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Unit 4 Rocks and minerals
Teaching Notes
4.3
Investigating calcium carbonate
Direct uses of limestone

Extracting iron from iron ore

Neutralizing acidity in soil - Powdered limestone is most often used to neutralize acidity in soil. It is
because it is cheaper than any form of lime (calcium oxide).

Manufacturing cement and concrete - Cement contains a mixture of calcium aluminate Ca(AlO2)2
and calcium silicate CaSiO3. It is made by mixing limestone with shale, heating the mixture slowly
and strongly, adding gypsum (calcium sulphate) and grinding up the final lumps of solid. When it is
mixed with water it dries into a hard solid. To produce concrete, cement is mixed with aggregate (a
mixture of stone chips and sand) and water is added. As the concrete ‘sets’, the compounds in the
cement bond to form a hard mass of hydrated crystals.

Manufacturing sodium carbonate - Sodium carbonate is an important industrial chemical. It is used
in the manufacture of soaps, detergents, dyes, drugs and other chemicals. Sodium carbonate is
manufactured using calcium carbonate, sodium chloride, carbon dioxide and ammonia.
Indirect uses of limestone
Limestone gives calcium oxide when heated. Calcium oxide is used:

to neutralize acidity in soil;

as a drying agent;

to make soda glass, by heating sand with soda (sodium carbonate) and calcium oxide.
Calcium oxide gives calcium hydroxide when water is added to it. Calcium hydroxide is used:

to neutralize acidity in soil;
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to make bleaching powder;

to purify water.
4.5
Formation of limestone caves
Weathering
There are two types of weathering: physical and chemical.
Physical weathering
It is the process in which rocks are broken down into smaller pieces but not chemically changed. For
example, plant roots grow into cracks on rocks and slowly prise them apart. On the other hand, water in
the cracks expands as it freezes in winter, forcing the cracks wider. The rocks break up eventually.
Chemical weathering
It is the process in which rocks are broken down by reactions with air and water and are themselves
chemically changed. The reaction between limestone and rain water to form soluble calcium
hydrogencarbonate is an example.
Erosion
The products of weathering are then moved by a transport agent. The process is called erosion.
For example, river water can be the transport agent. The more energy the water possesses, the
faster it can erode and the more material it can transport.
When the water slows down, it deposits the material as sediment. Some is deposited in the flood plain.
Some is carried out to sea and falls to the sea floor.
Hardness in water
How water acquires hardness
Calcium hydrogencarbonate is the most common cause of hard water. It forms when rain falls on rocks
of limestone and chalk. Similarly rainwater reacts with dolomite (CaCO3MgCO3) and gypsum
(CaSO42H2O) to give other compounds that make water hard.
Hardness in water can be divided into two types - temporary and permanent. Temporary hardness
is caused by the presence of dissolved calcium or magnesium hydrogencarbonate. It is called temporary
hardness because it can be removed easily by boiling. Permanent hardness is caused by the presence
of dissolved calcium or magnesium sulphate. Permanent hardness is so called because it is much more
difficult to remove and certainly cannot be removed by boiling.
Ways of softening hard water
Boiling
Temporary harness is easily removed by boiling. When heated the calcium hydrogencarbonate
decomposes, producing insoluble calcium carbonate.
Calcium hydrogencarbonate
heat
calcium carbonate + water + carbon dioxide
Addition of washing soda
Washing soda is sodium carbonate. It removes both temporary and permanent hardness by precipitating
calcium carbonate. For example, it reacts with calcium sulphate:
Sodium carbonate + calcium sulphate
calcium carbonate + sodium sulphate
Bath salts contain sodium carbonate to soften the bath water.
Ion exchange
In ion exchange, unwanted ions are removed by replacing them with ‘harmless’ ions.
A typical ion exchanger is a container full of small beads. The small beads are made of a special
plastic called ion exchange resin, which has the ‘harmless’ ions weakly attached to it. These ‘harmless’
ions are usually sodium ions. When hard water flows through, the calcium and magnesium ions in it
switch places with the sodium ions and attach themselves to the resin. The sodium ions are carried away.
When all the sodium ions have been removed from the resin, it can be regenerated by pouring a solution
of a suitable sodium salt through it.