 2

Main calcium and magnesium salts causing hardness

(i) Which two samples contain hard water?1. _____________________2. _____________________ [2]

(ii) Which one sample contains temporary hard water?

_______________________________ [1]

(iii) Explain how you worked out your answer to part (c)(ii).

_________________________________________________________________ [1]

(i) Which two samples contain hard water?1. A [1]2. D [1]

(ii) Which one sample contains temporary hard water?

A [1]

(iii) Explain how you worked out your answer to part (c)(ii).

Hardness removed [1] by boiling [1]

(iv) Explain how temporary hardness arises in rainwater. ______________________________

___________________________ [3]

(v) Give a balanced symbol equation for the reaction in part (c)(iv). ___________________________ [2]

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The two equations may be combined as follows:

CaCO3 + H2O + CO2 Ca(HCO3)2

(iv) Explain how temporary hardness arises in rainwater.

Reaction [1] between calcium carbonate / CaCO3 / limestone [1] (inrocks) and dissolved carbon dioxide / CO2 [1] in rain water.

(v) Give a balanced symbol equation for the reaction in part (c)(iv).

CaCO3 + H2O + CO2 → Ca (HCO3)2 or CaCO3 + H2CO3 → Ca(HCO3)2

d) The sample of permanent hard water may be softened using washing soda.

i) Explain, in terms of ions, how permanent hard water is softened using washing

soda. _________________________________ ______________________________ [3]

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Washing soda (sodium carbonate) Calcium + Sodium Calcium + Sodium Carbonate Carbonate Chloride Chloride  CaCl2 + Na2CO3 CaCO3 + Na2CO3

 This is the favoured method in industry.

The precipitation of Ca2+ and Mg2+ carbonates is instant

The sodium salts left behind in their place do not cause hardness

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d) The sample of permanent hard water may be softened using washing soda.

i) Explain, in terms of ions, how permanent hard water is softened using washing soda.

Precipitation/removal [1] of calcium ions/Ca2+ or magnesium ions/Mg2+ [1] by reaction with CO2-

3 ions [1] or the calcium ions/Ca2+ or magnesium ions Mg2+ [1] (in the hard water) react with the carbonate ions/CO2-

3 [1] (in the washing soda) to form a ppte/of solid (calcium carbonate) [1]

(ii) Give a balanced symbol equation for the reaction in part (d)(i).

____________________________________ [2]

(ii) Give a balanced symbol equation for the reaction in part (d)(i).

Equation: Ca2+ + CO2-3 → CaCO3

or Mg2+ + CO2-3 → MgCO3

or Full equation with Na2CO3 and CaSO4/CaCl2/MgSO4/MgCl2

(e) Give two advantages and two disadvantages of hardness in a town’s water supply.

Advantages: 1.____________________2.____________________

Disadvantages: 1. ___________________ 2. ___________________

[4]

e) Give two advantages and two disadvantages of hardness in a town’s water supply.

Advantages: tastes better, used to brew beer, used to tan leather, helps to prevent heart disease.

Any two. [2]

Disadvantages: produces scum (with soap) produces scale (when boiled), wastes soap, damages delicate fibres. [2]

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Advantages Disadvantages It tastes better It provides useful calcium ions

for the healthy growth of bones and teeth.

It is better for brewing beers and spirits

It is thought to reduce heart disease

Lime scale in pipes causes the inside of the pipe to be covered with insoluble carbonates.This layer of carbonate prevents the water in the pipe from coming into contact with the metal of the pipeand so prevents pipe corrosionand poisonous metal salts becoming dissolved in the water.

Lime scale furring of kettles and pipes.

The lime scale can build up on the inside of the pipe restricting the flow of water or causing a blockage.Radiators affected in this way radiate less heat into the room.This can happen in industry where hot water is used or in domestic heating systems (like the hot water in your house).

Soap is wasted because more soap is required for washing.

Scum can spoil special finishes on some fabrics.

From our “Learning Outcomes”

5a) The label below is taken from a bottle of sparkling mineral water and shows the presence and concentration of ions in mg per litre.

2003, Paper 2

(i) Which one of these ions is present in the greatest concentration?___________________________ [1]

(ii) Which of these ions is needed to make strong teeth and healthy bones?

___________________________ [1]

(i) Which one of these ions is present in the greatest concentration?Hydrogen carbonate [1]

(ii) Which of these ions is needed to makes strong teeth and healthy bones?

