Diffusion
and
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
Solute
molecule
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
More free water molecules
on this side of membrane
Partially-permeable
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.