The operation involved when the soluble material is largely on the surface of an insoluble solid and is merely washed off by the solvent is called

ANS:C - elutriation or elution

Elutriation, also known as elution, is a separation technique used to remove soluble material from the surface of an insoluble solid by washing it off with a solvent. This process is commonly employed in various industries, including chemical, pharmaceutical, and environmental sectors. Here's how elutriation works:

1. Preparation of the solid phase: The solid material containing the soluble component is usually in the form of a powder or granules. It may be placed in a container or a column, depending on the specific setup of the elutriation system.
2. Introduction of solvent: A solvent, typically a liquid, is introduced into the system. The solvent should be capable of dissolving the desired soluble component while leaving the solid material unaffected.
3. Percolation or washing: The solvent percolates through the solid material, washing off the soluble component from its surface. This process is facilitated by various mechanisms such as agitation, gravity, or pressure. As the solvent passes through the solid, it carries away the soluble material along with it.
4. Separation of phases: After the elution process, the solvent containing the dissolved solute is separated from the solid material. This separation can be achieved through filtration, centrifugation, or decantation, depending on the nature of the system.
5. Recovery of solute: The solvent containing the dissolved solute is then subjected to further processing to recover the solute in its purified form. This may involve additional separation techniques such as evaporation, crystallization, or distillation, depending on the properties of the solute and solvent.

Elutriation is commonly used in various applications such as the purification of pharmaceutical compounds, separation of contaminants from environmental samples, and extraction of valuable components from natural resources. It offers a simple and effective means of isolating soluble materials from solid matrices, making it a versatile technique in many industries.