Calcium [1]

(iii) This water is “sparkling”. What gas is used to make it fizzy?

_______________________________ [1]

(iv) Use the information from the label to give the name and formula of two compounds which could be present in this mineral

water. [4]Name ___________ Formula __________Name ___________ Formula __________

(iii) This water is “sparkling”. What gas is used to make it fizzy?

Carbon dioxide [1]

(iv) Use the information from the label to give the name and formula of two compounds which could be present in this mineral

water. [4]Name Calcium sulphate [1] Formula CaSO4[1]

Name Calcium chloride [1] Formula CaCl2 [1]

(v) An open bottle of sparkling mineral water was placed in the fridge for a day, and

another placed in sunlight. Which bottle of water will be less fizzy at the end of the day? Explain your answer.

___________________________________________________________________________________________________ [3]

(v) An open bottle of sparkling mineral water was placed in the fridge for a day, and

another placed in sunlight. Which bottle of water will be less fizzy at the end of the day? Explain your answer.

Bottle in sunlight [1] increase in temperature [1] decrease solubility of gases [1]

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It is a good idea to boil tap water to be used to refill a goldfish bowl.This removes any dissolved chlorine that may harm fish.However, it also removes the dissolved air.Leaving the water to cool and sit overnight will give air time to dissolve once again in the water

(vi) Describe how you would find the approximate pH of the sparkling mineral water.

________________________________________________________ [2]

(vi) Describe how you would find the approximate pH of the sparkling mineral water.

Universal solution /pH paper [1] compare colour to pH chart [1]

b) 25cm3 samples of water A, B, C and D were tested with soap solution. The volume of soap solution required to produce a lather that lasted for a minute was recorded. Fresh samples of each were boiled and tested again with soap solution. The results are shown below.

Water Sample

Volume of soap solution required

(cm3) before boiling

Volume of soap solution required (cm3) after boiling

A 5.0 5.0

B 1.0 1.0

C 11.0 6.0

D 9.0 1.0

i) What is meant by the term “hard water”?

_________________________ [2]

(ii) Which of the four samples contains temporary hardness only?_________________________ [1]

i) What is meant by the term “hard water”?

Does not later [1] easily with soap [1]

(ii) Which of the four samples contains temporary hardness only?Sample D [1]

(iii) Which of the four samples contains both permanent and temporary

hardness? Explain your answer.____________________________________________________________________________________ [3]

(iii) Which of the four samples contains both permanent and temporary

hardness? Explain your answer.Sample C [1] some [1] hardness remained / removed on boiling [1]

(iv) Give a balanced, symbol equation for the reaction taking place when

sample D is boiled.___________________________ [2]

(v) Ion exchange can be used to soften hard water. Explain, in terms of ions, how this method works.

_________________________________________________________ [3]

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Ion exchange resins Ca++(aq) + 2Na+(resin) Ca++(resin) + 2Na+(aq) The ion exchange column is packed with resin beadsThe beads are “charged with sodium and chloride ionsCalcium ions and magnesium ions leave the water and go into the resin beads in exchange for sodium ions.In view of the charges of the ions involved, two sodium ions leave the resin for each calcium or magnesium ion entering the resinThe water leaving the resin has no Ca++ or Mg++ ions and is soft waterEventually the resin runs out of Na+ ions.However, the resin can be “recharged” by flushing brine (concentrated NaCl) through it – which reverses the process described above.

From “learning Outcomes”

(iv) Give a balanced, symbol equation for the reaction taking place when

sample D is boiled.Ca(HCO3)2 → CaCO3 + H2O + CO2 [2]

(v) Ion exchange can be used to soften hard water. Explain, in terms of ions, how this method works.

Calcium ions in hard water [1] exchanged [1] with Na+ ions [1] on resin

3a) Enniskillen is a limestone region in Northern Ireland famous for its Marble Arch Caves. Water taken from a river near to the caves was found to be hard water, which can be softened by the addition of washing soda.

