Which of the following is not a homogenous mixture?

The correct option is Smoke.

To understand why smoke is not a homogeneous mixture, let us first define the difference between homogeneous and heterogeneous mixtures:
1. Homogeneous Mixture: A mixture in which the components are uniformly distributed throughout its volume, and there are no visible boundaries of separation. The composition is uniform throughout.
2. Heterogeneous Mixture: A mixture in which the components are not uniformly distributed, and distinct phases or boundaries can be seen. The composition varies from one part to another.

Let us analyze each of the given options:
- Brass: Brass is an alloy, which is a solid solution of copper and zinc. Since the metals are mixed uniformly at the atomic level, it is a homogeneous mixture.
- Air: Air is a mixture of gases like nitrogen, oxygen, carbon dioxide, and others. These gases are completely and uniformly mixed with each other, making air a homogeneous mixture.
- Aqueous solution of sugar: When sugar dissolves in water, the sugar molecules disperse uniformly among the water molecules. You cannot see separate boundaries between sugar and water, so it is a homogeneous mixture.
- Smoke: Smoke consists of tiny solid soot and ash particles suspended in a gas (air). This is a colloid, which is a type of heterogeneous mixture. The solid particles are distinct from the gaseous phase and are not uniformly distributed at the microscopic level. Therefore, smoke is not a homogeneous mixture.