CHEMISTRY O LEVEL(FORM THREE) NOTES – HARDNESS OF WATER

HARDNESS OF WATER

INTRODUCTION.

Water is very essential substance for all living things. Over 70% of the earth is water and there are different sources of water for daily use. These sources include rivers, lakes, wells, ponds, springs, and streams.

Water can also be obtained from taps that are supplied with treated from the local or nearest water works.

In some places, water lathers easily with soap, this water is said to be soft. In other places, the same amount of soap would give scum and very little lather. This water is said to be hard. Scum is an insoluble layer of impurities that accumulates at the surface of a liquid, especially water.

Hardness of water is caused by dissolved calcium and magnesium compounds. They include calcium, calcium hydrogen carbonate, magnesium sulphate and magnesium hydrogen carbonate. For example

Calcium sulphate + sodium stearate → calcium stearate + sodium sulphate

(Hardness) (Soap) (Scum)

The soap and the hardness join to form scum. The other compound sodium sulphate which is formed dissolves in water.

Calcium hydrogen carbonate is the most common cause of hardness of water. It forms when rain falls on the rocks of limestone and chalk, which are mainly composed of the insoluble calcium carbonate. As the rain falls through the air, it dissolves carbon dioxide to form
weak acidic solution. This solution is able to attack the calcium carbonate to form the soluble calcium hydrogen carbonate.

Water + carbon dioxide + calcium carbonate → calcium hydrogen carbonate

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

Dolomite and gypsum are other rocks that are sources of hard water.

TYPES OF HARDNESS OF WATER.

There are two types of hardness of water, namely Temporary and Permanent hardness of water.

Hardness caused by calcium hydrogen carbonate is called temporary hardness because it can be removed by boiling the water.

Ca (HCO
3)2 (aq) → H2o (l) + CO2 (g) + CaCO3 (s).

Hardness caused by other calcium and magnesium compounds is called Permanent Hardness.

This is because boiling does not affect it.

HOW TO REMOVE PERMANENT HARDNESS OF WATER

Permanent hardness of water can be removed by:

i) Distillation; This gets rid of both temporary and permanent hardness.

ii) Adding sodium carbonate (washing soda); This is added to the water to precipitate calcium carbonate. It removes both types of hardness of water for example, its reaction with sodium sulphate is;

Calcium sulphate+ sodium carbonate →calcium carbonate

CaSO4 (aq) + Na2CO3(aq) → CaCO3(s) + Na2SO4 (aq)

iii) Ion exchangers; These can remove both types of water hardness by removing all the calcium and magnesium ions in the water.

An ion exchanger is a container full of small beads. These are made of special plastic called ion exchange resin. This has ions for example, sodium ions that are weakly attached to it.

When hard water is passed through the ion exchanger, the calcium and magnesium ions in the water change places with the sodium ions and attach themselves to the resin. The calcium and magnesium ions are therefore left behind in the resin as the soft water flows out with the sodium ions.

After some time, all the sodium ions will have been replaced and the exchanger cannot remove hardness anymore. This is resolved by regeneration of the resin by pouring a concentrated solution of sodium chloride into the exchanger.

The sodium ion push the calcium and magnesium ions off the resin, making the ion exchanger ready for use again.

The Ca2+ from the hard water replaces the Na+ ions in the ions exchanger; softened water (with Na+ ions) leaves the ion exchanger to be used in the household.

The sample of water that do not lather easily are of hard water. These include the sea water and sometimes the tap water. The boiled, distilled and rain water easily form lather with soap and are therefore soft water.

ADVANTAGES AND DISADVANTAGES OF HARD WATER.

1) ADVANTAGES:

i) It tastes better due to dissolved compounds.

ii) It provides useful calcium for the growth of bones and tee
th.

iii) The formation of lime scale in pipes forms a sort of insulation which prevents the water in the tap from coming into contact with the metal of the pipe. This prevents pipe corrosion and poisonous metal salts from getting dissolved in the water.

2) DISADVANTAGES:

i) The temporary hardness of water causes limescale in water boilers, hot water pipes, kettles and other appliances, this makes them less efficient.
Scaling can also cause blockage in the appliances and has to be removed from time to time.

ii) Hard water needs more soap than soft water, so it means a lot of soap is wasted.

iii) Hard water leaves scum deposits that are difficult to wash out and causes damage various articles.

WATER TREATMENT.

Before water is used in the public water supply, it has to be treated, not to make it pure but to make it safe to drink.

