AQA C3.2
Water
S Jones 2012
Self review of learning outcomes:
Learning outcomes
Soft water readily forms lather with soap. Hard water reacts with soap to form scum and so more soap is needed to form lather. Soapless detergents do not
form scum.
Hard water contains dissolved compounds, usually of calcium or magnesium. The compounds are dissolved when water comes into contact with rocks.
There are two types of hard water. Permanent hard water remains hard when it is boiled. Temporary hard water is softened by boiling.
-.
Temporary hard water contains hydrogencarbonate ions (HCO3 ) that decompose on heating to produce carbonate ions which react with calcium and
magnesium ions to form precipitates.
Using hard water can increase costs because more soap is needed. When temporary hard water is heated it can produce scale that reduces the efficiency of
heating systems and kettles.
Hard water has some benefits because calcium compounds are good for the development and maintenance of bones and teeth and also help to reduce heart
disease.
Hard water can be made soft by removing the dissolved calcium and magnesium ions. This can be done by:
 adding sodium carbonate, which reacts with the calcium and magnesium ions to form a precipitate of calcium carbonate and magnesium carbonate
 using commercial water softeners such as ion exchange columns containing hydrogen ions or sodium ions, which replace the calcium and
magnesium ions when hard water passes through the column.
Water of the correct quality is essential for life. For humans, drinking water should have sufficiently low levels of dissolved salts and microbes.
Water filters containing carbon, silver and ion exchange resins can remove some dissolved substances from tap water to improve the taste and quality.
Chlorine may be added to drinking water to reduce microbes and fluoride may be added to improve dental health.
Pure water can be produced by distillation.
Red
Orange
Green
Use the PowerPoints, revision guides and the internet to help you complete the activities in this
booklet.
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Learning outcomes
□ I can define hard and soft water
□ I can state the ions which cause hardness
□ I can explain the advantages and disadvantages of hard water
Starter - Hard and soft water
In some parts of the country it is found that water does not easily form a
lather when shaken with soap (can be represented as sodium stearate,
NaSt). Water like this is called HARD water. Hard water contains dissolved
compounds that react with soap to form a scum (the solution is cloudy due
to the presence of an insoluble stearate compound). Water, which easily
forms a lather and no scum, is called SOFT water.
The table below shows what happens when 25 cm3 of each of the solutions
has 1 cm3 of soap solution at a time from a burette added, and then shaken
until a lather that lasts for at least thirty seconds is obtained. The
experiment was repeated but this time; first, the water had been boiled for
five minutes.
Volume of soap solution used (cm3)
Sample
Before boiling
After boiling
Distilled water
0.5
0.5
Water from limestone hills
10.5
1.0
Water from chalk streams
9.0
1.5
Distilled water, after shaking
with calcium sulphate and then
11.0
11.0
filtering
Conclusion: There are two types of hardness:
1. Temporary hard water – this can be softened by boiling the water.
2. Permanent hard water – this cannot be softened by boiling the water.
3. Hard water is water that reacts with soap to form a scum and so more
soap is needed to form a lather.
4. Soft water is water that readily forms a lather with soap and there is no
scum. Very little soap is needed to form a lather.
Which ions cause hardness?
The following table shows what happens when one level spatula of the
compound is dissolved with 2 cm3 of distilled water, then 2 cm3 of soap
solution is added and the test tube shaken for one minute.
Ions present
Height of lather (mm)
Solution used
Positive
Negative
Sodium sulphate
Sodium
Sulphate
19.0
Magnesium sulphate
Magnesium
Sulphate
3.0
Potassium chloride
Potassium
Chloride
21.0
Calcium chloride
Calcium
Chloride
1.0
Sodium nitrate
Sodium
Nitrate
20.0
Magnesium nitrate
Magnesium
Nitrate
2.0
Calcium nitrate
Calcium
Nitrate
1.0
Conclusion: From the table it is clear that hardness is associated with
positive ions, particularly those of calcium and magnesium. Hardness in
water is due to the presence of these two ions.
Main - How does hard water form?
