Question

Which of the following metal does not react with acid?
(A) Magnesium
(B) Sodium
(C) Gold
(D) Lead

Answer 1

Hint: Metals generally react with acid and give a salt and hydrogen gas. A metal which lies below hydrogen in the reactivity series will not react with any acid as it will be unreactive towards the hydrogen ions in the solution.

Complete step by step solution: Dilute acids often react with metals and produce a salt and hydrogen gas.
We know metals are compounds that react by losing their valence electrons to form positive ions. These positive ions when reacted with negative ions form ionic compounds which are known as salts.
When a metal piece is added to dilute acid, the metal forms its corresponding cation which reacts with the anion of the acid and forms a salt and hydrogen gas is evolved.
Now, let us go through the options given to us and find out whether the particular metal reacts with acid or not.
In the first option, we have magnesium.
Magnesium reacts with hydrochloric acid and forms magnesium chloride and hydrogen gas is evolved. We can write the reaction as-
$$Mg+2HCl\to MgC{l}_2+H_2(g)$$
In the next option, we have sodium. Sodium is very unstable and it reacts with almost anything. A piece of sodium in acid will cause a violent exothermic reaction. We can write the reaction as-
$$Na+2HCl\to 2NaCl+H_2(g)$$
The reaction is highly exothermic and causes the hydrogen gas to ignite and eventually explodes.
In the third option, we have gold. We know gold is a noble metal and it lies below hydrogen in the reactivity series and thus it is unreactive towards acids.
And lastly, we have the lead. It also reacts with hydrochloric acid and gives us lead chloride and hydrogen gas. We can write the reaction as-
$$Pb+2HCl\to PbC{l}_2+H_2(g)$$
We can understand from the above discussion that gold metal does not react with acids.

Therefore, the correct answer is option [C] Gold.

Note: Gold is not dissolved in acid but there is a special mixture of acids which is used to dissolve gold. The acid mixture is known as aqua-regia. It is a mixture of concentrated hydrochloric acid and concentrated nitric acid in 3:1 ratio. Nitric acid causes oxidation of hydrochloric acid and liberation of nascent chlorine which reacts with gold and gives its chloride. The metal can be recovered from the chloride whenever required.
$$\begin{array}{rl}& HN{O}_3+3HCl\to NOCl+2H_{2}O+2\left[Cl\right]\\ & Au+3\left[Cl\right]\to AuC{l}_3\end{array}$$