Animated Solution for Chemistry - Electrochemistry: The metal that cannot be obtained by the electrolysis of an aqueous solution of its salts is

Imagine you are setting up an electrolytic cell. You dissolve a metal salt in water, insert two electrodes, and connect a battery. You might expect the metal cations to simply swim to the cathode, grab some electrons, and deposit as pure metal. But there is a hidden twist! Because you are using an aqueous solution, water molecules ($H_{2}O$) are everywhere.

At the cathode (the negative electrode), a fierce competition begins. Both the metal cations ($M^{n+}$) and the water molecules want to gain electrons and undergo reduction. The ultimate question is: who wins this tug-of-war?

Standard Reduction Potential

In electrochemistry, the winner of this competition is decided by a strict rule: the species with the higher standard reduction potential ($E_{red}^{\circ }$) gets reduced preferentially.

If we look at the electrochemical series, metals that are placed below hydrogen (like Copper, Silver, and Chromium) have a positive or relatively high reduction potential. This means they are "hungry" for electrons.

For example, the reduction of Copper: Because $E_{C{u}^{2+}/Cu}^{\circ }$ is greater than the reduction potential of water, the $C{u}^{2+}$ ions easily win the competition and deposit as a beautiful solid layer on the cathode.

Now, let's look at Calcium ($Ca$). Calcium is an s-block metal, highly electropositive, and sits way at the top of the electrochemical series. Its standard reduction potential is extremely negative. It absolutely hates gaining electrons to become a neutral atom.

Because $E_{C{a}^{2+}/Ca}^{\circ }$ is much lower than the reduction potential of water, the water molecules step up and steal the electrons instead!

The reduction of water takes place:

Instead of getting a shiny deposit of Calcium metal, you will just see bubbles of Hydrogen gas ($H_2$) evolving at the cathode. The Calcium ions remain happily dissolved in the solution.

Because water is reduced in preference to Calcium, we can never obtain Calcium metal by electrolysing its aqueous solution.

This principle applies to all highly reactive metals, including all s-block elements (like Sodium, Potassium, Magnesium) and Aluminum. If you want to extract these metals, you must completely eliminate water from the system by electrolysing their molten (melted) salts instead. Therefore, the correct answer is Calcium (Option B).