Calcium oxide reacts with water to form calcium hydroxide. What type of reaction is this, and what experiment…

Question

Calcium oxide reacts with water to form calcium hydroxide. What type of reaction is this, and what experimental observation tells you that energy is involved in this process?

Accepted Answer

## Model Answer This is a **combination reaction** because two reactants (CaO and H₂O) combine to form a single product (calcium hydroxide). $$	ext{CaO}(s) + 	ext{H}_2	ext{O}(l) ightarrow 	ext{Ca(OH)}_2(aq) + 	ext{Heat}$$ **Experimental observation:** When water is added to calcium oxide, the beaker becomes hot (rise in temperature is felt on touching the beaker), showing that a large amount of heat is released. This makes it an **exothermic reaction**. *Source: Chapter 1, Section 1.2.1 Combination Reaction* --- ## Explanation - **1 mark** for correctly identifying it as a combination reaction (and optionally exothermic). - **1 mark** for the observation — touching the beaker and feeling the rise in temperature is the key experimental evidence examiners expect. Saying "heat is released" without linking it to the observation may cost you the mark. - Always write the balanced equation when the reaction is mentioned — it strengthens your answer and takes only one line.

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