Diffusionand

Osmosis

Diffusion

• Particles in a liquid or gas spread out…

• … from regions of high concentration…

• … to regions of low concentration…

• …until the particles are evenly spread out.

Dissolving KMnO4 crystal

• The difference between the regions of high concentration and low concentration is called the concentration gradient

• The steeper the concentration gradient, the faster diffusion takes place

High concentration gradient

Fast rate of diffusion

Low concentration gradient

Slow rate of diffusion

• Diffusion occurs because the particles in gases and liquids are moving.

Dissolving substances in water

• The molecules in liquid water are constantly moving

• When water molecules bump into particles of a soluble substance, they stick to them

Free moving water molecules

Sugar molecules in sugar lump

• When the water molecules move away…

… they carry particles of the solute with them

• Adding a solute to water reduces the amount of free water molecules

Free water molecules

Solutemolecule

Partially-Permeable Membranes

• A partially-permeable membrane will allow certain molecules to pass through it, but not others.

• Generally, small particles can pass through…

Partially permeable membrane

…but large particles cannot

Partially-permeable membraneMore free water molecules

on this side of membrane

Water-solute particle is too large to pass through membrane

Free water molecules diffuse in this direction

Osmosis

• Osmosis is the diffusion of free water molecules…

• … from a region of high concentration of free water molecules…

• … to a region of low concentration of free water molecules…

• …across a partially-permeable membrane…

• …until they are evenly spread out.