In the study of mixtures, which of the following is not a colloid but instead a true solution?

Introduction / Context:
Mixtures in chemistry can be classified into true solutions, colloids, and suspensions based on particle size and behaviour. Colloids have particle sizes between those of solutions and suspensions and show unique properties like the Tyndall effect. Recognising examples of colloids and non colloids helps in understanding real world systems such as milk, fog, and paint. This question asks you to identify which mixture listed is not a colloid but rather a true solution.

Given Data / Assumptions:
- The mixtures listed are sugar water, milk, paint, and fog. - Colloids have dispersed particles in the size range roughly 1 to 1000 nanometres. - True solutions have solute particles much smaller than colloidal particles and do not scatter light significantly. - We assume normal everyday examples of these mixtures.

Concept / Approach:
A true solution is homogeneous at the molecular level, and the solute particles are so small that they do not settle out and do not scatter light. Sugar dissolved in water forms such a solution. A colloid has larger dispersed particles that remain suspended and may scatter light; examples include milk (an emulsion), paint (a colloidal dispersion of pigments), and fog (tiny liquid droplets dispersed in air). Therefore, among the options, sugar water is the mixture that is a true solution rather than a colloid.

Step-by-Step Solution:
Step 1: Consider sugar water. When sugar dissolves completely in water, the sugar molecules are dispersed uniformly at the molecular level, giving a clear homogeneous solution. Step 2: Note that sugar water does not exhibit the Tyndall effect; a beam of light passes through without being scattered by particles. Step 3: Consider milk. Milk is an emulsion of fat globules and other substances in water, and it is a classic example of a colloid. Step 4: Consider paint, which is a colloidal dispersion of solid pigment particles in a liquid medium and shows typical colloid behaviour. Step 5: Consider fog, which consists of tiny liquid water droplets dispersed in air, another common example of a colloid. Step 6: Compare all four and identify sugar water as the only mixture that is a true solution, not a colloid.

Verification / Alternative check:
Textbooks on physical chemistry and school science consistently list milk, paint, and fog as examples of colloids: milk as an emulsion, paint as a sol, and fog as an aerosol. Sugar water, along with salt water, is normally given as an example of a true solution. Laboratory demonstrations of the Tyndall effect show that a flashlight beam is scattered in milk or fog but not in clear sugar water solutions. These observations confirm that sugar water is not a colloid.

Why Other Options Are Wrong:
Option B (Milk) is wrong as the answer because milk is a well known colloid, specifically an emulsion of fat in water. Option C (Paint) is incorrect because paint is a colloidal dispersion of pigment particles in a liquid medium. Option D (Fog) is incorrect because fog is a colloid consisting of tiny liquid droplets dispersed in air, often called an aerosol.

Common Pitfalls:
Students may confuse all cloudy or coloured mixtures with colloids and all clear mixtures with solutions without examining particle behaviour. While this rule often works, some solutions can be coloured and some colloids appear almost clear. Another pitfall is to forget that true solutions do not scatter light, whereas colloids do. To avoid mistakes, focus on particle size and properties such as the Tyndall effect rather than colour alone.

Final Answer:
The correct answer is: Sugar water.