A metal cup half-filled with dilute acid slowly develops bubbles on its inner surface and gradually thins. The diagram shows the bubbling. Which metal could...
Question 1 Report
A metal cup half-filled with dilute acid slowly develops bubbles on its inner surface and gradually thins. The diagram shows the bubbling. Which metal could the cup be made from?
Answer Details
Bubbles forming on the inside of the cup and the metal thinning show that the metal is dissolving in the acid and releasing hydrogen. Only a metal above hydrogen in the reactivity series can do this:
\[ \text{Zn} + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2 \]
Zinc sits above hydrogen, so it is steadily eaten away while hydrogen bubbles cling to the surface, which matches both observations.
Silver, gold and platinum all lie below hydrogen. None of them can displace hydrogen from a dilute acid, so a cup made from any of them would show no bubbles and would not lose thickness, which is precisely why they are used for coinage and jewellery. The gradual loss of metal is the key clue that the container itself is a reactant.
Download The App On Google Playstore
Everything you need to excel in your exams
