Which metal is the most powerful reducing agent in aqueous solution?

Lithium

The correct answer is Option (4) → Lithium

 Lithium is the most powerful reducing agent in aqueous solution.In aqueous solution, the strength of a reducing agent depends on the standard reduction potential of the metal in water. Although lithium has a very high ionization energy, it releases a large amount of hydration energy when Li⁺ ions are formed. This high hydration energy makes the overall reduction potential of lithium most negative, so lithium loses electrons most easily in aqueous solution and acts as the strongest reducing agent.

Option (1) → Potassium - Potassium is a strong reducing agent. Potassium loses electrons easily because of its low ionization energy, but the hydration energy of K⁺ is less than that of Li⁺. Therefore, in aqueous solution potassium is not the strongest reducing agent.

Option (2) → Sodium -Sodium is also a good reducing agent. Sodium has a negative reduction potential and can easily lose electrons, but its hydration energy is lower than that of lithium. Hence it is weaker than lithium as a reducing agent in aqueous solution.

Option (3) → Barium- Barium is a reducing metal from the alkaline earth metals. Although barium can lose electrons, its reduction potential is less negative compared with lithium in aqueous solution, so it is not the most powerful reducing agent.