[Jasim]

Not all solutions are translucent, some are opaque. If a substance does not dissolve, then it will be a cloudy solution. Two things are going on when a solid is put into a liquid.

1. The solid is now in a less concentrated form. If the liquid transmits a lot of light and the solution is very dilute, then it stands to reason that most of the light will be transmitted. Less light may be transmitted as the concentration is increased.

2. Solvation of the solid occurs. Let's take water as an example solvent. When a solid is dissolved in water, the water will form what is called a Solvation Shell around the molecule. That is how things dissolve in water. This shell helps to move light around the molecule, thereby causing transmittance of most of the light. If a solid is unable to dissolve in the water (meaning no solvation shell forms), then the solution is more likely to be opaque (cloudy).

Here is an image of the solvation shells that form around the salt NaCl when dissolved in water.