Hard Water


In some parts of the country, the tap water is described as hard water. This means that it is hard to get a lather with soap. Instead of forming a lather, soap forms a precipitate known as scum. Water in which soap lathers easily is known as soft water. Hard water contains soluble calcium and magnesium salts. Soap is the common name for sodium stearate (more properly, sodium octadecanoate). The soluble calcium ions and magnesium ions combine with stearate ions in the soap to form insoluble calcium and magnesium stearates. These compounds are the insoluble scum that floats on the water. For example, with calcium ions (magnesium ions are similar):

                calcium ions       +             stearate ions                                 scum

                (in solution)                        (in solution)                                                        (insoluble solid)


                Ca2+(aq)                +             2 C17H35CO2-(aq)                           Ca (C17H35CO2)2(s)

If you go on adding soap, eventually all the calcium ions and magnesium ions will be precipitated as scum. After that, soap will lather easily and will be able to work as an effective cleaning agent. The amount of soap that is required to make a lather can therefore be used to estimate the hardness of water.

Soapless detergents are able to work in hard water because their calcium and magnesium salts are soluble and scum is not formed. For many purposes, people prefer soapless detergents to soaps.

How does water become hard?

Rainwater dissolves carbon dioxide as it falls through the atmosphere. A small fraction of the dissolved carbon dioxide reacts with the water to produce carbonic acid, which is a weak acid.

Equation for the reaction of carbon dioxide and rainwater:






As this solution passes over and through rocks containing carbonates such as limestone (CaCO3) and dolomite (MgCO3) the weak acid in the rain attacks them and very slowly dissolves them.  The dissolved substances are called calcium hydrogencarbonate and magnesium hydrogencarbonate:

Equation for limestone:                                                                                                                                                      





Some of the rock strata may contain gypsum (hydrated calcium sulphate, CaSO4•2H2O), anhydrite (CaSO4) or kieserite (MgSO4•H2O) which are very sparingly soluble in water. It is the presence of the soluble calcium and magnesium ions that cause the water to be come hard.

Types of hardness

Hardness in water can be divided into two types: permanent hardness and temporary hardness. Temporary hardness can be removed easily by boiling the water. It is caused by the presence of dissolved calcium hydrogencarbonate {Ca(HCO3)2(aq)} or magnesium hydrogencarbonate {Mg(HCO3)2(aq)}. The reaction of magnesium hydrogencarbonate with soap (sodium stearate) is given below:


Word equation:                                                                                                                                                                    






Chemical equation:                                                                                                                                                             




Permanent hardness is so called because it is much more difficult to remove. It results from the presence of calcium sulphate (CaSO4), calcium chloride (CaCl2), magnesium sulphate (MgSO4) or magnesium chloride (MgCl2). These substances do not decompose when they are heated, hence they cannot be removed by boiling.


Removal of hardness

Temporary hardness can be removed from water by boiling. Calcium and magnesium hydrogencarbonates can only exist in solution and not as a solid. When heated, calcium and magnesium hydrogencarbonate solutions decompose producing insoluble carbonates. For example:


Word equation:                                                                                                                                                                    





Chemical equation:                                                                                                                                                             



Eventually, all of the calcium and magnesium ions will be removed from solution as a precipitate and none are left to react with the soap.

Incidentally, the stalactites and stalagmites found in underground caverns in limestone areas are formed in a similar way. Weakly acidic rainwater slowly dissolves the limestone. The resulting solution of calcium hydrogencarbonate decomposes as the water evaporates. This happens very slowly at the cool ambient temperatures and forms the distinctive pillars of calcium carbonate.

Temporary hardness can also be removed by any of the methods described below for permanent hardness.

Permanent hardness can be removed by the addition of washing soda crystals, by the use of ion exchange resins, or by distillation.

Addition of washing soda crystals, Na2CO3•10H2O

Washing soda (sodium carbonate decahydrate) can be regarded as a ‘carrier’ for the carbonate ion. The soluble sodium salt can be dissolved in the hard water. The carbonate ions will then react with the soluble calcium and magnesium ions. These will be precipitated as solid carbonates and can no longer cause hardness. For example:


Word equation:                                                                                                                                                                    





Chemical equation:                                                                                                                                                             



The sodium ions do not react with soap to form scum, so they do not give rise to hardness themselves.

Ion Exchange Resins (Water Softeners)

Hard water is passed through a column containing a suitable resin. As the water permeates through the resin, each of the calcium or magnesium ions causing the hardness is exchanged for two sodium ions:


Word equation:                                                                                                                                                                    




Chemical equation:                                                                                                                                                             



After a period of time, all of the sodium ions originally on the resin will have been exchanged. At this point, the ion exchange resin needs to be regenerated. This is done by adding a concentrated solution of table salt (sodium chloride, NaCl) which flushes the calcium and magnesium ions down the drain. This is why water softeners have to be frequently filled with salt.










The water is distilled away from the dissolved substances. This method is used to make ‘deionised’ water for use in laboratories but is too expensive to be used on a large scale.

Advantages and disadvantages of hard water





















How do soaps and detergents work?

The cleaning action of both soaps and detergents can be explained by using a simplified diagram of each type of molecule:

This highlights the key features of the molecules: they have an uncharged hydrocarbon tail, and a charged (ionic) head group. The hydrocarbon tail is lipophilic- i.e. it can form intermolecular bonds to grease molecules. The ionic head group is hydrophilic- i.e. it will be attracted to water molecules. Soap or detergent molecules can effectively surround a ball of grease on, for example, a dirty plate so that it dissolves in water. Once dissolved in water, the grease can be flushed down the sink as sewage. The molecules surround the grease such that the lipophilic tails stick into the ball of grease whilst the ionic head groups stick out into the solution. If a sufficient number of molecules are involved, the grease takes on the outward appearance of a very large ion. Ions are generally soluble in water. The difference between soaps and detergent is that the calcium and magnesium salts of soaps form a solid precipitate, scum. The calcium and magnesium salts of soapless detergents are water-soluble.