2005, Paper 1

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StalactitesWater constantly drips through cracks in the ceiling of limestone cavesCalcium hydrogencarbonate is present, dissolved in the water.A miniscule amount of the calcium hydrogencarbonate in the water undergoes decomposition as the drop hangs from the ceiling.This deposits a miniscule amount of insoluble calcium carbonate on the ceilingIt takes about 500 years to grow 1 cm StalagmitesWhere the drops of water strike the floor of the cave the same process occursPillarsWhen a stalactite and stalagmite eventually meet, they form a pillar

Ca(HCO3)2(aq) CaCO3(s) + H2O(l) + CO2(g)

 

This reaction should seem familiar.It explains why boiling removes temporary hardness in water

(i) What is meant by hard water?_______________________________ _______________________________ [2]

(ii) Describe how you would confirm that the river water contained temporary hardness, giving any observations you would make._____________________________________________________________________________________________ [4]Quality of Communication [2]

(i) What is meant by hard water?Does not lather readily [1] with soap [1] [2]

(ii) Describe how you would confirm that the river water contained temporary hardness, giving any observations you would make.Add soap [1], shake [1], (observe) no lather [1], boil [1], repeat [1] (observe) lather [1] (Max [4]) [4]Quality of Communication [2]

(iii) Explain, in terms of ions, how the addition of washing soda can soften hard water._______________________________ _______________________________ _______________________________ _______________________________ [4]

(iii) Explain, in terms of ions, how the addition of washing soda can soften hard water.Carbonate ions / CO3

2- (from washing soda) [1], react [1] with calcium ions/Ca2+ (from hard water) to produce a precipitate/ insoluble [1] calcium carbonate [1] Ca2+ + CO3

2- → CaCO3 (s) [4]

b) Water vapour makes up approximately 1% of air. Substances such as sodium

hydroxide undergo changes when they are in contact with moist air.

Describe what would be observed if sodium hydroxide pellets were exposed to moist air for several days.

_______________________________ _______________________________ _______________________________ [4]

b) Water vapour makes up approximately 1% of air. Substances such as sodium

hydroxide undergo changes when they are in contact with moist air.

Describe what would be observed if sodium hydroxide pellets were exposed to moist air for several days.

White [1] pellets form a colourless [1] liquid / solution [1] eventually form white [1] flakes / powder [1] (Max [4]) [4]

c) The solubility of sodium nitrate in water at temperatures between 0°C and 60°C is

given in the table below.

Temperature (°C) 0 10 20 30 40 50 60

Solubility (g/100g water) 72 78 84 95 104 112 124

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From “Learning Outcomes”

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It is a good idea to boil tap water to be used to refill a goldfish bowl.This removes any dissolved chlorine that may harm fish.However, it also removes the dissolved air.Leaving the water to cool and sit overnight will give air time to dissolve once again in the water

(i) Explain what is meant by the term solubility._____________________________________ _____________________________________ [4]

(ii) If a saturated solution of sodium nitrate containing 25g of water is cooled from 50°C to 20°C, calculate the mass of sodium nitrate which would crystallise. [3]

(i) Explain what is meant by the term solubility.Mass [1] which saturates [1] 100g of water [1] at a particular temperature [1] [4]

(ii) If a saturated solution of sodium nitrate containing 25g of water is cooled from 50°C to 20°C, calculate the mass of sodium nitrate which would crystallise. [3]

Solubility at 50C per 25g of water = = 28g [1]

Solubility at 20C per 25g of water = = 21g [1]Mass recrystallised in 28 – 21 = 7g [1]or 112 -84 = 28g, 28/4 = 7gSubtraction [1] Division [1] Answer [1]

4

112

4

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A mixture X containing 10g of sodium nitrate at 30°C is prepared. After careful stirring, some solid is seen to remain at the bottom of the flask

(iii) Calculate the maximum mass of sodium nitrate which would dissolve in 10g of water 30°C.

_______________________________ [1]

A mixture X containing 10g of sodium nitrate at 30°C is prepared. After careful stirring, some solid is seen to remain at the bottom of the flask

(iii) Calculate the maximum mass of sodium nitrate which would dissolve in 10g of water 30°C.

9.5g [1]

(iv) Explain why the solution formed when mixture X is prepared is saturated.

_______________________________ _______________________________ [2]

(iv) Explain why the solution formed when mixture X is prepared is saturated.

More added [1] than can dissolve [1] [2]

5a) Water which contains substances that are bad for health is polluted water. One of

the main causes of this pollution is the presence of nitrates.

(i) What is the main source of these nitrates?

_______________________________ [1]

2007, Paper 2

5a) Water which contains substances that are bad for health is polluted water. One of

the main causes of this pollution is the presence of nitrates.

(i) What is the main source of these nitrates?

Fertilisers [1]

(ii) State two medical problems associated with high levels of nitrates in drinking water.

1. _____________________________ [1]2. _____________________________ [1]

(ii) State two medical problems associated with high levels of nitrates in drinking water.

