Water notes

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What’s in the water we drink?

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. The hardness of water can be measured by titration with soap solution. Hard water contains dissolved compounds, usually of calcium or magnesium. These 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.

(i) Adding sodium carbonate, which reacts with the calcium and magnesium ions to form a precipitate of calcium carbonate and magnesium carbonate

(ii) 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. Drinking water fit for humans should have sufficiently low levels of dissolved salts and microbes. Water of the correct quality is produced by:

Choosing an appropriate source

Passing the water through filter beds to remove any solids

Sterilising with chlorine.

Water filters containing carbon, silver and ion exchange resins can remove some dissolved substances from tap water to improve the taste and quality.

Examination questions may give information about water filters so that comparisons can be made.

Pure water can be produced by distillation. However large amount of energy needed for distillation and, as a consequence, of the high costs involved this is not an economically viable method.

1a The Water Cycle and Water as a Resource

Water is the most abundant substance on the surface of our planet and is essential for all life. Water in rivers, lakes and the oceans is evaporated by the heat of the Sun (endothermic). The water vapour formed rises into the atmosphere, cools and forms clouds of condensation (exothermic). Eventually this gives rain and snow 'precipitation' which on melting returns to the rivers, seas and oceans. This is known as the water cycle.

Water is an important raw material and has many uses. It is used as a solvent and as a coolant both in the home and in industry. It is used in many important industrial processes including the manufacture of sulphuric acid.

Seawater/brine is a valuable resource e.g. large scale evaporation in 'salt pans' (using fuel burning or solar energy) to produce 'sea salt' sodium chloride NaCl, the water also contains lots of other salts including bromides from which the element bromine is extracted.

Water Treatment and pollution - domestic and industrial contexts

There are various undesirable materials that need to be removed from water before it is fit for domestic consumption. They include colloidal clay, microscopic organisms, chemicals which cause tastes or odours and Acidic substances:

Drinking water is made fit for domestic home consumption by

(i) allowing sedimentation to occur, where larger insoluble particles settle out,

(ii) passing it through sand filter beds to remove finer solid particles,

(iii) treating with chlorine to kill bacteria,

(v) aluminium sulphate is added to coagulate colloidal clay

(vi) Carbon slurry absorbs molecules causing 'tastes' and 'odours'.

(vii) Adding lime slurry to neutralise the water if it is too acid.

The use of artificial fertilisers results in many natural waters being contaminated with dissolved nitrate and ammonium ions. Dissolved nitrate ions can have harmful effects on babies and so the levels of nitrate are carefully monitored. Nitrates may be carcinogenic. The ions from this pollution are not easy to remove on a large cost-efficient scale.

An ion-exchange filter can remove these and other ions which can cause problems e.g. calcium and magnesium which cause hardness in water and iron compounds.

Hard and Soft Water

HARD and SOFT WATER: Many compounds dissolve in water without chemical change but may have a variety of consequences. Water which readily gives a lather with soap (not detergents) is described as soft water. Detergents usually give a good lather with any water.

This means the water does not readily give a good lather with soap and so wastes soap as well as causing a 'scum'! Though it does not affect soapless detergents. The 'scum' is due to the formation of insoluble calcium and magnesium salts formed by a reaction between the soap molecules and calcium and magnesium ions.

Most hardness is due to water containing dissolved calcium or magnesium compounds.

The hard water is formed when natural waters flow over ground or rocks containing calcium or magnesium compounds. e.g. Chalk and limestone, mainly calcium carbonate CaCO3 with some magnesium carbonate too. Or Gypsum rock deposits, which are mainly calcium sulphate CaSO4, and magnesium sulphate which was called 'Epsom Salts', formula MgSO4.7H2O, because it crystallised out of evaporated spring water from Epsom on the chalk downs of southern England.

Calcium sulphate (slightly soluble) and magnesium sulphate (very soluble) are washed out of rock formations.

Insoluble calcium carbonate (in limestone, chalk) and insoluble magnesium carbonate both dissolve in acid rainwater to form soluble hydrogencarbonates . Naturally carbonated water (dissolved carbon dioxide makes water acidic so it reacts with the carbonate),

insoluble calcium carbonate + water + carbon dioxide ==> soluble calcium hydrogencarbonate

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

The simplest test for 'hardness' is to shake the water with an old fashioned 'soapy'* soap.

Soft water readily forms a lather with soap but hard water does not.

Hard water forms a scum from the dissolved calcium or magnesium compounds.

The scum is a precipitate formed from insoluble calcium and magnesium soap salts, instead of a nice frothy lather. Eventually with enough soap, a lather does form, when all the calcium and magnesium ions have been precipitated as a 'scum salt'! However, it does mean a lot of soap is wasted! A modern detergent is sometimes called a 'soapless soap' or soapless detergent. Its advantage is that no insoluble salt 'scum' is formed because the Ca and Mg salts of it are soluble. So modern detergents e.g. like 'washing up liquids' give a lather with any water which is more acceptable for dish washing.

Using hard water can increase costs because more soap is needed to make a useful 'washing lather' and hard water often leads to deposits (lime scale) forming in heating systems and kettles which require cleaning at times.

The 'lime scale' is usually caused by the thermal decomposition of the dissolved hydrogencarbonates producing insoluble calcium carbonate (so it does remove some of the temporary hardness before washing! and chemically, it is the opposite of the carbonated water dissolving action above)

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

However there is a plus side to the deposition! The coating on the inner surface of the pipe work prevents corrosion and the dissolving of potentially poisonous salts of copper or lead into the water supply. The lime scale can be removed by any acid (hydrogen ion solution) treatment which dissolves the calcium carbonate.

calcium carbonate + hydrochloric acid ==> calcium chloride + water + carbon dioxide

Hard water can be made soft by removing the dissolved calcium and magnesium ions.

If due to calcium/magnesium hydrogencarbonates it is removed by boiling (see above).

Adding enough 'soapy' soap, see above, but the water is best treated before the washing!, so its not the desired solution with the scum and all that!

The addition of sodium carbonate (as 'washing soda' crystals), which dissolves and precipitates out the calcium or magnesium ions as their insoluble carbonates(s) formed.

calcium sulphate + sodium carbonate ==> calcium carbonate + sodium sulphate

CaSO4(aq) + Na2CO3(aq) ==> CaCO3(s) + Na2SO4(aq)

or more simply ionically: Ca2+(aq) + CO32-(aq) ==> Ca2+CO32-(s) (called an 'ionic equation')

Packs of ion exchange resins can hold or release ions in an ion exchange process.

Permanently hard water means the hardness cannot be removed by boiling e.g. when caused by dissolved magnesium or calcium sulphate.

Temporary hard water means it is softened by boiling e.g. when caused by magnesium hydrogencarbonate or calcium hydrogencarbonate.

HOWEVER, on a positive point hard water contains dissolved compounds that are good for health. Hard water often provides calcium compounds that help the development of strong bones and teeth and help to reduce heart illnesses and also traces of other essential elements like iron and iodine.