Question

A metal which doesn’t react with cold water but reacts with steam to liberate ${{H}_{2}}$ gas is :
(A) Na
(B) Mg
(C) Au
(D) Fe

Answer 1

Hint: It is found that the metals that react with steam form oxides. And hydrogen gas is also evolved during the reaction. And when heat is also applied then there is speed up in the reaction observed.

Step by step solution:
-The metals like magnesium and sodium when react with steam then their hydroxides are generated:
\[Mg\left( g \right)+2{{H}_{2}}O\left( g \right)\to Mg{{\left( OH \right)}_{2}}\left( aq \right)+{{H}_{2}}\left( g \right)\]
 \[2Na\left( s \right)+2{{H}_{2}}O\left( g \right)\to 2NaOH\left( aq \right)+{{H}_{2}}\left( g \right)\]
- If we pass steam over hot iron then the steam will react with the iron to produce a black, magnetic oxide of iron called as triiron tetraoxide, $F{{e}_{3}}{{O}_{4}}$$F{{e}_{3}}{{O}_{4}}$, we can see it from the reaction:
\[3Fe\left( s \right)+4{{H}_{2}}O\left( g \right)\to F{{e}_{3}}{{O}_{4}}\left( s \right)+4{{H}_{2}}\left( g \right)\]
- The hydrogen which is produced in the reaction is swept away by the stream of steam.
- We can see that under different conditions, the products of this reaction will also react together. Hydrogen which is passed over hot triiron tetraoxide reduces it to iron. Steam is also produced.
\[F{{e}_{3}}{{O}_{4}}\left( s \right)+4{{H}_{2}}\left( g \right)\to 3Fe\left( s \right)+4{{H}_{2}}O\left( g \right)\]
- This time the steam produced in the reaction is swept away by the stream of hydrogen. Unlike other reactions between metals and steam, these reactions are found to be reversible, but under the conditions normally used, they become one way reactions. It is also found that the products are not left in contact with each other, so the reverse reaction can happen.
- It is also found that the reaction between the steam and iron is much slower than the reactions between steam and other metals higher in reactivity series.
- It is also found that heat often has to be applied to the iron in the test tube to imitate the reaction with steam and to speed up the reaction.
Hence, we can conclude that option (d) is correct, that is, Fe is the metal which doesn’t react with cold water but reacts with steam to liberate ${{H}_{2}}$ gas .

Additional information:
- We can see that if metals react with cold water then various hydroxides are produced.
- We have already seen that metals that react with steam form oxides.
- Hydrogen is always produced when a metal reacts with cold water or steam.
- There are various metals such as lead, silver, copper and gold that don't react with water at all.

Note:
- Metals like iron, aluminium and zinc don’t react with cold or hot water. But they react with steam to form hydrogen and metal oxide. Whereas, Mg and Na react with steam to liberate hydrogen and metal hydroxide.