Water (WJEC GCSE Chemistry)

   Their results are shown in the table. 

potassium   sodium   nickel   magnesium    tin

Mass = ........................................................ g

Potassium sulfate, K₂SO₄, is soluble in water.

Hard water can be softened using different methods, including boiling the water and passing it through an ion exchange column

Two hard water samples, A and B, were tested for hardness, using soap solution. Both samples were tested before and after boiling, and after passing through an ion exchange column.

State the type (or types) of hard water that each sample contains. Give your reasoning.

A   ..................

B   ..................

Higher Tier Only

Sodium carbonate (washing soda) can also be used to soften hard water. It reacts with the magnesium compounds in the water.

Write a balanced symbol equation for the reaction between sodium carbonate and magnesium chloride, to form sodium chloride and magnesium carbonate.

The water used in our homes is treated to make it safe to drink.

State the main stages involved in the treatment of the public water supply. Describe the purpose of each stage.

You do not need to refer to fluoridation in your answer.

Fluoride is sometimes added to water supplies.

State why fluoride is added and give one reason why some people oppose the addition of fluoride to water.

Reason for adding fluoride .........................................................................

Reason why some people oppose its addition ............................................................................

This advert was published to encourage people to save water by taking a shower instead of a bath.

Use the data in the advert to calculate the percentage of water saved by taking a shower for 8 minutes rather than running a full bath.

Percentage of water saved = ……....................... %

The following table shows the solubility of potassium permanganate in water at temperatures between 30°C and 60°C.

Plot these values on the grid below. Draw a suitable line.

Use the graph to calculate the mass of crystals formed when a saturated solution in 500 g of water is cooled from 65°C to 30°C.

Mass of crystals = ………........................ g

Water samples A, B, C and D were tested for hardness using the apparatus shown.

Soap solution was added 1 cm³ at a time to each sample and the volume required to produce a permanent lather on    shaking was recorded. Each sample was tested before and after boiling.

The results are shown in the table.

[2]

   Water sample ............................................................

   Explanation

Discuss the benefits and drawbacks of living in a hard water area.

Fluorine exists naturally as the fluoride ion. It is found in soil, water, foods and several minerals, such as fluorapatite and fluorite.

Fluoride ion concentration in seawater averages 1.3 ppm (parts per million). In fresh water, the natural range is typically between 0.01 and 0.3 ppm. In some parts of the world, fresh water contains fluoride ion levels which are dangerous and can lead to health problems.

In the early 1930s, scientists found that people who were brought up in areas with naturally fluoridated water had up to two-thirds fewer cavities compared to those who lived in areas where the water was not fluoridated. Several studies since then have repeatedly shown that when fluoride is added to people’s drinking water in areas where natural levels are low, tooth decay decreases.

However, many European countries which do not fluoridate their water do not have a higher incidence of dental decay than countries which do so. It was also found that in Germany and Finland, decay rates either remained stable or continued in their downward trend after they stopped adding fluoride to their drinking water.

Figure 1 shows data about the effect of fluoridation of drinking water on the mean number of decayed, missing and filled teeth (DMFT) and the amount of fluorosis seen.

Figure 2 shows the change in mean DMFT in three regions of Australia over a four year period.

Describe the effects of adding varying concentrations of fluoride ions to drinking water

Tick (✓) the statement below that best describes why Germany stopped fluoridating its water supplies.

Ten years after stopping adding fluoride there was no increase in tooth decay
They found that adding fluoride caused fluorosis
Natural water supplies already contain fluoride in a high concentration
Studies showed that areas with no fluoridation did not have higher levels of decay than areas that did fluoridate

Tick (✓) the box which gives one definite conclusion that can be drawn using only the data in Figure 2.

Fluoridation has no effect on levels of decay
People have reduced their intake of sugary foods over this period
More than one factor affects levels of decay
Fluoridation is the main cause of falling levels of decay

‘Mass medication’ is an argument often given to oppose fluoridation of water supplies.

Explain what is meant by the term mass medication.

The graphs below show the solubilities of sodium chloride and potassium chlorate in water at different temperatures.

[1]

 ....................................... °C

[2]

 
relative formula mass = .......................................

[2]

 
percentage by mass = ....................................... %

Potassium chlorate forms potassium chloride and oxygen on heating.

Balance the symbol equation that represents the reaction taking place. 

2KClO₃  →  KCl + O₂

One out of six people today do not have access to safe fresh water. Estimates suggest fresh water supplies will be a major problem for half the countries of the world by 2025 and by 2050 about 75 % of the world’s population will experience a serious scarcity of the resource.

As the world’s population increases not only will the demand for drinking water and water for sanitation increase, but the demand for water to produce more food will increase. As climate change makes rainfall less predictable and droughts more common, a growing number of countries are turning to desalination.

One method of desalination, called reverse osmosis, uses a thick membrane to filter salt from seawater. But this system requires an extremely high pressure to force water through the membrane and therefore uses a lot of energy.

Researchers in the USA have come up with a new approach using a different kind of filtration material - sheets of graphene - a one-atom-thick form of the element carbon. This new material can be far more efficient and possibly less expensive than existing desalination methods. Sheets of graphene are perforated with precisely-sized holes which only let the water molecules through.

[1] 

water is becoming scarce
each person is drinking more water
climate change affects the availability of drinking water

[1]

water molecules contain atoms and not ions
sheets contain holes which are bigger than ions
sheets contain holes which are bigger than water molecules but smaller than sodium and chloride ions
sheets contain holes which let molecules through but not ions
molecules are smaller than ions

The total volume of water on the Earth is 1.4 × 10⁹ km³, 97% of which is seawater. Most of the remaining 3% is bound up in ice at the poles, leaving 0.3% available as fresh water.

