What is the limestone cycle, and how is limestone used to make useful materials?
Thermal decomposition of limestone, the reactions of the limestone cycle, and the uses of limestone, quicklime, slaked lime and cement.
A focused answer to the WJEC GCSE Science Double Award Unit 2 topic on limestone, covering thermal decomposition, the reactions of the limestone cycle, and the uses of limestone, quicklime, slaked lime and cement.
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What this dot point is asking
WJEC Double Award Unit 2 wants you to describe the thermal decomposition of limestone, the reactions of the limestone cycle, and the uses of limestone, quicklime, slaked lime and cement.
What limestone is
Thermal decomposition
Thermal decomposition means a single compound is broken down by heat into simpler substances.
The limestone cycle
The reactions link together in the limestone cycle:
- Limestone (calcium carbonate) is heated to give quicklime (calcium oxide) and carbon dioxide.
- Quicklime + water gives slaked lime (calcium hydroxide), releasing a lot of heat.
- Slaked lime dissolves a little in water to give limewater (calcium hydroxide solution).
- Limewater reacts with carbon dioxide to form calcium carbonate again (turning cloudy), completing the cycle.
The reaction of limewater with carbon dioxide is the standard test for carbon dioxide: the gas turns limewater cloudy (milky).
Limestone and acids
Like all carbonates, limestone (calcium carbonate) reacts with acids. The reaction produces a salt, water and carbon dioxide: for example, calcium carbonate + hydrochloric acid to calcium chloride + water + carbon dioxide. This is why acid rain slowly damages limestone buildings and statues, dissolving the stone and leaving them worn. It is also the reason carbonates fizz when acid is added, which can be used as a test for a carbonate. Knowing this general reaction (carbonate + acid) links limestone to the acids and salts content of the course.
Uses of limestone and its products
- Limestone: a building stone; crushed for roads; used to make cement (heated with clay), concrete (cement, sand and gravel) and glass (with sand and sodium carbonate).
- Quicklime and slaked lime: alkaline, so they are used to neutralise acidic soil in farming and to treat acidic water and industrial waste.
- Limewater: used to test for carbon dioxide.
The advantages and drawbacks of quarrying
Getting limestone means quarrying it, which brings benefits and problems that exam questions ask you to balance. Quarrying provides useful building materials and jobs for local people, and the products (cement, concrete, glass) are widely needed. However, quarrying destroys habitats, creates noise and dust, brings extra lorry traffic, and leaves a scar on the landscape. After quarrying, the site can be restored by replanting and landscaping. Being able to give an advantage and a disadvantage, and reach a balanced conclusion, is a common evaluation question.
Try this
Q1. Name the gas given off when limestone is thermally decomposed. [1 mark]
- Cue. Carbon dioxide.
Q2. What is the chemical name for slaked lime? [1 mark]
- Cue. Calcium hydroxide.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC style4 marksDescribe the reactions that turn limestone into quicklime and then into slaked lime, giving the word equations.Show worked answer →
A Unit 2 structured question worth 4 marks. Reward: heating limestone (calcium carbonate) thermally decomposes it to quicklime (calcium oxide) and carbon dioxide: calcium carbonate to calcium oxide + carbon dioxide (2); adding water to quicklime makes slaked lime (calcium hydroxide): calcium oxide + water to calcium hydroxide (2). Markers credit each correct word equation. A common error is to confuse quicklime (calcium oxide) with slaked lime (calcium hydroxide).
WJEC style3 marksGive one use of limestone in the construction industry and explain why slaked lime is useful in farming.Show worked answer →
A Unit 2 application question. Reward: limestone (or cement/concrete made from it) is used as a building material for walls, roads or foundations (1); slaked lime (calcium hydroxide) is alkaline, so it is spread on fields to neutralise acidic soil (1), helping crops grow better (1). Markers credit a construction use and the neutralisation of acidic soil. A common error is to give a vague "it is used a lot" answer without the chemistry.
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