Question

Can rusting of iron nails occur in distilled water? Justify your answer.

Answer 1

Hint: Rusting of electrolysis is an electrochemical process. Therefore it needs anode and cathode points where oxidation and reduction can take place respectively. Understanding the rusting of iron chemically is therefore significant.

Complete step-by-step answer:
-Rusting of iron is a chemical process where iron oxidised to ferric oxide in the presence of oxygen and hydronium ions. The overall reaction that happens during this phenomenon can be summarized below:
\[\begin{align}
  & At\text{ Anode: 2}Fe(s)\to 2F{{e}^{2+}}\text{(aq)+4}{{\text{e}}^{-}}\text{ } \\
 & At\text{ Cathode: }{{O}_{2}}(g)+4{{H}^{+}}(aq)+4{{e}^{-}}\to 2{{H}_{2}}O(l) \\
 & Overall\text{ reaction: 2}Fe(s)+{{O}_{2}}(g)+4{{H}^{+}}(aq)\to 2F{{e}^{2+}}\text{(aq)+2}{{H}_{2}}O(l) \\
\end{align}\]
-Anode is the electrode where oxidation takes place while reduction takes place at the cathode.
 -Anode iron is oxidised into ferrous ions in the presence of oxygen and protons as shown above. Oxidation is the loss of electrons, and reduction is the gain of electrons. The four electrons lost by iron reduce oxygen to water. The ferrous ions are further oxidised into ferric ions ($F{{e}^{3+}}$) in a similar process. This results in formation of the brown coating that we all are so familiar with, which is commonly termed as “rust”.
-As mentioned above this all can only happen when water molecules are present in dissociated form that is-
\[{{H}_{2}}O(l)\to {{H}^{+}}(aq)+O{{H}^{-}}(aq)\]
-Distilled water is a pure form of water. It is obtained by vaporising liquid water at its boiling temperature. The minerals and other impurities stay behind and only the liquid water is obtained when the steam is cooled down. It turns out that a sample of “ideal” distilled water does not conduct electricity. This happens because conduction requires the presence of charged species called as ions which are absent in distilled water. It is only when water contains minerals and other impurities it becomes a good conductor of electricity. The dissociation of water molecules also occurs when an external ion is present.
-Thus if distilled water is completely free of such impurities then it will not contain hydronium ions (${{H}_{3}}{{O}^{+}}$) and hydroxide ions ($O{{H}^{-}}$) which are a necessity for the corrosion process to occur as shown in the equation above. But this is seldom the case, as a distilled water sample always contains some amount of impurities. And if it is left open then gases like carbon dioxide will get dissolved and the following reaction will take place-
\[\begin{align}
  & C{{O}_{2}}+{{H}_{2}}O\to {{H}_{2}}C{{O}_{3}} \\
 & {{H}_{2}}C{{O}_{3}}\to {{H}^{+}}+HC{{O}_{3}}^{-} \\
\end{align}\]
As you can see a proton or hydronium ion is generated. This can also result when other gases such as oxides of sulphur or other non-metals get dissolved in water.
Observing all the facts above it can be concluded that rusting of iron well is possible inside distilled water provided the reaction container is left open or somehow the reaction mixture contains a small amount of impurities.

Note: Distilled water is sometimes not pure enough for laboratory purposes so they are again subjected to distillation. The water obtained as a result is referred to as double distilled water which does not contain any impurities. If this is taken in a container which is covered, preventing any external agents from reacting with it and an iron nail is dropped, it is very likely that no rusting will take place. While for the water which is distilled only ones, chances are there that the iron nail will rust, although after a long time, if the same conditions are provided as mentioned above.