Calculate the volume of the Earth’s available fresh water. Give your answer in standard form. 

One piece of survival equipment found on life rafts is a ‘solar still’. Solar stills are used to remove soluble salts from seawater to form drinkable water.

State and explain the physical changes that occur in the formation of drinkable water from seawater. Include the name given to the overall process taking place. 

Higher Tier Only

The diagram shows an ion exchange column used for softening permanent hard water.

[2]

[2]

Describe and explain a method to test whether a water sample is soft water, temporary hard water or permanent hard water. 

(You do not need to include reference to fair testing in your answer)

The table below gives information about the concentration of ions in drinking water from four different locations.

[1]

Write the formula of sodium sulfate.  .................

[1]

Compound 1 ..................................

Compound 2 ..................................

State the location where you would expect to find the least amount of tooth decay.

Give a reason for your choice.

The graphs below show the results of research on the effect of water fluoridation on the teeth of children aged 12 years in the United States.

Use the graph to find the decrease in the mean DMFT between 1967 and 1992.

Decrease in the mean DMFT = ..............................................

Does the evidence from the graph support the following statement? Give a reason for your answer.

“Fluoridation of drinking water is responsible for the decrease in tooth decay among 12 year-olds.”

Give two reasons why some people oppose the fluoridation of drinking water.

Reason 1 ..................................

Reason 2 ..................................

The table below shows the amount of soap solution required by different samples of water to form a permanent lather. In each case 25 cm³ of the water samples were used and the soap solution was added 1 cm³ at a time.

Two pupils, David and Haf, calculated the mean value for sample B. David calculated a value of 13.5 and Haf calculated a value of 12. Show how both values were obtained. State which is the better value to use and give a reason for your choice.

State which of water samples A, B and C is the least hard.

Water sample . . . . . . . . . . . . . . . . . . . . . . . . . . .

State which of water samples A, B and C contains both temporary and permanent hardness. Give the reason for your answer.

Water sample .................

Reason ...................................................

Name an ion which causes hardness in water.

The following graph shows the mean numbers of decayed, missing and filled teeth (DMFT) in 12 year-olds in three areas of Australia in 2008, 2010 and 2012. One area has been fluoridated for 20 years, one is newly fluoridated and the other is unfluoridated.

Describe what the graph tells us. Use this information and your knowledge of fluoridation to explain why some people support fluoridation of water supplies but others oppose it.

The grid below shows the solubility curves for four ionic compounds.

[1]

 ............................................................ °C

[3]

Mass = ........................................... g

[1]

[1]

[2]

Potassium nitrate reacts with aluminium hydroxide to produce aluminium nitrate and potassium hydroxide.

Balance the symbol equation for the reaction taking place.

 KNO₃ + Al(OH)₃  Al(NO₃)₃ +  KOH

Most water in Wales is sourced from surface water and ground water. Ground water is the water beneath the surface of the Earth, consisting mainly of surface water that has filtered through the rocks. It is the source of water found in springs and wells. Human activity causes surface water to be more contaminated than ground water.

The diagram below shows the three main stages in the treatment of surface water before it enters the mains water system on its way to the taps in your home.

large insoluble particles sink to the bottom
soluble particles are removed
small fine insoluble particles are removed
fluoride is added to the water

[1]

[2]

[2]

A family uses a water butt to collect rainwater in order to reduce their use of mains water.

They use a total of 450 litres of water on a given day. The chart shows the volumes of mains water and collected rainwater used for various purposes on that day.

Olivia investigated the relative hardness of three samples of water, A, B and C.

She added soap solution to each sample separately and recorded the volume of soap solution needed to get a permanent lather. Her results are shown in the table.

Describe how you would repeat this investigation, including enough detail to show that you will be carrying out a fair test. Assuming that you get similar results to Olivia, state and explain the conclusion you would reach.

In the UK, drinking water is produced from fresh water.

Lakes and rivers are common sources of fresh water.

Name another source of fresh water.

Explain how drinking water is obtained from fresh water.

Pure water can be obtained from sea water by the process of distillation.

Explain why this is not used in the UK to produce drinking water.

Reverse osmosis is another method of desalination of fresh water which is a process that uses a membrane as shown in the diagram.

Suggest what the role of the membrane is in reverse osmosis and why this is an expensive process.

A student investigated the amount of dissolved solids contained in different water samples.

They also measured the pH of the water samples.

Their results are shown in the table below.

What would you expect the pH and mass of dissolved solids to be in a sample of pure water?

Explain your answer.

How could the student confirm that the final water sample is pure?

Calculate the mass of the dissolved solids in 100 cm³ of sea water?

Use the results from table in part (a). 

Waste water, including sewage and agricultural waste water, requires treatment before being released into the environment.

Name two substances that must be removed from waste water prior to it being released.

Treatment of waste water produces liquid effluent and solid sewage sludge.

What two processes does waste water go through in order to produce the effluent and sludge?

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

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 the information in part (c) in your answer.

Drinking water contains dissolved solids which are not harmful to drink in small quantities.

Describe a method that could be used to find out the amount of dissolved salts contained in different water samples.

Permanent hard water can be softened. 

Explain how adding sodium carbonate, (Na₂CO₃) softens permanent hard water.  

Higher tier only

State the ionic equation for the reaction that occurs when sodium carbonate reacts with hard water. 

Higher tier only

Boiling is a method of removing temporary hardness.

Explain how boiling will remove hardness.

Include relevant equations in your answer.