To classify a solid solution, we must first define solutions based on their phases:
- A solution is a homogeneous mixture. In a solution, the solute is evenly distributed throughout the solvent. Solutions can exist in solid, liquid, or gaseous phases.
- Solid solutions are mixtures where both the solute and the solvent are solids. These commonly occur as alloys, where solute metal atoms are integrated into the solvent metal's atomic structure.
Next, let's analyze each option:
- Camphor in N$_2$ gas: This involves a solid (camphor) and a gas (N$_2$), forming a gaseous solution, not a solid one.
- Oxygen gas in water: This is a liquid solution; a gas (oxygen) is dissolved in a liquid (water).
- Ethanol in water: This is also a liquid solution, with both components in the liquid phase.
- Amalgam of mercury with sodium: This is a solid solution. Mercury, typically liquid at room temperature, forms a solid phase with sodium. This represents a metal alloy, with the solution existing in a solid state.
Therefore, the correct example of a solid solution from the given choices is the Amalgam of mercury with sodium.