When rain falls, it reacts with carbon dioxide in the air to form carbonic acid:
H20 (l) + CO2 (g)  H2C03 (aq)
When this dilute solution of carbonic acid flows over limestone, marble or
chalk it reacts with calcium carbonate in the rocks to form calcium
hydrogencarbonate:
CaCO3 (s) + H2C03 (aq)  Ca(HCO3)2 (aq)
in limestone
in rain
in hard water
Unlike calcium carbonate, calcium hydrogencarbonate is soluble in water
and the presence of the calcium ions makes the water hard. Calcium
carbonate in chalk, limestone and marble is the main cause of hard water. In
some areas calcium sulphate can occur as gypsum (CaSO4.2H20) and
anhydrite (CaSO4). In this form these minerals will cause the water to
become hard. Calcium sulphate is only slightly soluble in water but enough
will dissolve to make the water hard.
Advantages/disadvantages of hard water
Advantages
Disadvantages
Supplies calcium
Wastes soap because some of the soap forms
compounds required by
scum with the impurities in the water. This
the body for bones and
increases costs because more soap is needed
teeth. They also help to
to be used.
reduce heart illnesses.
Has a better taste than
Scum (calcium or magnesium stearate)
soft water.
formed leaves marks on clothes and baths.
Better for brewing beer.
Cause a layer of ‘fur’ in kettles and scale in
boilers and pipes. Scale (limescale – calcium
carbonate) in pipes may block pipes and make
radiators less efficient.
Lead compounds in pipes
Can spoil special finishes on fabrics.
are less soluble in hard
water.
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Learning outcomes
□ I can explain how hard water is soften
□ I can explain how to treat water
□ I can explain what pure water is
Starter - How is hard water softened?
In some areas, substances that cause hardness in the water must be
removed. This is called water softening. In order to soften hard water, we
must take out the calcium and magnesium ions. This can be achieved in
various ways:
1) By boiling - boiling decomposes calcium hydrogencarbonate forming
insoluble calcium carbonate. Ca(HCO3)2 (aq)  CaCO3 (s) + H20 (l) + CO2 (g)
This removes the hardness (temporary) caused by calcium
hydrogencarbonate, but boiling does not remove the hardness (permanent)
caused by calcium sulphate.
2) By adding washing soda - washing soda and bath salts contain sodium
carbonate (Na2CO3). Adding these to hard water removes all the calcium
ions as a precipitate of calcium carbonate.
Ca2+ (aq) + C032 -(aq) CaCO3 (s)
3) By ion-exchange - the most convenient way of softening water is to use an
ion-exchange column. The hard water passes through a column containing a
special substance called a resin. The resin contains sodium ions, which are
displaced by calcium ions as hard water passes through the column. The
sodium ions do not cause hardness, so the water is now soft.
Ca2+ (aq) + 2Na+ (s)  Ca2+ (s) + 2Na+ (aq)
Main - Water Treatment
Diagram of how water is made safe to drink
The hard water contains the calcium
ions.
This hard water is being poured
through the ion-exchange column.
The ion-exchange column contains
sodium ions.
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The liquid in the ion exchange resin
swaps ions, The calcium ions causing
the hardness in the water leave the
water and stick on the resin. The
sodium ions from the resin enter the
water to replace the calcium ions.
The liquid leaving the ion exchange
column does not contain any calcium
ions and therefore is not called hard
water.
The water from rivers is first stored in reservoirs where the process of
purification starts. The water may contain
 Bacteria
 Dissolved substances e.g. nitrates, sulfates, gases from the air, calcium
and magnesium compounds form rocks.
 Undissolved substances e.g. mud, grit, sand, twigs, dead plants.
Before the water is safe to drink most of the above must be removed – that
is the water is cleaned. There are three steps to this cleaning process.
a) Since the water is in reservoirs it is still. This allows the larger particles
of dirt to settle out. At the surface oxygen and sunlight break down
other impurities and kill some bacteria.
b) From the storage reservoirs it is filtered through beds of fine sand to
remove all of the undissolved substances. These filters can become
clogged and have to be replaced.
c) It is treated with chlorine, which dissolves and kills any remaining
bacteria.
The water that flows from taps is clean but not quite pure. It still contains
dissolved substances, which were not removed at the waterworks. Some
dissolved substances can be removed by passing the water through filters
made of carbon, silver and ion exchange resins. This helps to improve the
taste and the quality.
