INTRODUCTION
Natural fractional crystallization takes place in Kenya/East Africa at:
(i) Lake Magadi during extraction of soda ash(Sodium carbonate) from Trona(sodium sesquicarbonate)
(ii) Ngomeni near Malindi at the Indian Ocean Coastline during the extraction of common salt(sodium chloride).
Extraction of soda ash from Lake Magadi in Kenya
Rain water drains underground in the great rift valley and percolate underground where it is heated geothermically.
The hot water dissolves underground soluble sodium compounds and comes out on the surface as alkaline springs which are found around the edges of Lake Magadi in Kenya.
Temperatures around the lake are very high (30-40oC) during the day.
The solubility of trona decrease with increase in temperature therefore solid crystals of trona grows on top of the lake (upto or more than 30metres thick)
A bucket dredger mines the trona which is then crushed ,mixed with lake liquor and pumped to washery plant where it is further refined to a green granular product called CRS.
The CRS is then heated to chemically decompose trona to soda ash(Sodium carbonate)
Chemical equation
2Na2CO3.NaHCO3.2H2O(s) -> 3Na2CO3 (s) + CO2(g) + 5H2O(l)
Soda ash(Sodium carbonate) is then stored .It is called Magadi Soda. Magadi Soda is used :
- make glass
- for making soapless detergents
- softening hard water.
Common salt is colledcted at night because its solubility decreases with decrease in temperature. It is used as salt lick/feed for animals.
Extraction of common salt from Indian Ocean at Ngomeni in Kenya
Oceans are salty.They contain a variety of dissolved salts (about 77% being sodium chloride).
During high tide ,water is collected into shallow pods and allowed to crystallize as evaporation takes place.The pods are constructed in series to increase the rate of evaporation.
At the final pod ,the crystals are scapped together,piled in a heap and washed with brine (concentrated sodium chloride).
It contains MgCl2 and CaCl2 . MgCl2 and CaCl2are hygroscopic. They absorb water from the atmosphere and form a solution.
This makes table salt damp/wet on exposure to the atmosphere.
24.Some water form lather easily with soap while others do not.
Water which form lather easily with soap is said to be “soft”
Water which do not form lather easily with soap is said to be “hard”
Hardness of water is caused by the presence of Ca2+ and Mg2+ ions.
Ca2+ and Mg2+ ions react with soap to form an insoluble grey /white suspension/precipitate called Scum/ curd. Ca2+ and Mg2+ ions in water come from the water sources passing through rocks containing soluble salts of Ca2+ and Mg2+ e.g. Limestone or gypsum
There are two types of water hardness:
(a)temporary hardness of water
(b)permanent hardness of water
(a)temporary hardness of water
Temporary hardness of water is caused by the presence of dissolved calcium hydrogen carbonate/Ca(HCO3)2 and magnesium hydrogen carbonate/Mg(HCO3)2
When rain water dissolve carbon(IV) oxide from the air it forms waek carbonic(IV) acid i.e.
CO2(g) + H2O(l) -> H2CO3(aq)
When carbonic(IV) acid passes through limestone/dolomite rocks it reacts to form soluble salts i.e.
In limestone areas; H2CO3(aq) + CaCO3(s) -> Ca(HCO3)2 (aq)
In dolomite areas; H2CO3(aq) + MgCO3(s) -> Mg(HCO3)2 (aq)
(b)permanent hardness of water
Permanent hardness of water is caused by the presence of dissolved calcium sulphate(VI)/CaSO4 and magnesium sulphate(VI)/Mg SO4 Permanent hardness of water is caused by water dissolving CaSO4 and MgSO4 from ground rocks.
Hardness of water can be removed by the following methods:
(a)Removing temporary hardness of water
(i)Boiling/heating.
Boiling decomposes insoluble calcium hydrogen carbonate/Ca(HCO3)2 and magnesium hydrogen carbonate/Mg(HCO3)2 to insoluble CaCO3 and MgCO3 that precipitate away. i.e
Chemical equation
Ca(HCO3)2(aq) -> CaCO3 (s) + CO2(g) + H2O(l)
Mg(HCO3)2(aq) -> MgCO3 (s) + CO2(g) + H2O(l)
(ii)Adding sodium carbonate (IV) /Washing soda.
Since boiling is expensive on a large scale ,a calculated amount of sodium carbonate decahydrate /Na2CO3.10H2O precipitates insoluble Ca2+(aq) and Mg2+(aq) ions as carbonates to remove both temporary and permanent hardness of water .This a double decomposition reaction where two soluble salts form an insoluble and soluble salt. i.e.
