Diffusion and Osmosis

Dissolving KMnO4 crystal 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

Partially permeable membrane Partially-Permeable Membranes A partially-permeable membrane will allow certain molecules to pass through it, but not others. Partially permeable membrane Generally, small particles can pass through… …but large particles cannot

Free water molecules diffuse in this direction Partially-permeable membrane More 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.

Answer to Brian teaser 1 What is the difference between ‘melt’ and ‘dissolve’? Melting and dissolving are not the same. Melting is a change of state from solid to liquid. Such as, ice in a cup melting into water. This is usually due to temperature Dissolving occurs when a substance is added to a liquid (solvent) and is intimately dispersed in it at the molecular level e.g. sugar dissolving in water. The sugar spreads out in between the water molecules. Dissolving is due to molecules of the solvent mixing with the molecules of the solute. Temperature can speed up this process because the molecules move faster and you are able to dissolve more producing a saturated solution or even a super saturated solution

In melting only one substance is involved and the liquid and solid are the same material. Heat is needed for melting to occur. Dissolving involves two materials; the resulting solution is a mixture of both.