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When a few pieces of calcium oxide (CaO) are added to water, calcium hydroxide forms with a large release of heat. Which equation correctly represents this combination reaction?
ACaO + CO$_2$ $\longrightarrow$ CaCO$_3$
BCaO + H$_2$O $\longrightarrow$ Ca(OH)$_2$ + Heat
CCa(OH)$_2$ $\longrightarrow$ CaO + H$_2$O
DCaCO$_3$ $\longrightarrow$ CaO + CO$_2$
Answer & Solution
Correct answer: B. CaO + H$_2$O $\longrightarrow$ Ca(OH)$_2$ + Heat
1. The reactants given are calcium oxide and water; the product is calcium hydroxide.
2. So CaO + H$_2$O combine to form Ca(OH)$_2$, releasing heat.
3. The equation CaCO$_3$ $\longrightarrow$ CaO + CO$_2$ is the decomposition of limestone, not this formation.
4. The option splitting Ca(OH)$_2$ is the reverse direction, and the CaO + CO$_2$ option uses CO$_2$, which is not a reactant here.
_Source: Balbharati (Maharashtra Board) Class 10 Science & Technology, Ch 3 "Chemical Reactions and Equations", p.45_
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