Chapter 1 · Question 6
What is a combination reaction? Describe the reaction of quicklime (calcium oxide) with water, give the balanced chemical equation, and explain its application in whitewashing walls. Is this reaction exothermic?
Q6
What is a combination reaction? Describe the reaction of quicklime (calcium oxide) with water, give the balanced chemical equation, and explain its application in whitewashing walls. Is this reaction exothermic?
Answer Revealed
Direct Answer:
A combination reaction is a chemical reaction in which two or more substances (elements or compounds) combine to form a single product. Quicklime () reacts vigorously with water to form slaked lime () with the release of a large amount of heat, so it is exothermic. Balanced equation: . In whitewashing, a solution of slaked lime is applied to walls. It slowly reacts with from air to form a thin, hard layer of calcium carbonate (), which gives the walls a shiny finish after 2-3 days.
Simple Explanation
In a combination reaction, two or more things join to make just one product — like mixing two ingredients to get a single substance. Quicklime () is a white powder that, when water is added, gets very hot and turns into slaked lime (). This slaked lime is painted on walls, where it slowly absorbs from the air and forms calcium carbonate — the same material as marble — creating a hard, shiny white finish.
Exam-Ready Structure
Combination reactions are one of the fundamental reaction types, and the quicklime-water reaction is a classic NCERT example: 1. Definition: A combination reaction is a reaction in which two or more reactants (elements or compounds) combine to form a single product. General form: . 2. The reaction of calcium oxide with water: (Equation 1.13). Calcium oxide (quicklime) reacts vigorously with water, producing calcium hydroxide (slaked lime) and releasing a large amount of heat — the beaker becomes hot to touch. This makes it both a combination reaction and an exothermic reaction. 3. Application in whitewashing: (a) Slaked lime, , is dissolved in water to make a suspension for whitewashing walls. (b) Once applied to the wall, reacts slowly with carbon dioxide present in the air: (Equation 1.14). (c) The thin layer of calcium carbonate () formed on the wall surface is hard, adheres well, and gives the wall a characteristic shiny finish after 2-3 days. (d) Calcium carbonate is also the chemical name of marble. 4. Other combination reaction examples from the NCERT chapter: burning of coal () and formation of water ().
Key Points
- Combination reaction: two or more reactants → single product ()
- Quicklime + water: (exothermic)
- Slaked lime on walls reacts with air :
- (calcium carbonate) is the chemical name for marble, forms a shiny coating
- Burning of coal () and formation of water are also combination reactions