Limestone

Limestone is mainly calcium carbonate, CaCO₃. When it is heated, it breaks down to form calcium oxide and carbon dioxide. Calcium oxide reacts with water to produce calcium hydroxide.

Limestone and its products have many uses, including being used to make mortar, cement, concrete and glass.

Thermal decomposition

Metal carbonates such as calcium carbonate break down when heated strongly. This is called thermal decomposition. Here are the equations for the thermal decomposition of calcium carbonate:

calcium carbonate  calcium oxide + carbon dioxide

CaCO₃ CaO + CO₂

Other metal carbonates decompose in the same way. Here are the equations for the thermal decomposition of copper carbonate:

copper carbonate  copper oxide + carbon dioxide

CuCO₃ CuO + CO₂

Notice that in both examples the products are a metal oxide and carbon dioxide. The carbon dioxide gas can be detected using limewater. Limewater turns cloudy white when carbon dioxide is bubbled through it.

Metals high up in the reactivity series - such as calcium - have carbonates that need a lot of energy to decompose them. Metals low down in the reactivity series - such as copper - have carbonates that are easily decomposed. This is why copper carbonate is often used at school to show these reactions. It is easily decomposed, and its colour change, from green copper carbonate to black copper oxide, is easy to see

Quicklime and slaked lime

For your exam, you need to know how quicklime and slaked lime are obtained from limestone.

Making quicklime

If limestone is heated strongly, it breaks down to form calcium oxide and carbon dioxide. Calcium oxide is also called quicklime. It is yellow when hot, but white when cold.

Here are the equations for this reaction:

calcium carbonate  calcium oxide + carbon dioxide

CaCO₃ CaO + CO₂

This is a thermal decomposition reaction.

Making slaked lime

Calcium oxide reacts with water to form calcium hydroxide, also called slaked lime.

Here are the equations for this reaction:

calcium oxide + water → calcium hydroxide

CaO + H₂O → Ca(OH)₂

A lot of heat is produced in the reaction, which may even cause the water to boil.

Summary

Using common names instead of chemical names, this is what happens:

limestone  quicklime + carbon dioxide

quicklime + water → slaked lime

Uses of limestone

Limestone, quicklime and slaked lime are all used to neutralise excess acidity - which may be caused by acid rain - in lakes and in soils.

Limestone is used as a building material, and to purify iron in blast furnaces. It's also used in the manufacture of glass, and of cement (one of the components of concrete).

The main uses of limestone and its products

• Limestone (CaCO₃) can be used as a building material and in the manufacturing of iron.
• Glass - heated with sand and soda (sodium carbonate).
• Cement - heated with clay in a kiln.
  • Concrete - mixed with sand, water and crushed rock
  • Mortar - mixed with sand and water
• Quicklime - heated.
  • Slaked lime (Calcium Hydroxide Ca(OH)₂) - mixed with water
    • Lime motar - mixed with water

Glass

Glass is made by melting sand and then cooling it. Flat sheets of glass for windows are made by floating molten glass on a layer of molten tin.

Glass manufacturers add sodium carbonate to sand during the manufacturing process, to reduce the melting temperature of the sand and so save energy. The sodium carbonate decomposes in the heat to form sodium oxide and carbon dioxide, but this makes the glass soluble in water. Calcium carbonate (limestone) is therefore also added, to stop the glass dissolving in water. The calcium carbonate decomposes in the heat to form calcium oxide and carbon dioxide. About 90 per cent of glass is soda-lime glass, or bottle glass.

Environmental, social and economic considerations

The limestone industry

You need to be able to evaluate some of the effects of the limestone industry.

The main advantages and disadvantages of the limestone industry

Advantages	Disadvantages
Limestone is a valuable natural resource, used to make things such as glass and concrete.	Limestone quarries are visible from long distances and may permanently disfigure the local environment.
Limestone quarrying provides employment opportunities that support the local economy in towns around the quarry.	Quarrying is a heavy industry that creates noise and heavy traffic, which damages people's quality of life.

Advantages and disadvantages of various building materials

Limestone, cement and mortar slowly react with carbon dioxide dissolved in rainwater, and wear away. This damages walls made from limestone, and it leaves gaps between bricks in buildings. These gaps must be filled in or "pointed". Pollution from burning fossil fuels makes the rain more acidic than it should be, and this acid rain makes these problems worse.

Concrete is easily formed into different shapes before it sets hard. It is strong when squashed, but weak when bent or stretched. However, concrete can be made much stronger by reinforcing it with steel. Some people think that concrete buildings and bridges are unattractive.

Glass is usually brittle and easily shattered, but toughened glass can be used for windows. While glass is transparent and so lets light into a building, buildings with lots of glass can be too hot in the summer.