Diffusion is the process of movement of a substance (solid, liquid, or gas) from the region of higher concentration to the region of lower concentration so as to spread uniformly. It is a natural and physical process that occurs in all states of matter, i.e. solid, liquid, and gases. Diffusion helps in the movement of substances in and out of the cells. The molecules move from a region of higher concentration to a region of lower concentration until the concentration becomes equal throughout.

Some key points about diffusion in chemistry class 9 are:

• Definition: The process of movement of a substance (solid, liquid, or gas) from the region of higher concentration to the region of lower concentration so as to spread uniformly is called diffusion.

• Examples: Diffusion occurs in all states of matter. For example, in gases, smoke coming out of factories is seen only near the mouth of chimneys. After rising high, it mixes up with air and vanishes out. In liquids, when two liquids are kept together, they get mixed because of diffusion. For instance, when salt solution is kept in water, this makes the whole solution salty after some time.

• Factors affecting diffusion: The rate of diffusion depends on various factors such as temperature, pressure, concentration gradient, and surface area.

• Importance: Diffusion is an important process occurring in all living beings. It helps in the movement of substances in and out of the cells.

In summary, diffusion is the process of movement of a substance from a region of higher concentration to a region of lower concentration so as to spread uniformly. It is a natural and physical process that occurs in all states of matter and is important for the movement of substances in and out of cells.