1a The Water Cycle
Water as a Resource
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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.

1b Water Treatment and pollution - domestic and industrial
contexts
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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 o
(i) allowing sedimentation to occur, where larger insoluble o particles settle out,
(ii) passing it through sand filter beds to remove finer solid o o particles,
(iii) treating with chlorine to kill bacteria,
(iv) adding small amounts of sulphur dioxide to remove
excess toxic chlorine
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 ( see below ).
Iron in water is a non-harmful but an aesthetic nuisance impurity: o o readily soluble iron(II) when exposed to air form rusty brown insoluble iron(III) hydroxide or hydrated iron(III) oxide compounds. These stain yellow/orange/brown washing laundry and white plumbing facilities!
The iron(III) ions also form inky black compounds with the tannic acids in tea and giving it a 'metallic' taste.

o
Cooked vegetables turn brown (complex compounds with phenols).
 Colloidal clay: A colloid consists of one substance (or mixture of substances) very finely dispersed in another substance (or a mixture of substances) without a new true solution forming.
 Examples of colloids that is the fine dispersion of one substance in another without a new solution forming: o A sol is a solid dispersed in a liquid e.g. tiny particles of clay in water. o A foam is a gas dispersed in a liquid e.g. a well shaken soap solution or shaving cream foam. o An emulsion is a liquid dispersed in another liquid e.g. (i) milk
(aqueous solution + insoluble, but dispersed fats), (ii) French dressing in salads (based on vinegar + olive oil, but these do reform the oil and aqueous layers quite easily which is why they are shaken before use) and (iii) margarines contain emulsifiers to stop the salty water from separating out and
mayonnaise also contains an emulsifier to stop the oil and aqueous based components separating out.
1c Hard and Soft Water
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HARD and SOFT WATER: Many compounds dissolve in water without chemical change but may have a variety of consequences! o
Water which readily gives a lather with soap (not detergents) is described as soft water.
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Note: Detergents usually give a good lather with any
water.
Some of these dissolved substances make the water hard. o
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.

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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. o
The hard water is formed when natural waters flow over ground or rocks containing calcium or magnesium compounds. o o o e.g. Chalk and limestone, mainly calcium carbonate CaCO with some magnesium carbonate too.
3 or Gypsum rock deposits, which are mainly calcium sulphate
CaSO
4
, and magnesium sulphate which was called 'Epsom Salts', formula MgSO
4
.7H
2
O, 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 o e.g. naturally carbonated water (dissolved carbon dioxide o o makes water acidic so it reacts with the carbonate) ... insoluble calcium carbonate + water + carbon dioxide ==> soluble calcium hydrogencarbonate
CaCO
3(s)
+ H
2
O
(l)
+ CO
2(g)
==> Ca(HCO
3
)
2(aq)
The simplest test for 'hardness' is to shake the water with
an old fashioned 'soapy'* soap. o
* The term 'soapy soap' is NOT a joke! e.g. the blocks of
'household' soap based on sodium stearate, sodium palmitate (from palm oil) or sodium oleate (from olive oil).

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The amount of hardness in water sample can be estimated by titrating it with soap solution and noting what volume of soap solution is needed to produce a lather.
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.
Hard water can be made soft by removing the dissolved
calcium and magnesium ions. o
If due to calcium/magnesium hydrogencarbonates it is o removed by boiling
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, a plus 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. http://www.docbrown.info/page01/AqueousChem/AqueousChem.h
tm