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
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
• 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
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