Fluoride compounds are added to toothpastes and to water supplies to help
prevent tooth decay. The arguments for and against adding fluorides to
water supplies are complicated. One of the arguments against adding
fluoride to water is that people should be able to choose to take extra
fluoride or not. In high doses fluoride can cause cancer and bone problems
in humans, so some people believe that fluoride shouldn’t be added to
drinking water
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Questions and activities
Produce a mind map revision aid about this topic.
State whether the sentences below are true or false.
1. Water which passes over rocks can become hard.
2. Water can be softened by removing chloride and carbonate ions from
the water.
3. Adding sodium chloride is one way of removing hardness from water.
4. Scale is formed when soap is used with hard water.
5. You can remove the hardness from water by adding sodium carbonate.
6. Water hardness is caused by Ca2+ and Mg2+ ions.
7. Soapless detergents do not form scum.
8. Less soap is needed to form a lather with hard water.
9. Permanent hardness can be caused by dissolved calcium sulfate.
10. Temporary hardness can be caused by hydrogencarbonate ions
Hard water can cause the build-up of scale in pipes, boilers and kettles. Why can
this be a problem with kettles?
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An ion exchange column can be used to remove the hardness from water.
Explain how hard water becomes soft when it is passed through an ion exchange
column.
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Does using an ion exchange column work for permanent hardness, temporary
hardness of both?
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In an experiment to compare the hardness of three different water sources, a
soap solution was added to samples using a burette. Five drops were added at a
time until a lasting lather was formed. Fresh samples of the water were boiled
and the experiment was repeated.
When the samples of hard water were boiled in a beaker, a white precipitate
formed.
Ca(HCO3)2 (aq)  CaCO3 (s) + H20 (l) + CO2 (g)
1. Give the chemical name of the white precipitate formed.
___________________________________________________________
The results for the experiment are shown in the table below:
Drops of soap solution needed to
Drops of soap solution needed
Source
produce a lather using unboiled
to produce a lather using boiled
sample
sample
A
35
5
B
30
15
C
5
5
Write the letters in the gaps below to complete the sentences about the results.
2. Source ______ and _______ are hard water
3. Source ______ contains both temporary and permanent hardness.
4. Source ______ contains only temporary hardness.
5. Source ______ is soft water.
6. Explain how you can tell which source contains permanent hardness.
___________________________________________________________
___________________________________________________________
___________________________________________________________
7. Give the name of a chemical that can be added to permanent hard
water to soften it.
___________________________________________________________
8. Suggest which source contains the most temporary hardness.
___________________________________________________________
9. Explain your answer.
___________________________________________________________
___________________________________________________________
___________________________________________________________
___________________________________________________________
___________________________________________________________
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Drinking water needs to be good quality. Which type of water is most pure?
Circle your answer.
tap water
river water
distilled water
sea water
Explain why this type of water is not generally used as drinking water.
_________________________________________________________________
mud.
Water from reservoirs is treated in a water treatment works. Sequence these
steps.
Chlorine is added to kill any remaining harmful microbes
The water passes through a mesh screen to remove bits like twigs
1
Chemicals are added to make solids and microbes stick together and
fall to the bottom.
The water is filtered to remove all solids
Now try C3.2 Past Exam Questions
Chlorine and fluoride can be added to drinking water. Use the words below to
fill in the gaps in the passage.
harmless toxic cancer bone tooth decay disease heart disease
Fluoride is sometimes added to drinking water because it helps reduce
_____________________________. Chlorine is added to water to prevent
________________. However, high doses of fluoride have been linked to
__________________ and __________________ problems. Chlorine can react
with natural substances in water to produce ______________ by-products.
Using these sentences produce a flow chart which explains how water is treated
so that it is safe to drink.
The water then enters a settlement
The water now looks clean, but still
tank, where sand and soil fall to the
contains bacteria which could be
bottom.
harmful. A small amount of chlorine is
added to kill any harmful bacteria.
The water is now stored in large tanks The pH of the water is checked and
and service reservoirs to be pumped
corrected to ensure it is neutral.
to homes, schools and businesses.
Aluminium sulfate and lime are added The water enters the treatment works,
to the water. This makes small
passing through a cage-like metal
particles of dirt clump together and
screen. This catches large objects such
sink. The sludge is collected and
as sticks and leaves.
thrown on a landfill site where it forms
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The water then passes through a fine
sand filter, so no particles of dirt
remain