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GCSE Chemistry - Earth Sciences

Earth Sciences - The Calcium Cycle

(1) Limestone :

Calcium is a very common element in nature. It occurs most commonly in limestone rock as calcium carbonate, CaCO3. Limestone itself is used in many industrial processes, e.g. to make iron in the Blast Furnace, as a component of cement and in the manufacture of glass.

Limestone is commonly mines in open-cast mines, where the rock is blasted out of the ground, leaving massive pits. This is a relatively straight-forward and economically cheap way of mining; though it has a great impact on the local environment, with the pits requiring a lot of money to blend them into the local area once all the limestone has been extracted.

(2) Quicklime :

When calcium carbonate is heated strongly in a slowly rotating kiln (see the diagram below)

it undergoes thermal decomposition to give calcium oxide, CaO, also known as quicklime, and carbon dioxide gas.

Equations -

calcium carbonate → calcium oxide + carbon dioxide

CaCO3(s) → CaO(s) + CO2(g)

(3) Slaked Lime :

A hydration reaction is one where water is added to a compound to make a new compound. When water is added to calcium oxide, calcium hydroxide, Ca(OH)2, is produced. This process used to be known as slaking lime and so calcium hydroxide is also known as slaked lime.

Equations -

calcium oxide + water → calcium hydroxide

CaO(s) + H2O(l) → Ca(OH)2(s)

Calcium hydroxide in a solid form is used to neutralise acidity on farm land and in lakes affected by acid rain.

A dilute aqueous solution of calcium hydroxide is known as limewater and is used the laboratory to test for carbon dioxide gas. When carbon dioxide gas reacts with calcium hydroxide a white precipitate is formed. This precipitate is calcium carbonate

Equations -

calcium hydroxide + carbon dioxide → calcium carbonate + water

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

(4) Summary :

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Earth Sciences - Rocks

(1) Igneous :

Igneous rocks are formed when molten rock, magma, cools down and solidifies. This forms rocks with a glassy appearance.

There are two different types of igneous rocks - intrusive and extrusive.

Intrusive igneous rocks are formed when the magma is forced up into the earth's crust, but does not break out. The rock slowly cools, forming large interlocking crystals. An example of an intrusive igneous rock is granite.

Extrusive igneous rocks are formed when the magma is forced up through the earth's crust onto the earth's surface in a volcanic eruption. The rock cools quickly in air and forms small interlocking crystals. An example of an extrusive igneous rock is basalt.

(2) Metamorphic:

When different types of rocks are buried underground during tectonic activity, i.e. earthquakes, they can be compressed by the pressure of the rocks above them and heated by the earth's core. This pressure and heat can change the rock's texture and structure, without the rock first being melted. Thus the resulting rocks are called metamorphic.

Metamorphic rocks are composed of interlocking crystals and are generally layered, and can be spilt, or cleaved, along these layers.

Examples of metamorphic rocks are slate, which is formed from mudstone, and marble, which is formed from limestone.

(3) Sedimentary:

Sedimentary rocks are formed over millions of years by the building up of layers of sediment, i.e. small particles of solids, as water washes over and slowly erodes larger rocks. The weight of these layers squeezes the water out and salts crystallize out of the water, acting like a glue and sticking the small solid particles together.

Examples of sedimentary rocks are sandstone, which is formed from grains of sand, and limestone, which is formed from the crushed shells of dead creatures.

Sedimentary rocks often contain fossils of prehistoric plants and animals, and fossils contained in the same layer will come from the same age; so dating of fossilized remains can be achieved.

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written by Dr Richard Clarkson : © Saturday, 1 November 1997

updated : Sunday, 29th April, 2012

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