Diffusion is defined as which of the following?

• A. Movement of particles from an area of high concentration to an area of low concentration.
• B. Movement of particles from an area of low concentration to an area of high concentration.
• C. The dissolution of a solute.
• D. The mixing of two liquids.

Answer: (A)

Diffusion is the passive spread of particles as they move randomly, from areas of higher concentration to areas of lower concentration, until the distribution becomes uniform. It doesn’t require energy input, and the rate goes up with higher temperature and with smaller particles.

Think of a drop of ink spreading through water or perfume gradually filling a whole room—that spreading happens because individual particles are moving in all directions and more move away from crowded areas than into them.

The idea of moving from low to high concentration would need energy input and isn’t how diffusion works. Dissolution is about solute particles dissolving in a solvent, which is a different step in forming a solution. Simply mixing two liquids can involve diffusion, but diffusion specifically describes the movement down a concentration gradient that leads to spreading.