Diffusion is a general term referring to the net movement of atoms and molecules along a concentration gradient, from an area of concentration to an area of concentration.

Higher concentration to an area of lower concentration

Explanation:

When you open a perfume bottle at a corner of a room, after a while, its fragrance can be perceived across a distance at the other end of the room. This is because, molecules of the compound in the fragrance have moved from the area of higher concentration in the perfume bottle, across a concentration gradient to a region of lower concentration at the other end of the room. This is diffusion.

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a) the particles of the chair are strongly held together.