Sodium and potassium react violently with cold water, while calcium's reaction is comparatively less vigorous. State two observations that distinguish sodium's reaction with cold water from calcium's reaction, and give reasons for these differences.
Generated by claude-sonnet-4-6 · 2026-06-26 01:05 · grounding rag
Model Answer
Two observations distinguishing sodium's reaction from calcium's:
- Vigour/Speed: Sodium reacts very vigorously (violently) with cold water, often catching fire, whereas calcium's reaction is comparatively slower and less vigorous.
- Movement on water: Sodium moves rapidly/erratically on the water surface (may melt into a ball) due to the large amount of heat produced, while calcium sinks and reacts steadily, producing bubbles.
Reasons: Sodium is higher in reactivity than calcium in the activity series — it loses its single valence electron more readily. Calcium has two valence electrons to lose, making it less reactive. Both form hydroxides and hydrogen gas:
$$2\text{Na} + 2\text{H}_2\text{O} \rightarrow 2\text{NaOH} + \text{H}_2\uparrow$$
$$\text{Ca} + 2\text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 + \text{H}_2\uparrow$$
Source: Chapter 3, Activity series / reactivity of metals with water
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Explanation
- Examiners expect two distinct observations (vigour, fizzing/fire, movement) and a reason linked to reactivity/electronic configuration — losing 1 electron (Na) vs 2 electrons (Ca).
- Writing the balanced equations earns marks and shows understanding.
- "Sodium catches fire" is a valid observation for full credit.
- Do not confuse CaO + H₂O (combination reaction) with Ca metal + H₂O — the question is about the metal reacting with water.