Which one of the following metals will not reduce \(H_2O\)?

Cu

The correct answer is option 3. Cu.

To understand why certain metals do or do not reduce water, it’s useful to look at their reactivity and the conditions required for such reactions. Here's a detailed explanation:

Reaction of Metals with Water

1. Lithium (Li):

Reactivity: Lithium is an alkali metal and is highly reactive.

Reaction with Water: 

\(2 \text{Li} + 2 \text{H}_2\text{O} \rightarrow 2 \text{LiOH} + \text{H}_2\)

Lithium reacts vigorously with water at room temperature to form lithium hydroxide and hydrogen gas. The reaction is quite exothermic, releasing heat.

2. Iron (Fe):

Reactivity: Iron is a transition metal and is less reactive than alkali and alkaline earth metals.

Reaction with Water: With Steam:

\(\text{Fe} + 3 \text{H}_2\text{O} \rightarrow \text{Fe(OH)}_3 + 3 \text{H}_2 \)

With Cold Water: Iron does not react with cold water under normal conditions.

Iron only reacts with water when it is in the form of steam and at high temperatures. The reaction with cold water does not occur readily.

3. Calcium (Ca):

Reactivity: Calcium is an alkaline earth metal and is also quite reactive with water, though not as much as alkali metals.

Reaction with Water:

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

Calcium reacts with water to form calcium hydroxide and hydrogen gas. This reaction occurs at room temperature, though it is less vigorous compared to alkali metals.

4. Copper (Cu):

Reactivity: Copper is a less reactive metal and is located below iron in the reactivity series.

Reaction with Water: Copper does not react with water at room temperature or even with steam under normal conditions.

Copper’s reactivity is low, and it does not have the necessary activity to displace hydrogen from water.

Summary

Highly Reactive Metals (Li, Ca): These metals react readily with water, producing metal hydroxides and hydrogen gas.

Less Reactive Metals (Fe): Iron reacts with water only under specific conditions (high temperature and steam), not with cold water.

Low Reactivity Metals (Cu): Copper does not react with water at all.

Therefore, Copper (Cu) is the metal that will not reduce water because it does not react with water under normal conditions.