Understanding Separation Techniques for Colloids

The question asks about a specific technique used to separate colloidal particles from smaller particles like ions. This separation method uses a special membrane, such as one made from cellophane or collodion.

Let's look at the options provided and understand what each technique involves:

• Diffusion: This is the movement of particles from an area of higher concentration to an area of lower concentration. While it involves movement of particles, it's not primarily used for separating colloids from ions using a membrane based on size differences in this context.
• Dialysis: This is a process that utilizes a semi-permeable membrane. This membrane has pores that are large enough to allow small ions, solvent molecules, and smaller solute particles to pass through, but too small to allow larger colloidal particles to escape. When a solution containing colloidal particles and small ions is placed inside a bag made of this membrane and immersed in pure solvent (like water), the small ions and solvent molecules can move across the membrane, while the colloidal particles are retained inside the bag. This process effectively purifies the colloid by removing the small impurities. Cellophane and collodion are common materials used for such membranes.
• Osmosis: This is the movement of solvent molecules across a semi-permeable membrane from a region of lower solute concentration to a region of higher solute concentration. It's mainly about solvent flow driven by concentration differences, not about separating different types of solute particles based on size.
• Viscosity: This is a physical property of a fluid that measures its resistance to flow. It is not a technique used for separating components of a mixture.

Why Dialysis is the Correct Technique

Based on the description in the question – separating colloidal particles from small ions using a semi-permeable membrane like cellophane or collodion – the technique that fits perfectly is Dialysis. It is the standard method for purifying colloidal solutions by removing crystallizable impurities (small ions, small molecules) which can pass through the membrane, while the colloidal particles are retained.

Therefore, Dialysis is the technique specifically designed for this type of separation based on the size of particles relative to the pore size of the semi-permeable membrane.