The water from our taps looks similar no matter where you live in this country.

However differences become obvious when we use soap to wash

In certain places when we wash with soap, the water forms a rich lather easily, but others it is more reluctant to – this is because the water is HARD.

Hard water makes it MORE difficult to wash, but also more difficult to clean the bath or sink when finished, because hard water contains dissolved substances which react with soap to form SCUM.

The scum floats on the water and will stick to the bath.The scum formed by hard water is not the problem when washing

clothes.That’s because modern detergents do not produce scum in hard

water. They are called SOAPLESS DETERGENTS.

SUMMARY SCUM FORMS WITH HARD WATER AND SOAP

SCUM DOES NOT FORM WITH HARD WATER AND SOAPLESS DETERGENTS (e.g. during washing)

hard watersoft water

SCUMlather

WHAT IS HARD WATER?ANSWER : Hard water tends to contain calcium and magnesium compounds (i.e. calcium and magnesium ions Ca2+ and Mg2+ ions).HOW IS WATER MADE HARD?ANSWER:• Rain water can run over rocks containing these

ions• Once dissolved in the water hard water then get

into our rivers and streamsTwo examples of rocks which causes hardness in water are:

1. Calcium sulphate (OR Gypsum)2. Calcium carbonate (OR Limestone)

Ca2+ Mg2+

Ca2

+

Mg2

+

hard water

NOTE: Limestone (calcium carbonate) however is almost totally

insoluble in water but as rain falls through the air, carbon dioxide becomes dissolved in it, making the water acidic.

Limestone reacts with this slightly acidic water to form calcium hydrogen carbonate which is soluble in water, so the calcium ions (Ca2+) become dissolved in the water making it hard.

i.e. an equation to explain this is shown below:

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

This soluble compound contains Ca2+

ions which makes the water hard

Since the CO2 from the air dissolves in the rain water, it makes the water acidic and hence this water fall into our steams and rivers making that acidic too!

HOW DOES SCUM FORM? In terms of economic factors, hard water is more expensive

because more soap is required or the same wash.The soap reacts with the magnesium and calcium ions in the

water, forming SCUM Only after all of the calcium and magnesium have reacted can the

soap begin to form a lather – and this is why so much more soap is needed per wash.

The equation below shows exactly how the calcium ions react with soap:

Sodium stearate + Ca2+ ions Calcium stearate precipitate + Na+ ions (soap) Calcium ions Scum soluble in water

NOTE – this also works with magnesium ions:

Sodium stearate + Mg2+ ions Magnesium stearate precipitate + Na+ ions (soap) Magnesium ions Scum soluble in water

Ca2+

Mg2

+

Calcium ions from water + Stearate ions from soap Calcium Stearate

Stearate

(Soluble)

(Soluble)

(Insoluble)SCUM

Ca(HCO3)2

Scum

Heat

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

Limescale

LIMESCALE

Scum is not the only problem hard water can potentially cause. Pipes can suffer from scale(also limescale). Scale is also common in heating elements and other parts of our

hot water system. Pipes which have a lot of scale eventually block up and stop

functioning. The same problem occurs in kettles. The element inside a kettle

can `fur` up. When it happens to them, they tend to become less energy

efficient – by being more slow and heating to lower temperatures – because scale is such a poor conductor of heat.

It takes longer to boil and water and takes more energy, this therefore costs more money each time you use the kettle.

Eventhough there are many disadvantaged of using hard water, there are also some advantages.

Calcium ions in drinking water help in the development of strong bones and teeth

There is also evidence which suggests that hard water helps to reduce heart disease.

Hard and soft water

You might think when you turn on a tap at home or in school that what comes out is just water, but it isn’t entirely. Tap water contains a number of dissolved chemicals, all of which are harmless, and some of which are actually beneficial to our health. However, they do change the nature of the water. Water that contains no calcium or magnesium ions is known as soft water. Water that contains dissolved calcium and magnesium salts is known as hard water. Hard water is split into two further categories: permanent hardness and temporary hardness. In this practical activity you will investigate the differences between these three types of water.

Method

1. Pour a 4 cm depth of water sample A into a test tube.2. Collect 10 cm3 of soap solution in a small beaker.3. Add a drop of soap solution to the test tube. Stopper the tube

and shake it. If no lather (foamy bubbles) appear, add another drop, stopper and shake again. Continue until a lather appears that lasts for at least 5 seconds. Note the number of drops that you have used and note the appearance of the water mixture in the test tube. Record this in Table 1.

