Question

Iron nails are taken in three test tubes-$\mathrm{A}$ having anhydrous ${\mathrm{CaCl}}_2$, $\mathrm{B}$ having boiled water, and a layer of oil and $\mathrm{C}$ having water. Nails in which test tube will undergo corrosion? Give reason.

Answer 1

1. Corrosion- Definition

It is a phenomenon in which oxidation of a substance takes place in presence of air and moisture and the substance turns into an undesirable product.

It is generally a redox reaction in which oxidation(loss of electrons) and reduction(gain of electrons) take place simultaneously.

2. Example

Rusting of iron
Iron gets rusted in presence of air and moisture.
$4\mathrm{Fe}\left(\mathrm{s}\right)+3\mathrm{O}{}_2(\mathrm{g})+\mathrm{xH}{}_2\mathrm{O}(\mathrm{l})\to 2\mathrm{Fe}{}_2\mathrm{O}{}_3.\mathrm{xH}{}_2\mathrm{O}(\mathrm{s})$
Iron oxygen water Rust

Here oxidation state of $\mathrm{Fe}$ in $\mathrm{Fe}$ is $0$ and that of $\mathrm{O}$ in $\mathrm{O}{}_2$ is $0$ at the reactant side but the oxidation state of $\mathrm{Fe}$ in $\mathrm{Fe}{}_2\mathrm{O}{}_3$ is $+3$ and that of $\mathrm{O}$ in $\mathrm{Fe}{}_2\mathrm{O}{}_3$ is $-2$ at the product side.
⦁ So, we can say that $\mathrm{Fe}$ is getting oxidized since its oxidation number is increasing from $0$ to $+3$ and $\mathrm{O}{}_2$ is getting reduced since its oxidation number is decreasing from $0$ to $-2$ .
So, iron gets oxidized and oxygen gets reduced.

Here rust formed is an undesirable product.

3. For rusting of the iron presence of both air and moisture is compulsory can be proved with the help of an experiment:

1. Take three test tubes, three iron nails, and three cork.
2. In test tube A add calcium chloride(drying agent) it soaks all the moisture present in a test tube place one iron nail in this and covers tightly with the help of cork.
3. In test tube B add boiled water and oil to a test tube and place one iron nail in this and cover tightly with the help of cork.
4. In a test tube, C adds water and places one iron nail in this, and covers it tightly with the help of cork.

4. Observation:

i. In test tube A no change occurs because there is no presence of water only air is available.

ii. In test tube B no change occurs because there is no presence of air [ boiled water(free from oxygen) and oil (does not allow oxygen to react with iron nail since oxygen does not dissolve in oil)] only water is available.

iii. In test tube C change will occur because both air and moisture are available. So here grey iron nails will turn brown due to the formation of rust.