It is observed that Calcium on reaction with water floats on its surface. Explain why it happens. Also write a balanced chemical equation for the reaction that occurs. What happens when the aqueous solution of the product of this reaction reacts with Carbon dioxide gas ? Write a balanced chemical equation for the reaction.
Generated by claude-sonnet-4-6 · 2026-06-15 06:49 · grounding rag
Model Answer
Calcium floats on water because the hydrogen gas ($H_2$) produced during the reaction lifts it to the surface. Also, calcium is less dense than water. The heat evolved is not enough to ignite the gas, so it just bubbles and causes calcium to float.
Reaction of calcium with water:
$$Ca(s) + 2H_2O(l) \rightarrow Ca(OH)_2(aq) + H_2(g)$$
The product formed is calcium hydroxide $[Ca(OH)_2]$. When its aqueous solution reacts with carbon dioxide, calcium carbonate is formed:
$$Ca(OH)_2(aq) + CO_2(g) \rightarrow CaCO_3(s) + H_2O(l)$$
Source: Chapter 1, Chemical Reactions and Equations
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Explanation
- Floating reason: Two points are expected — hydrogen gas evolution (buoyancy effect) and low density of calcium. Many students write only one; mention both for full marks.
- The first equation is not directly in the passage (Ca + H₂O is standard Class 10 chemistry); the passage gives the CaO + H₂O reaction, but the Ca(OH)₂ + CO₂ equation (1.14) is directly from the text — make sure to reproduce it exactly.
- Note the state symbols: $CaCO_3$ is a precipitate $(s)$, which is important.
- This is a 3-mark question: one mark for the explanation of floating, one for each balanced equation.