Limestone

Limestone is found in many parts of the United Kingdom, notably the Yorkshire Dales, Derbyshire Peaks, Sussex Downs and Mendip Hills. As well as giving a beautiful and varied countryside, limestone is a very useful raw material.

Limestone is one form of calcium carbonate (CaCO3); however, calcium carbonate is also found in nature as chalk, calcite and marble. It is the second most abundant mineral in the Earth’s crust after different types of silicates.

Deposits of chalk were formed from the shells of dead marine creatures that lived many millions of years ago. In several places in the UK, chalk was covered with other types of rock and, therefore, it was put under great pressure. This caused the relatively soft chalk to change into the harder material, limestone.

Marble formed in places where the chalk was not only subjected to high pressures but also to a high temperature.

About 80 million tonnes of limestone are quarried in Great Britain each year.

Direct use of limestone

Limestone has a variety of uses but the most important ones are:

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Manufacture of iron and steel

In the blast furnace, limestone is used to remove earthy and sandy materials found in the iron ore. First of all, limestone is thermally decomposed by the heat of the furnace into calcium oxide (lime or, sometimes, quicklime) and carbon dioxide:

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The calcium oxide then reacts with sand (SiO2) to form slag (calcium silicate, CaSiO3) which is a liquid at the temperatures of the furnace and can easily be removed:

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Manufacture of cement

Limestone (or chalk) is mixed with clay in a heated rotary kiln. The material produced is called cement. It contains a mixture of calcium aluminate [Ca(AlO2)2] and calcium silicate (CaSiO3). The dry product is ground to a powder and mixed with a little calcium sulphate (CaSO4) to slow down the setting rate of the cement. When water is added to the mixture, complex chemical changes occur, resulting in a hard interlocking mass of crystals of hydrated calcium aluminate and silicate.

Manufacture of glass

When it is cooled, a hot solution of sugar in water can form crystals of sugar or stay clear and viscous like treacle or honey. Honey is a super-cooled liquid because it doesn’t form crystals at the temperature one would expect. In the same way, a mixture of silicon dioxide and metal oxides forms glass, which is also a super-cooled liquid that has not crystallised Glass may crystallise over a period of many years and then become brittle. On the other hand, pieces of ancient Egyptian glass have remained uncrystallised for over 4000 years.

One way of making glass is to mix a saturated solution of brine with limestone to make sodium carbonate decahydrate:

2NaCl(aq, sat.)  +  CaCO3(s)      Na2CO3•10H2O(s)  + CaCl2(aq)

The sodium carbonate is then heated with sand (SiO2) and limestone to form glass. A typical glass contains (by weight) 70% silicon dioxide, 15% sodium oxide (Na2O) and 10% calcium oxide (CaO), with 5% other metal oxides. The sodium and calcium are added as carbonates but lose carbon dioxide to form oxides in the glass. The glass formed in this process softens at 650°C allowing it to be shaped.

There are two distinct constituents of glass. The silicon oxide forms a strong network. The metal oxides help destroy this three-dimensional lattice and lower the melting point by about 1000°C. Without the metal oxides, silicon oxide or silica is a crystalline material called quartz which melts at 1700°C.

Indirect uses of limestone

Lime manufacture

When limestone is heated to 1000°C it undergoes thermal decomposition to form calcium oxide (lime, or sometimes, quicklime) and carbon dioxide:

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This reaction is reversible. The reaction is also an important industrial process and takes place in a lime kiln.

Calcium oxide (lime) is a base and is used by some farmers to spread on fields and neutralise the acidity of the soil. It exhibits properties typical of a base, forming a salt and water when reacting with an acid. For example, with hydrochloric acid:

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Large amounts of calcium oxide are converted into calcium hydroxide [Ca(OH)2] which is also called slaked lime.



Manufacture of slaked lime (calcium hydroxide)

Calcium hydroxide is a cheap industrial alkali. It is used in large quantities for:

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Calcium hydroxide (slaked lime), a white powder, is made by adding an equal amount of water to calcium oxide (lime). A strongly exothermic reaction (slaking) occurs. If a piece of quicklime in a dish has a few drops of water added to it, nothing will appear to happen for a time. Then water vapour is seen to come off, whilst a hissing sound as the water drops on indicates it is becoming hot. It commences to expand and crack, and finally crumbles to a powder, slaked lime:

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A weak solution of calcium hydroxide in water is called limewater. Calcium hydroxide (slaked lime) is mixed with sand to form mortar. Limewater is used to test for carbon dioxide.

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On building sites calcium hydroxide, slaked lime, is mixed with sand to give mortar. Mortar is used to hold bricks together. As mortar is exposed to air, it becomes gradually harder as it reacts with carbon dioxide in the air to form calcium carbonate:

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Space for Additional Notes on Qualitative Analysis

 

 

 

 

 

 

 

 

 

 

 

 

 



A Review of the Limestone Topic

Each topic in the Vth Form Chemistry course is divided into a series of 'learning targets'. These are listed below for work on Limestone. For each 'learning target' you should make an assessment of how you are progressing, in terms of increasing your knowledge and developing a clear understanding of the principles. You should assess your progress on a 1 - 3 scale as follows:

                1=           I feel confident about this aspect of the work and I am encountering few                       problems.

                2=           I am making reasonable progress, but I have encountered a few difficulties and                         feel that I need to go over these particular areas again.

                3=           I am finding this aspect of the topic difficult.

Remember, be fair to yourself - be honest!!

Learning Targets

a)    I appreciate that limestone, chalk and marble are naturally-occurring forms of

        calcium carbonate                                                                                                                                                    

b)    I can describe the decomposition of calcium carbonate on heating to produce

        calcium oxide and carbon dioxide and know that this is an important industrial

        process                                                                                                                                                                        

c)     I can recall that calcium carbonate is used in the manufacture of glass, cement

        and iron                                                                                                                                                                      

d)    I can describe the effect of water on calcium oxide and I appreciate that the solution

        produced (limewater) is alkaline                                                                                                                        

e)     I can recognise the difference between a solution and a suspension                                                       

f)     I understand why calcium oxide and calcium hydroxide are used to neutralise soil

        acidity and that a salt is formed in the reaction                                                                                             

Ideally, all of your responses will be ‘1’. However, this is rarely the case first time through! If you have written a ‘2’ anywhere, you may wish to read through your notes again or look at the relevant page in your text book. If you have written ‘3’ as a response to any of the questions, see me for further help. Alternatively, you may wish to attend the Chemistry Surgery which takes place in the Chemistry Department during Pursuits Periods (Mondays and Fridays, 2.00- 2.45pm). A member of the department will be on hand to sort out your difficulties.