Limestone is a type of rock mainly calcium carbonate, CaCO3, which when heated breaks down to form calcium oxide and carbon dioxide. Calcium oxide reacts with water to produce calcium hydroxide.
Limestone is quarried (dug out of the ground) and used as a building material. It is also used in the manufacture of cement, mortar and concrete.
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).
Calcium carbonate breaks down when heated strongly. This is thermal decomposition - a reaction where a substance is broken down into at least two other substances by heat.
calcium carbonatecalcium oxide + carbon dioxide
Other metal carbonates decompose in the same way, including sodium carbonate, magnesium carbonate and copper carbonate.
Metals high up in the reactivity series (such as sodium, calcium and magnesium) have carbonates that need a lot of energy to decompose them. Indeed, not all the carbonates of group 1 metals decompose at the temperatures reached by a Bunsen burner. 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 thermal decomposition. It is easily decomposed and its colour change, from green copper carbonate to black copper oxide, is easy to see.
Products from calcium carbonate
If calcium carbonate is heated strongly, it breaks down to form calcium oxide and carbon dioxide. Calcium oxide is yellow when hot, but white when cold.
Calcium oxide reacts with water to form calcium hydroxide, which is an alkali. A lot of heat is produced in the reaction, which may even cause the water to boil.
calcium oxide + water → calcium hydroxide
Uses of limestone
Carbonates react with acids to produce carbon dioxide, a salt and water.
calcium carbonate + hydrochloric acid → carbon dioxide + calcium chloride + water
Since limestone is mostly calcium carbonate, it is damaged by acid rain. Sodium carbonate, magnesium carbonate, zinc carbonate and copper carbonate also react with acids: they fizz when in contact with acids, and the carbon dioxide released can be detected using limewater.
When limestone is heated strongly, the calcium carbonate it contains decomposes to form calcium oxide. This reacts with water to form calcium hydroxide, which is an alkali. Calcium hydroxide is used to neutralise excess acidity, eg. in lakes and soils affected by acid rain.
Cement is made by heating powdered limestone with clay. Cement is an ingredient in mortar and concrete: Mortar, used to join bricks, is made by mixing cement with sand and water. Concrete is made by mixing cement with sand, water and aggregate (crushed rock)
Limestone as building material
Limestone, cement and mortar slowly react with carbon dioxide dissolved in rainwater and wear away. This damages walls made from limestone, and 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.
Quarrying limestone is big business but the need for limestone has to be balanced against the economic, environmental and social effects.
Advantages and disadvantages:
- Limestone is a valuable natural resource, used to make things such as glass and concrete.
- 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.
- Limestone quarries are visible from long distances and may permanently disfigure the local environment.
The limestone cycle