Which of the following types of reaction represents formation of slaked lime from calcium oxide and water?

Question

Examoogle · Accepted Answer

The correct answer is Option D: Combination reaction. The formation of slaked lime from calcium oxide and water is a combination reaction. In this reaction, calcium oxide ($CaO$), also known as quicklime, reacts vigorously with water ($H_2O$) to form a single product, calcium hydroxide ($Ca(OH)_2$), known as slaked lime, along with the release of a large amount of heat.

Step-by-Step Solution & Concepts:
The formation of slaked lime from calcium oxide and water is a combination reaction. In this reaction, calcium oxide ($CaO$), also known as quicklime, reacts vigorously with water ($H_2O$) to form a single product, calcium hydroxide ($Ca(OH)_2$), known as slaked lime, along with the release of a large amount of heat.

Working Principle: When solid calcium oxide is added to water, an exothermic chemical reaction takes place. Two reactants combine to form a single product without any byproducts, which defines a combination reaction. The chemical equation is given by: $CaO(s) + H_2O(l) ightarrow Ca(OH)_2(aq) + 	ext{Heat}$.

Key Formulas: $CaO_{(s)} + H_2O_{(l)} ightarrow Ca(OH)_{2(aq)} + 	ext{Heat}$ — Reaction of quicklime with water; $Ca(OH)_{2(aq)} + CO_{2(g)} ightarrow CaCO_{3(s)} + H_2O_{(l)}$ — Whitewashing reaction forming calcium carbonate

Key Points: Calcium oxide ($CaO$) is commonly known as quicklime or burnt lime.; Calcium hydroxide ($Ca(OH)_2$) is known as slaked lime.; This is an exothermic combination reaction where two or more reactants combine to form a single product with the release of heat.; Slaked lime solution is used for white-washing walls because it reacts slowly with carbon dioxide in the air to form a shiny layer of calcium carbonate ($CaCO_3$).

Advantages: Produces a clear suspension used effectively for traditional wall painting.; Simple one-step combination process.

Disadvantages: Vigorous exothermic reaction can cause boiling and splattering if water is added improperly to large amounts of quicklime.

Applications: Whitewashing of walls in buildings.; Neutralization of acidic soils in agriculture.; Manufacturing of bleaching powder.

Additional Info: Option A (Decomposition reaction) involves a single reactant breaking down into two or more products, which is the opposite of this reaction.. Option B (Precipitation reaction) involves the formation of an insoluble salt when two solutions are mixed.. Option C (Displacement reaction) involves a more reactive element displacing a less reactive element from its compound.

Answer: D is correct — Combination reaction

Exam Tip: Always remember that combination reactions usually release heat (exothermic), whereas decomposition reactions generally absorb heat or require energy (endothermic).

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Option A: Decomposition reaction

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Option B: Precipitation reaction

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Option C: Displacement reaction

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