The various stages include:

1. Filtration to remove any large objects such as algae or parasites.

2. Precipitation of mineral salts such as iron salts using alum (calcium hydroxide).

3. Filtration through a sand bed- this is a biological process. A gelatinous layer is used ton trap unwanted particles.

4. Chlorination. Treating with chlorine kills harmful bacteria in the water- this is a disinfectant.

5. Fluoridation. Adding sodium fluoride in small quantities. Fluorides harden the enamel on teeth and reduce dental problems. Not all water is treated in this way.

As much as a supply of clean, safe water is essential to ensure good health, we must not forget that safe disposal of used water is of equal importance. This becomes especially important when people live close together in towns and cities.

In a city it is essential to have a system for the disposal of used water. We call this a sewage system.

The processes involved in sewage treatment plants are outlined below.

1. The sewage that enters a treatment plant contains debris that might damage the pumps and machinery such materials are removed by screens or vertical bars. After removal, this debris is burned or buried.

2. The waste water then passes through a grinder where leaves and other organic materials are reduced in size for efficient treatment and removal later.

3. Next, grit is removed in grit chambers. The grit is removed and the disposed of as sanitary landfill.

4. With grit removed, the water passes into a sedimentation tank, in which organic materials settle out and are drawn off for disposal. An alternative to sedimentation that is used in the treatment of some waste water is flotation, in which air is forced into the waste water under pressure of 1.75 to 3.5kg mâ»². The waste water, supersaturated with air, is then discharged into an open tank, there the rising air bubbles cause the suspended solid to rise to the surface, where they are removed.

EXTRACTION OF METALS

OCCURRENCE OF METALS.

The metals which are most in reactivity are extracted from the sea, for example calcium is extracted from limestone, chalk and marble in the sea.

– Metals of medium reactivity are found in the form of oxides and sulphides

– Example Aluminium, Zinc, Iron and Tin

– Least reactive metals are found as free uncombined elements, for example Gold, Silver etc.

Main natural/forms of metals.

Metals in order of reactivity Main form in nature

Potassium potassiumchloride

Sodium sodium chloride

Calcium calcium carbonate which is limestone / chalk

Aluminium aluminium oxide

Zinc zinc sulphide

Iron iron oxide or iron sulphide

Tin tin oxide

Lead lead sulphide

Copper copper pyrites (CuFeS2)

Mercury mercury sulphide

Silver silver sulphide

Gold occurs as a free element

Platinum occurs as a fee element

LOCATION OF METALS IN TANZANIA
Tanzania is a country of mineral rich land.A varieties of metals investigated to exist in many parts of Tanzania. A mass medias of Tanzania are very familiar to some important places like Nyamongo, Buzwagi, Bulyanhulu etc. Those places are very important for the economical fluctuation and personal financial enhancement. Many people migrate there, as a means solving their way of life.

METALS	LOCATION
1. Copper	– Morogoro
2. Tin	– Misemyi Kagera
3. Phosphate	– Minyingu in the lake Narton Manyara
4. Uranium	– Namtumbo Ruvuma
5. Iron Haematite ore	– Liganga place – Ludewa district Njombe
6. Gold	– Chunya Mbeya – Nyarugusu Geita – Nyamongo Tarime district, Mara – Bariadi – Buzwagi and Bulyahulu Shinyanga

EXTRACTION OF METALS

Extraction is the process whereby metals are obtained from their ores. The ores are reduced to respective metals in the process. The metallic ions gain electrons and become the corresponding atom

-The extraction can be done in two methods;
1. Electrolytic reduction
2. Chemical reduction

Table shows some metals and their methods of extraction

	METAL	NAME OF ORE	EXTRACTION METHOD
Very reactive metals	Potassium Sodium Magnesium Aluminium	Pota – chloride Rock salt (NaCl) Magnesium chloride Bauxite	Electrolysis
Less reactive metals	Zinc Tin, Iron Lead Copper	Zinc Blender Hematite, magnetite Galena Copper pyrites	Chemical reduction
Least reactive metals	Gold	Uncombined	Purified by electrolysis

STAGES OF EXTRACTION OF METALS.