Blue baby syndrome [1] (stomach) cancer [1] any associated medical problem e.g. Kidney disease or liver disease

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Blue Baby Syndrome - An illness that begins when large amounts of nitrates in water are ingested by an infant and converted to nitrite by the digestive.

The nitrite then reacts with oxyhemoglobin (the oxygen-carrying blood protein) to form metheglobin, which cannot carry oxygen

b) In some parts of Northern Ireland the tap water is described as hard water.

Hardness in water can be temporary or permanent.

(i) What is meant by the term hard water?

_______________________________ _______________________________ [2]

b) In some parts of Northern Ireland the tap water is described as hard water.

Hardness in water can be temporary or permanent.

(i) What is meant by the term hard water?

Water that does not later easily [1] with soap [1] [2]

(ii) Name a compound which causes temporary hardness in water.

_______________________________ [1]

(iii) Write a balanced symbol equation to show how temporary hardness can be removed from water by boiling.

_______________________________ [2]

(ii) Name a compound which causes temporary hardness in water.

Calcium or magnesium hydrogencarbonate

(iii) Write a balanced symbol equation to show how temporary hardness can be removed from water by boiling.

Ca(HCO3)2 → CaCO3 + CO2 + H2O [2][1] [1]

(iv) Permanent hardness may be removed by adding washing soda to the water.

Explain, in terms of ions, how washing soda can soften water.

_______________________________ _______________________________ _______________________________ _______________________________ _______________________________ [5]

(iv) Permanent hardness may be removed by adding washing soda to the water. Explain, in terms of ions, how washing soda can soften water.

Carbonate ions / CO32- [1] from washing

soda [1] react with [1] calcium ions Ca2+ [1] from (hard) water [1] to produce a precipitate / solid / insoluble CaCO3 [1]or Ca2+ + CO3

2- → CaCO3 [1] [1] (s)[1] From fromhard washingwater [1] soda [1]

(v) State one disadvantage associated with hard water.

_______________________________ [1]

c) Some substances dissolve very well in water and are said to have a high solubility. Define the term solubility.

_______________________________ _______________________________ _______________________________ [4]

(v) State one disadvantage associated with hard water.

Scum formation/wasteful of soap/furring of kettle elements / blockage of hot water pipes / limescale

c) Some substances dissolve very well in water and are said to have a high solubility. Define the term solubility.

Mass [1] which dissolves and saturates [1] 100g of water [1] at a particular temperature [1]

or Maximum [1] mass [1] which dissolves in 100g of water [1] at a particular temperature [1] [4]

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Calcium + Sodium Calcium + SodiumChloride Stearate Stearate Chloride(Hardness) (Soap) (Scum)

d) The solubility curves for three potassium salts are shown on the graph opposite.

Study the solubility curves and answer the following questions.

(i) Which potassium salt is the least soluble at 40°C?

_______________________________ [1]

d) The solubility curves for three potassium salts are shown on the graph opposite.

Study the solubility curves and answer the following questions.

(i) Which potassium salt is the least soluble at 40°C?

Potassium chloride [1]

(ii) What is the minimum mass of water needed to dissolve 180g of potassium bromide at 80°C?

______________________________________________________________ [2]

(ii) What is the minimum mass of water needed to dissolve 180g of potassium bromide at 80°C?

Solubility of potassium bromide at 80°C = 90 [1] g/100g H2O180g (90 x 2) dissolves in 100 x 2 = 200g [1] of water [2]

(iii) When 1kg of water saturated with potassium chloride is cooled from 80°C to 50°C, what mass of solid crystallises out of solution?

____________________________________________________________________________________________________________________________ [4]

(iii) When 1kg of water saturated with potassium chloride is cooled from 80°C to 50°C, what mass of solid crystallises out of solution?

Solubility at 80°C = 50 [1] g/100g of water

Solubility at 50°C = 40 [1] g/100g of water

50 – 40 = 10g [1] will crystallise out of 100g of water.10 x 10 = 100g [1] will crystallise out of 1kg of water([2] for values at each temperature; [1] for subtraction; [1] for multiplication) [4]

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Do consider using others in this Series.

1.Atomic Structure and Bonding.2.Inorganic: Metal Chemistry3.Inorganic: Non-metal Chemistry4.Redox and Electrolytic Chemistry5.Organic Chemistry6.Calculations in Chemistry7.Hard Water8.Acids Bases & Salt Preparations9.Kinetics and Energetics10.Miscellaneous

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