Very small insoluble particles suspended in a liquid can best be separated from the liquid by which of the following methods?

Introduction / Context:
In school level chemistry and laboratory practice, you learn various methods of separating mixtures: filtration, decantation, crystallisation, distillation, and centrifugation. The choice of method depends on the properties of the mixture, such as particle size and solubility. This question asks which method is most suitable for separating very small insoluble particles from a liquid, where simple methods like decantation or filtration may not be effective on their own.

Given Data / Assumptions:

• The particles are insoluble but very small and suspended in a liquid (a suspension or colloid like mixture).
• The options include crystallisation, fractional distillation, centrifugation, and decantation.
• We assume no special filters are available that can easily trap very fine particles by ordinary gravity filtration.

Concept / Approach:
Centrifugation involves spinning a mixture at high speed so that denser particles experience a strong outward force, causing them to move toward the bottom of the container and form a sediment or pellet. The clear liquid (supernatant) can then be poured off. This method is particularly useful for separating very small insoluble particles that might remain suspended for a long time or pass through ordinary filter paper. Crystallisation is used to separate dissolved solids from a solution by forming crystals; it is not appropriate for insoluble particles already present. Fractional distillation is used for separating liquids with different boiling points, not for removing solid particles from a liquid. Decantation involves gently pouring off the clear liquid from a settled solid, but it is effective mainly when particles are large enough to settle quickly by gravity; for very fine particles, settling may be too slow or incomplete. Therefore, centrifugation is the best choice here.

Step-by-Step Solution:

Verification / Alternative check:
Laboratory protocols for separating blood cells from plasma, or for clarifying suspensions such as milk or bacterial cultures, routinely use centrifuges. These examples involve very small particles (cells, fat globules, or bacteria) that do not settle quickly by gravity alone. Educational materials on separation of mixtures describe centrifugation as the method of choice for suspensions where the solid particles are too fine for easy filtration or rapid settling. These real world uses confirm that centrifugation is the correct answer for this type of problem.

Why Other Options Are Wrong:

Crystallisation is wrong because it is used to recover dissolved solids from a solution by forming crystals, not to separate undissolved fine particles already present as a suspension.

Fractional distillation is incorrect because it separates liquid mixtures based on boiling point differences, not solid particles from liquids.

Decantation is also wrong in this context because it depends on particles settling by gravity, which is inefficient for very small particles that remain suspended for long periods.

Common Pitfalls:
Students may confuse crystallisation with general solid liquid separation or may think of decantation as a solution for all solid suspensions without considering particle size. Another pitfall is not distinguishing between separating dissolved substances and separating suspended solids. To avoid these errors, remember that centrifugation is specifically designed to separate very small insoluble particles from liquids by applying high rotational force, making it the correct choice here.

Final Answer:
Therefore, the best method to separate very small insoluble particles from a liquid is centrifugation.