There are two stages in the process of extraction of metals from their ores;
1. Ore purification
2. Extraction of the metal
3. Refining of the metal

1. ORE PURIFICATION
After mining the ore from particular process, the ore should be purified. There are almost three methods that can be used to purify the ore.
a) Dressing
b) Calcination
c) Roasting

a) Dressing
Is the method used to remove impurities without decomposing any component within the ore. Impurities like sand , limestone, quartz and silicate should be removed by;
– Hand picking
– Blowing
– Filtrating solids
– Sedimentation and Decaution
– Dressing also known as concentration.

b) Calcination
Calcination is a process in which ore is heated in the absence of air (to avoid oxide product) below its melting point to expel water from a hydrated oxide or expel carbondioxide from a carbonate.

c) Roasting
Roasting is a method used to purify ore by heating in the presence of air. Some time the ore may be mixed with other chemicasl. The roasting needed to make great change to oxide or chlorination.

2. REDUCTION OF OXIDES TO METALS

Carbon reduces oxides of zinc (Zn) and all metals below Zinc (Zn) in the E.C.S metals above Zinc (Zn) in the E.C.S require very high temperature for reduction. So many metals are obtained through electrolysis.

EXTRACTION OF SODIUM

Sodium is extracted by electrolysis of a mixture of molten sodium chloride and calcium chloride. The calcium chloride is added in order to reduce the temperature from over 8000c to 6000c.

The cell used is Down’s cell, sodium is liberated as the cathode, while chlorine is liberate at the anode

The two elements are separated from each other by cylindrical iron gauze

EXTRACTION OF ALUMINIUM

Aluminium is extracted by electrolysis from its ore (oxide Al2O3) which occur as hydrated aluminium oxide (bauxite) Al2O3.2H2O.

The purified bauxite is dissolved in molten cryolite (Na3AlF6) and is electrolyzed. The anode and cathode are both carbon

USES OF ALUMINIUM

1. it’s used for making kitchen vessels such as pots, pans etc.

2. it’s used in moving parts of machines and in engines example pistons and cylinders.

3. it’s used for making objects that must be as light as possible

Example parts of aero planes, railways trains and truck, buses, lorries, tankers, furniture and car etc

4. in over head tension cables for distribution of electric power

5. Other uses are packing materials for cigarette, sweets, biscuits etc.

EXTRACTION OF IRON AND STEEL

The main ores of iron are haematite, Fe2O3 and Magnetite Fe3O4, iron II carbonate, FeCO3 also occurs. The ore is heated to expel water. It is then loaded into the top of blast of furnace together with coke and limestone

Iron is extracted from its ore by reduction in a blast furnace

STAGE 1;

Near the base of the furnace where a hot air blast enters by the tuyeres, coke burns in air to produce carbon dioxide, providing the main source of heat for the process

STAGE 2;

A little higher in the furnace the carbon dioxide is reduced by hot coke to carbon monoxide

STAGE 3;

At higher stage still carbon monoxide reduces the ore to iron, some reduction may also occur directly by carbon

As it settles in the furnace the iron melts and drops into the bottom of the funnel

THE FUNCTION OF THE LIMESTONE (CaCO3)

The limestone is heated and decomposes quick lime (CaO) and CO2

The quicklime (CaO) reacts with acid impurities in the ore to form molten slag

Slag and iron are tapped off separately at frequent intervals. The iron being cast into “pigs” and is known as pig iron or cast iron. It contains many impurities like C, P, Si, Mn, S and thus it is brittle. It can however be cast by pouring into moulds as liquid to produce many articles in which great strength is necessary, Si, Mn, S and thus it is brittle. It can however be cast by pouring into moulds as liquid to produce many articles in which great strength is necessary.
Example; Railings, water pipes, bases of Bunsen, cookers, stoves etc

WROUGHT IRON:

It is the purest form of commercial iron. It contains 99% iron and less than 0.25% of carbon, to make wrought iron, cast iron is melted and stirred in a furnace carbon in the cast iron is oxidized away of gas (CO and CO2) while other impurities
Example Mn and Si are oxidized to a slag

Function of limestone- to remove impurities

(i) The wrought iron is strong and malleable. It can be shaped by hammering at very high temperature, 10000c.

(ii) It is used for making nails, sheets, chains, gates, farm machines etc.

STEEL

This is an alloy of iron with 0.15% to 0.17% or carbon. Steel is obtained by oxidizing the impurities in molten pig iron using a jet or “tonnage oxygen” directly vertically down the metal; after removing of impurities a calculated mass of carbon and (Ni, Co, Cs, Mn, It needed) is added.

PROPERTIES OF METALS

PHYSICAL PROPERTIES OF METALS AS COMPARED TO NON – METALS

PROPERTY	METALS	NON – METALS
Thermal or electrical conductivity	High	Low
Lustre	High	Low
Sonority	High	Low
Ductility	High	Low
Tensile strength	High	Low
Malleability	High	Low