4. Repeat the procedure for water samples B and C. Again record your results in the Table 1.

Two of the waters tested are hard water and the other is soft water. Soft water forms a good lather readily. Hard water requires more soap to form a lather. This lather tends to be poor and disappears quickly. White bits or scum will also form in the test tube with the lather.

1. Use this information to determine which of the waters are hard and which is soft. Record this in Table 2

2. Confirm your results by testing the three water samples with non-soapy detergent. Once again the soft water will form a lather readily whereas the hard water will not. You should notice that with non-soapy detergent you won’t see any scum forming on the hard water.

Temporary hard water is softened by boiling. In this next part of the practical you are going to determine which of the two hard water samples has temporary hardness and which has permanent hardness.

1. Take a 100 cm3 sample of the each of the two hard waters. Fill in their letters in the left-hand column of Table 2 and label each 500 cm3 beaker so that you will know which is which. Pour each sample into the correct 500 cm3 beaker and boil each for 5 minutes. Make sure the flammable soap solution is not near the Bunsen burner flame. Allow to the waters to cool. Describe the appearance of the two beakers of water after cooling.

2. Make sure the Bunsen burner is off. Repeat steps 1–3, adding the soap solution drop by drop. Record the results for each sample in Table 2.

3. Use your results to decide which one of the samples is temporary hard water and which one is permanent hard water.

Water sample

Soap solution Non-soapy detergent

Hard or soft water?

Number of drops needed to form a

lather

Appearance of water mixture on

adding soap

Number of drops needed to form a

lather

Appearance of water mixture on adding detergent

A 1 Lather was formed quickly and to the

top

1 Lather was formed quickly and to the

top

B 9 Lather was not very clear

8 Lather was not very clear

C 10+ Scum formed - No scum formed

Water sample Appearance of water in beaker after boiling

Number of drops of soap solution needed to form a lather

Appearance of water mixture

Permanent or temporary hardness?

B Limescale formed on the rim of beaker 2 Lovely lather formed

C No limescale formed - Stayed the same

Questions1.Identify the three samples as soft water or water with temporary

hardness or permanent hardness.ANSWER A = soft, B = temporary hard and C = permanent hard water.

2. What ions/substances are responsible for temporary hardness and permanent hardness of water?ANSWERThe substance responsible for temporary hardness is calcium hydrogencarbonate. Permanent hardness is caused by the presence of calcium ions and magnesium ions.

3. When temporary hard water is boiled, a reaction occurs that removes the chemical responsible for causing the hardness. Write a word equation and a balanced symbol equation for this reaction. [HT]ANSWER

calcium hydrogencarbonate → calcium carbonate + carbon dioxide + water

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

4. What is the common name given to the product of the reaction?

ANSWER – The product of this reaction, calcium carbonate is deposited as a white solid on the inside of kettles and hot pipes. We call this white solid `scale`

5. What problems can it cause in kettles and hot water pipes?

ANSWER – Scale reduces the efficiency of boilers and kettles and can be dangerous if it blocks pipes.

ADVANTAGES AND DISADVANTAGES OF HARD WATER

Disadvantages: Difficult to form a lather with soap. Scum forms that wastes soap. Scale forms inside kettles and pipes. This wastes energy when

the kettle is boiled and can block pipes.

Advantages: Some people prefer the taste. Calcium in water is good for

children’s teeth and bones. Helps to reduce heart disease.

Removing hardness from water

Soft water does not contain the dissolved salts that produce scum and scaleWater can be made soft by removing the Ca2+ and Mg2+ which are the ions which makes it soft.ADVANTGAES OF MAKING WATER SOFTWe don’t waste soapWe don’t get scum when washing And heating our water won`t make scale

TEMPORARY HARD WATERBoiling temporary hard water (like tap water) removes hardness in

water and hence makes the water softThe Ca2+ or/and Mg2+ ions are removed (as scale) So water which can have its hardness removed is known as

TEMPORARY HARD WATER. HOW DOES BOILING REMOVE TEMPORARY HARDNESS?This is how heating removes temporary hardness in water:

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

Causes temporary hardness!

