Chapter 3 · Question 4

Describe how different metals react with water. Classify the metals into groups based on their reactivity with cold water, hot water, and steam. Write chemical equations for sodium, calcium, and iron reacting with water/steam.

Answer Revealed

Direct Answer:

Metals react with water to produce metal oxide/metal hydroxide and hydrogen gas. Classification: (a) React vigorously with cold water: Potassium and sodium — reaction is violent and exothermic; the evolved hydrogen catches fire.

2\text{Na} + 2\text{H}_2\text{O} \rightarrow 2\text{NaOH} + \text{H}_2

. (b) React with cold water (less vigorous): Calcium — the heat is insufficient to ignite hydrogen; Ca floats due to H₂ bubbles.

\text{Ca} + 2\text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 + \text{H}_2

. (c) React with hot water only: Magnesium —

\text{Mg} + \text{H}_2\text{O} \rightarrow \text{MgO} + \text{H}_2

(or Mg(OH)₂). (d) React with steam only: Aluminium and iron.

3\text{Fe} + 4\text{H}_2\text{O(g)} \rightarrow \text{Fe}_3\text{O}_4 + 4\text{H}_2

. (e) No reaction with water: Lead, copper, silver, and gold.

Simple Explanation

The more reactive a metal, the more easily it attacks water. Sodium and potassium are so reactive they burst into flames the moment they touch cold water. Calcium reacts with cold water but more calmly, and it floats because hydrogen bubbles stick to it. Magnesium needs hot water. Aluminium and iron need steam — they ignore cold and hot water entirely. Lead, copper, silver, and gold just sit there — they will not react with water at all.

Exam-Ready Structure

The reaction with water is a key criterion for establishing the reactivity series of metals, as detailed in NCERT Activity 3.10: 1. General principle: Metal + Water → Metal oxide + Hydrogen. If the oxide is soluble, it dissolves further: Metal oxide + Water → Metal hydroxide. 2. The metals can be classified into five reactivity tiers based on their behaviour with water in different states: (a) Violent reaction with cold water: Potassium and sodium react explosively. Equations:

2\text{K} + 2\text{H}_2\text{O} \rightarrow 2\text{KOH} + \text{H}_2 + \text{heat}

;

2\text{Na} + 2\text{H}_2\text{O} \rightarrow 2\text{NaOH} + \text{H}_2 + \text{heat}

. The reaction is so exothermic that the hydrogen gas evolved catches fire. (b) Less vigorous reaction with cold water: Calcium. It sinks then floats due to hydrogen bubbles adhering to its surface. Equation:

\text{Ca} + 2\text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 + \text{H}_2

. (c) Reaction with hot water only: Magnesium does not react with cold water but reacts with hot water to form Mg(OH)₂ and H₂ — it also floats due to H₂ bubbles. (d) Reaction with steam only: Aluminium and iron require steam (gaseous water) to react.

2\text{Al} + 3\text{H}_2\text{O(g)} \rightarrow \text{Al}_2\text{O}_3 + 3\text{H}_2

;

3\text{Fe} + 4\text{H}_2\text{O(g)} \rightarrow \text{Fe}_3\text{O}_4 + 4\text{H}_2

. Activity 3.10 sets up a steam apparatus with a glass-wool soaked in water, heated to produce steam that passes over the metal sample. (e) No reaction with water in any form: Lead, copper, silver, and gold are below hydrogen in the reactivity series and do not displace hydrogen from water.

Key Points

Reactivity with water: K, Na > Ca > Mg > Al, Fe > Pb, Cu, Ag, Au
K and Na: violent reaction with cold water, H₂ catches fire; stored in kerosene
Ca: reacts with cold water but H₂ does not ignite; floats due to H₂ bubbles
Mg: reacts only with hot water (not cold); Al and Fe: react only with steam
Fe + steam: $3\text{Fe} + 4\text{H}_2\text{O(g)} \rightarrow \text{Fe}_3\text{O}_4 + 4\text{H}_2$
Pb, Cu, Ag, Au: do not react with water at all

Describe how different metals react with water. Classify the metals into groups based on their reactivity with cold water, hot water, and steam. Write chemical equations for sodium, calcium, and iron reacting with water/steam.