(i)with temporary hard water
Chemical equation
Na2CO3 (aq) + Ca(HCO3) 2 (aq) -> NaHCO3(aq) + CaCO3 (s)
Na2CO3 (aq) + Mg(HCO3) 2 (aq) -> NaHCO3(aq) + MgCO3 (s)
Ionic equation
CO32- (aq) + Ca2+ (aq) -> CaCO3 (s)
CO32- (aq) + Mg2+ (aq) -> MgCO3 (s)
(ii)with permanent hard water
Chemical equation
Na2CO3 (aq) + MgSO4 (aq) -> Na2SO4 (aq) + MgCO3 (s)
Na2CO3 (aq) + CaSO4 (aq) -> Na2SO4 (aq) + MgCO3 (s)
Ionic equation
CO32- (aq) + Ca2+ (aq) -> CaCO3 (s)
CO32- (aq) + Mg2+ (aq) -> MgCO3 (s)
(iii)Adding calcium (II)hydroxide/Lime water
Lime water/calcium hydroxide removes only temporary hardness of water from by precipitating insoluble calcium carbonate(IV).
Chemical equation
Ca(OH)2 (aq) + Ca(HCO3) 2 (aq) -> 2H2O(l) + 2CaCO3 (s)
Excess of Lime water/calcium hydroxide should not be used because it dissolves again to form soluble calcium hydrogen carbonate(IV) causing the hardness again.
(iv)Adding aqueous ammonia
Aqueous ammonia removes temporary hardness of water by precipitating insoluble calcium carbonate(IV) and magnesium carbonate(IV)
Chemical equation
2NH3 (aq) + Ca(HCO3) 2 (aq) -> (NH4) 2CO3(aq) + CaCO3 (s)
2NH3 (aq) + Mg(HCO3) 2 (aq) -> (NH4) 2CO3(aq) + MgCO3 (s)
(v)Use of ion-exchange permutit
This method involves packing a chamber with a resin made of insoluble complex of sodium salt called sodium permutit.
The sodium permutit releases sodium ions that are exchanged with Mg2+ and Ca2+ ions in hard water making the water to be soft. i.e.
Na2X(aq) + Ca2+ (aq) -> Na+ (aq) + CaX(s)
Na2X(aq) + Mg2+ (aq) -> Na+ (aq) + MgX(s)
Hard water containing Mg2+ and Ca2+
Ion exchange resin such as
Sodium permutit
Na+ ions replace Mg2+
and Ca2+ to make the water soft.
When all the Na+ ions in the resin is fully exchanged with Ca2+ and Ng2+ ions in the permutit column ,it is said to be exhausted.
Brine /concentrated sodium chloride solution is passed through the permutit column to regenerated /recharge the column again.
Hard water containing Mg2+ and Ca2+
Ion exchange resin as
Sodium permutit
Na+ ions replace Mg2+
and Ca2+ to make the water soft.
(vi)Deionization /demineralization
This is an advanced ion exchange method of producing deionized water .Deionized water is extremely pure water made only of hydrogen and oxygen only without any dissolved substances.
Deionization involve using the resins that remove all the cations by using:
(i)A cation exchanger which remove /absorb all the cations present in water and leave only H+ ions.
(ii)An anion exchanger which remove /absorb all the anions present in water and leave only OH– ions.
The H+(aq) and OH–(aq) neutralize each other to form pure water.
Chemical equation
H+(aq) + OH–(aq) -> H2O(l)
When exhausted the cation exchanger is regenerated by adding H+(aq) from sulphuric(VI)acid/hydrochloric acid.
When exhausted the anion exchanger is regenerated by adding OH–(aq) from sodium hydroxide.
Advantages of hard water
Hard water has the following advantages:
(i)Ca2+(aq) in hard water are useful in bone and teeth formation
(ii) is good for brewing beer
(iii)contains minerals that cause it to have better /sweet taste
(iv)animals like snails and coral polyps use calcium to make their shells and coral reefs respectively.
(v)processing mineral water
Disadvantages of hard water
Hardness of water:
(i)waste a lot of soap during washing before lather is formed.
(ii)causes stains/blemishes/marks on clothes/garments
(iii)causes fur on electric appliances like kettle ,boilers and pipes form decomposition of carbonates on heating .This reduces their efficiency hence more/higher cost of power/electricity.