Water Pollution

Water is very good at dissolving substances. It is therefore very unusual to find pure water on this planet. As water falls through the atmosphere and down onto and through the surface of the Earth, it dissolves a tremendous variety of substances. Chemical fertilisers washed off the surrounding land will add nitrate (NO3-) and phosphate (PO4-) ions to the water. It may also contain human waste as well as insoluble impurities such as grit, bacteria, oil and lead ‘dust’ from the exhausts of lorries and cars.

All these as well as natural impurities must be removed from water before it can be used and recent legislation has imposed strict guidelines on the amounts of various substances allowed in drinking water.

Most drinking water in the UK is obtained from lakes and rivers where pollution levels are low. The process of water treatment involves sedimentation, filtration and chlorination:










1. Impure water is first passed through screens to filter out floating debris.

2. Filtration through coarse sand traps larger insoluble particles. The sand also contains specially grown microbes which remove some of the bacteria.

3. A sedimentation tank has chemicals known as flocculants, for example alum (aluminium potassium sulphate), added to it to make the smaller particles, which remain in the water, stick together and sink to the bottom of the tank.

4. These particles are removed by further filtration through fine sand.

5. Finally, a little chlorine gas is bubbled into the water to kill any remaining bacteria. This sterilises the water.

Sewage Treatment

Sewage is waste water from homes and industry. After we have used the water it must be treated again before it can be returned to rivers, lakes, and seas.

Sewage contains waste products such as human waste and washing-up debris as well as everything else that is put down a drain or sink. The processes involved in the treatment of sewage are:


















1. Large screens remove large pieces of rubbish.

2. Sand and grit are separated in large sedimentation tanks. The sand and grit often contain large amounts of useful chemicals which, by the action of selected microbes, can be used as fertilisers.

3. The impure water is then removed and sent to a trickling filter, where it is allowed to drain through gravel on which microbes have been deposited. These kill off any remaining bacteria in the water.

4. Finally, the water is chlorinated and returned to a river.

Fluoridation of water supplies

In trace concentrations in drinking water, the fluoride ion (F-) helps growing bodies develop teeth that are particularly resistant to decay. Tooth enamel is mostly the mineral hydroxyapatite, [Ca3(PO4)2]3•Ca(OH)2. When fluoride ion is available during the development of enamel, a much harder mineral tends to form instead- fluorapatite, [Ca3(PO4)2]3•CaF2. Hydroxyapatite contains hydroxide ions which are more avidly attacked by acids (produced by mouth bacteria feeding on sugars) than are the fluoride ions in fluorapatite. Hence teeth whose enamel includes fluorapatite resist decay better. To obtain this protection for their children, many communities add fluoride ion to their water at extremely low concentrations, at or below parts per million (ppm). At higher concentrations, for example 2-3ppm, fluoride can cause mottling of the teeth, so careful control of the fluoride level in the water is important. World-wide, about 230 million people drink fluoridated water.[D.R.B3] 


Space for Additional Notes on Hard Water


























Space for Additional Notes on Hard Water





























A Review of the Hard Water Topic

Each topic in the Vth Form Chemistry course is divided into a series of 'learning targets'. These are listed below for work on Hard Water. For each 'learning target' you should make an assessment of how you are progressing, in terms of increasing your knowledge and developing a clear understanding of the principles. You should assess your progress on a 1 - 3 scale as follows:

                1=           I feel confident about this aspect of the work and I am encountering few                       problems.

                2=           I am making reasonable progress, but I have encountered a few difficulties and                         feel that I need to go over these particular areas again.

                3=           I am finding this aspect of the topic difficult.

Remember, be fair to yourself - be honest!!

Learning Targets

a)    I  can define hard water as water that does not readily lather with soap

        but gives scum                                                                                                                                                          

b)    I appreciate that hard water wastes soap but does not affect soapless detergents                              

c)     I recall that dissolved calcium and magnesium salts cause hardness                                                    

d)    I understand that hardness results from contact with limestone, gypsum and chalk                        

e)     I can recall and explain that boiling removes temporary hardness but not

        permanent hardness                                                                                                                                               

f)     I can recall and explain that both types of hardness can be removed by ion exchange

        and the addition of sodium carbonate                                                                                                              

g)    I understand industrial and domestic problems caused by deposition of limescale                          

h)    I understand that the presence of hardness in water causes the inner surface of

        pipework to become coated with limescale, which prevents corrosion and the entry

        of copper or lead ions into the water supply                                                                                                   

i)     I can describe the treatment of water to make it suitable for domestic use                                             

j)      I understand the need for sedimentation, filtration and chlorination                                                     

k)    I understand the reason for fluoridation                                                                                                          

Ideally, all of your responses will be ‘1’. However, this is rarely the case first time through! If you have written a ‘2’ anywhere, you may wish to read through your notes again or look at the relevant page in your text book. If you have written ‘3’ as a response to any of the questions, see me for further help. Alternatively, you may wish to attend the Chemistry Surgery which takes place in the Chemistry Department during Pursuits Periods (Mondays and Fridays, 2.00- 2.45pm). A member of the department will be on hand to sort out your difficulties.

Diagram p.901 Philip Matthews 'Advanced Chemistry' [D.R.B2]

Diagram p.135 Wilford and Earl [D.R.B3]