Question

Noble metals like Au, Pt dissolve in a mixture of

• A. 1 part of Conc. HCl + 1 part of $HNO_3$
• B. 1 part of Conc. $H_2SO_4$ + 1 part of $HNO_3$
• C. 1 part of Conc. $HNO_3$ + 3 parts of Conc. HCl
• D. 3 parts of Conc. $HNO_3$ + 1 part of Conc. HCl \bigskip

Hint:

Aqua Regia means "Royal Water". Composition: \[ HNO_3:HCl = 1:3 \] It dissolves noble metals such as Au and Pt.

Answer 1

The Correct Option is C

Concept: Gold and platinum are noble metals. They are highly resistant to oxidation and do not dissolve in ordinary acids. A special acid mixture called Aqua Regia is required to dissolve them.

Step 1: Recall the composition of Aqua Regia.
Aqua Regia is prepared by mixing: \[ 1 \text{ part concentrated } HNO_3 \] with \[ 3 \text{ parts concentrated } HCl \] \[ \boxed{\text{Aqua Regia} = HNO_3 : HCl = 1 : 3} \]

Step 2: Understand why Aqua Regia dissolves noble metals.
Nitric acid acts as an oxidizing agent and converts Au into ionic form. Hydrochloric acid provides chloride ions which stabilize the metal ions as complex ions. For example: \[ Au \rightarrow Au^{3+} \] and \[ Au^{3+}+4Cl^- \rightarrow [AuCl_4]^- \] Thus gold dissolves completely.

Step 3: Compare with the options.
Only option (C) contains: \[ 1 \text{ part } HNO_3 + 3 \text{ parts } HCl \] which is exactly Aqua Regia. \[ \boxed{\text{Option (C)}} \]