AI-generated practice question — model-generated for extra practice, not a previous-year CBSE board question.
Potassium and sodium are highly reactive metals. When they react with cold water, the reaction is so vigorous and exothermic that the hydrogen gas evolved immediately catches fire.
In contrast, calcium's reaction with water is less violent and the heat evolved is not sufficient to ignite the hydrogen gas produced. Hence calcium does not catch fire.
$$2\text{Na(s)} + 2\text{H}_2\text{O(l)} \rightarrow 2\text{NaOH(aq)} + \text{H}_2\text{(g)} + \text{heat (large)}$$
$$\text{Ca(s)} + 2\text{H}_2\text{O(l)} \rightarrow \text{Ca(OH)}_2\text{(aq)} + \text{H}_2\text{(g)} + \text{heat (less)}$$
Source: Chapter 3, Section 3.2.2
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The key distinction examiners look for is reactivity/heat evolved, not the gas produced (all three produce H₂). Award-winning answers state that K and Na react violently/exothermically enough to ignite H₂, while Ca's reaction is less exothermic so H₂ does not catch fire. Including the equations earns full marks for a 2-mark question.