Which of the following everyday mixtures is best classified as a suspension?

The correct option is Chalk powder mixed with water.

To understand why this is the correct answer, let us define a suspension and compare it with the other options provided:

1. What is a Suspension?
A suspension is a heterogeneous mixture in which the solute-like particles do not dissolve but remain suspended throughout the bulk of the medium, left floating around freely. The particle size in a suspension is relatively large (typically greater than 100 nm or 10^-7 m), making them visible to the naked eye. If left undisturbed, the suspended particles will eventually settle at the bottom due to gravity. Furthermore, these particles can be separated from the mixture using a simple filtration process.

2. Analyzing the Options:
- Salt solution prepared in water: When salt (NaCl) is mixed with water, it completely dissolves to form a homogeneous mixture called a true solution. The salt particles dissociate into individual sodium and chloride ions, which are extremely small (less than 1 nm) and do not settle down or scatter light.
- Sugar completely dissolved in water: Similar to salt, sugar molecules completely disperse and intermingle with water molecules to form a stable, homogeneous true solution. The particles do not settle down over time.
- Copper sulphate dissolved in water: Copper sulphate dissolves thoroughly in water to yield a clear blue, homogeneous true solution. The copper and sulphate ions remain uniformly distributed and do not settle.
- Chalk powder mixed with water: Chalk is primarily made of calcium carbonate (${\mathrm{CaCO}}_3$), which is insoluble in water. When chalk powder is mixed with water, it forms a turbid, heterogeneous mixture. The chalk particles remain suspended in the water, making the liquid cloudy. If the mixture is allowed to stand still for some time, the chalk particles will settle to the bottom. Thus, it perfectly matches all the characteristics of a suspension.

Therefore, among the given mixtures, chalk powder mixed with water is best classified as a suspension.