Solubility is practically applied in

Analyze the question: We are asked which process practically applies the principle of solubility, which is the ability of a solute to dissolve in a solvent.

Explain the concept: Solubility depends on "like dissolves like". Polar solutes dissolve in polar solvents such as water, and non-polar solutes dissolve in non-polar solvents such as hexane or ether. Different substances also dissolve to different extents in different solvents.

Check each option:

• A. Fractional distillation: This separates liquids by differences in boiling point, not solubility. ❌
• B. Determination of pH: pH measures the concentration of hydrogen ions (acidity or alkalinity). It is not an application of solubility. ❌
• C. Determination of saturation in hydrocarbons: This tests for double or triple bonds, for example with bromine water or acidified KMnO₄ (a chemical reaction). ❌
• D. Solvent extraction: This separates a substance by dissolving it in a solvent in which it is more soluble. Examples are extracting oil from palm kernels or groundnuts using hexane, and extracting iodine from water using an organic solvent. This is a direct application of solubility. ✅

Answer: The correct option is D, Solvent extraction.

Tip: When you see "solubility" in an application question, think of dissolving: solvent extraction, crystallization, and making solutions. Distillation is tied to boiling points, and chromatography to differential adsorption and solubility.