Hardness removed as scale

NOTE: The same process occurs with Mg2+ ions

Mg(HCO3)2(aq) MgCO3(s) + H2O(l) + CO2(g)

Types of hard water Temporarily Hard

Co2

Ca2+Limestone

Soluble Calcium Hydrogencarbonate

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

Calcium and magnesium salts from some salts such as sulphates, are not removed by heating

These type of solutions are called PERMANENT HARD WATERThe water remains hard even after heating

Types of hard water Permanently Hard

Calcium Sulphate

GypsumAnhydrite

Heat

CaSO4(aq)

Cannot be softened

by heating

HOWEVER DESPITE THIS THERE ARE TWO WAYS IN WHICH HARD WATER CAN BE SOFTENED

METHOD 1 – to add sodium carbonate to it. Sodium carbonate is also known as washing soda When added to hard water the following reaction takes place:

Ca2+(aq) + CO32+ CaCO3(s)

`hardness` From sodium carbonate

NOTE: This reaction is very similar to the formation of scale when temporary hard water is heated. However, this reaction

is much quicker and energy is not wasted.

Ca2+

Mg2+

Na2CO3

Sodium CarbonateCalcium ions + Sodium Carbonate

Sodium ions + Calcium Carbonate

METHOD 2 – USING AN ION EXCHANGE COLUMN

Here the Ca2+(aq) and Mg2+(aq) ions are removed using an ion exchange column

A ion exchange column is normally packed with a resin containing Na+ ions (sodium ions)

As the hard water passes through the column the Na+ exchanges places with the Ca2+ or Mg2+ ions in the hard water

This makes the water coming out softThis is the way in which domestic water softening units workThe resin can be recharged with sodium ions after they have been

exchanged with Ca2+ or Mg2+ ions The resin can be washed with salt (sodium chloride solution).This puts the ions back inThis is why water softeners must be kept topped with salt.

Install a water softenerIon exchange column

contains resins

Ca2+ Mg2+

Na+

Exchanged for

1. Pour the distilled water into a test tube filling it a third of the way up.2. Fill the syringe with soap and carefully squeeze two drops into the test tube. Put a

bung in the test tube and shake it vigorously for ten seconds. 3. Place the test tube in the test tube rack and use the marker pen to indicate the

highest point on the test tube that the bubbles reached. Use the ruler to measure the height of the bubbles and record this in the results table.

4. Repeat the whole experiment in exactly the same way using the five prepared calcium chloride solutions and the five prepared sodium chloride solutions in place of the distilled water.

5. If time allows, repeat each of the experiments and calculate an average value for the height of bubbles obtained in each experiment.

6. Use your results to plot two line graphs showing the height of bubbles obtained against the concentration of each dissolved compound.

Concentration of calcium chloride solution (mg/l)

Height of bubbles (cm) Concentration of sodium chloride solution (mg/l)

Height of bubbles (cm)

0 (distilled water) 7 0 (distilled water) 7

20 5 20 7

60 3.5 60 7

100 2.5 100 7

140 1.5 140 7

180 0.5 180 7

QUESTION - Name one control variable that is being kept constant in this experiment.ANSWER - Any from: amount of soap used; amount of solution used; time involved in shaking

QUESTION - Why should a line graph be plotted from the results?ANSWER - A line graph should be plotted because all the variables are continuous.

QUESTION - Why was the distilled water used at the start of this experiment?ANSWER - It provides a baseline representing water with no compounds dissolved in it. It is a control.

QUESTION - Which, if any, of the two sets of solutions used shows the greatest concentration range? How would it be possible to recognise any anomalous values obtained in the experiment?ANSWER - Neither. The range is 180 mg/l (accept 160 mg/l) for both.

Anomalous values would not fit the trend line shown by the graph.

QUESTION - What effect does increasing the concentration of calcium chloride have on the hardness of the water?ANSWER - Increasing the concentration of calcium chloride causes the water to become harder.

QUESTION - What effect does increasing the concentration of sodium chloride have on the hardness of the water?ANSWER - Increasing the concentration of sodium chloride has no effect on the hardness of the water

QUESTION - How, if at all, would the pattern of results change if the amount of soap solution added to each solution was doubled?ANSWER - The pattern would be the same (but the lines would be moved up).

QUESTION - Calcium chloride solution contains dissolved calcium ions and chloride ions whereas sodium chloride solution contains dissolved sodium and chloride ions. Which ion, or ions, does the experiment suggest is responsible for making water hard? Justify your answer.

ANSWER - Neither sodium nor chloride ions can be responsible for making water hard since the sodium chloride solutions give (or should give) the same amount of lather as distilled water. Calcium ions must be responsible for making water hard since calcium chloride makes (or should make) water hard whereas sodium chloride does not