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Chemistry revision notes – Earth Materials

Limestone is a rock of which was once the bones and shells of sea animals. These died and collected as sediments at the bottom of seas, oceans and lakes.

Chemical name: Calcium carbonate

Chemical formula: CaCo3

Alternative names: Chalk, marble

Limestone can be used to prepare a number of new materials. For this to happen limestone must undergo endothermic decomposition.

Thermal decomposition is used on a large scale in industry to manufacture chemicals.

When limestone undergoes thermal decomposition it produces Quicklime (CaO)

Quicklime is produced in refractory-lined kilns. In most kilns, limestone is fed in at the top of the shaft and quicklime is drawn from the bottom.

Air is blown in at the base and burns with fuels part of the way up the kiln. The products of combustion of the fuel is mostly carbon dioxide gas which escapes through vents at the top of the shaft.

Various fuels are used e.g. coil, oil, natural gas and coke depending on the local fuel costs. The temperature created by these fuels reaches over 1000°C in the middle of the kiln.

Most modern kilns produce up to 300 tonnes/day of quicklime. Most quicklime is sold as a granular product but there is a growing market for the finely crushed solid.

Limestone is one of the world’s most common minerals. Because it is cheap and readily available it forms an important part of many other minerals.

Once limestone has been converted to quicklime, the quicklime is used to form slaked lime.

Slaked lime (calcium hydroxide) is made by adding a little water to quicklime.

Powdered limestone:

  • Put in lakes to neutralize acids in the water
  • Put on soil to neutralize acid soil.

All rocks are made up of different minerals. In other words, minerals are the chemical constituents of rocks although you can’t easily tell what their chemical composition is from their names.

Sedimentary rocks are made of sediment formed when all other types of rock wear down (erosion) or break down because of the effects of weather (weathering). The particles of rock are transported by water or wind or ice to a low point on the earth (a sea or a valley). Gradually, over millions of years, they are converted to rock.

The particles of rock form layers of sediment. Gradually more and more layers form on top of each other over thousands of years. The weight of the layers on top causes the bottom layers to be pressed together or compressed so the particles fuse together. This is called CEMENTATION.

Over millions of years the layers of rock build up into what are called strata. Particles can be rubbed of sedimentary rock because it is the softest type of rock.

Different sized particles form different kinds of rock. Very fine particles like mud form mudstone. Larger grains like sand form sandstone. Gravel and pebbles form conglomerates.

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Sedimentary rocks like Shale can be formed under the sea also. The shells from millions of tiny marine animals can build up on the sea floor. These shells are made from calcium carbonate. When these shells become slowly compressed together they form the rock limestone. When acid is added to this rock, it fizzes.

As sediments collects, the bodies of plants or animals fall into them. The bodies become buried and gradually turn to rock as there is no oxygen gas to allow them to rot. Over millions of years these bodies turn into fossils.

Fossils are only found in ...

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