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Science — CBSE Class 10 board question

Q1. [1]
When an element 'X' reacts with water, it starts floating. Identify the element 'X' :
  1. A Potassium
  2. B Calcium
  3. C Sodium
  4. D Iron
Previously asked in CBSE board exam
2026 31/1/1 Q23
Generated by claude-sonnet-4-6 · 2026-06-15 06:45 · grounding rag
Model Answer

Answer: B — Calcium

Calcium reacts with cold water and the hydrogen gas bubbles formed stick to its surface, causing it to float.

Source: Chapter 3, Section 3.2.2

Explanation

The key detail from the passage is: "Calcium starts floating because the bubbles of hydrogen gas formed stick to the surface of the metal." Sodium and potassium also react with water but they react violently (hydrogen catches fire) — they don't simply float. Iron does not react with cold or hot water at all. Calcium is the correct answer because it reacts moderately and floats due to hydrogen bubbles.

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