1. Colloids
• Tyndall effect
– Light is scattered by particles suspended in
water
– Use the Tyndall effect to differentiate between a
true solution and a suspension
• True solution - a homogeneous mixture
– ions and molecules are too small to scatter visible light
• Suspension - particles are suspended in water
– the particles are too large to be dissolved, and so are
large enough to scatter light.
2. Colloids
• Colloid
– aka a colloidal dispersion
– tiny particles are suspended in some medium
• the particles are single large molecules or
• the particles are groups of molecules or ions from 1
to 1000 nm.
5. Colloid
• What stabilizes a colloid?
– Electrostatic repulsion
• A colloid is neutral
• BUT, when a colloid is placed in an electric field, the
particles all migrate to the same electrode.
• The colloid attracts ions of the same charge which
surround the colloid particle.
• Ions of the opposite charge surround the first layer of
ions.
• Being surrounded by ion, the colloid particles repel
other colloid particles, and so are unable to aggregate
to precipitate out
6. Colloids
• To destroy a colloid (coagulation)
– heat the colloid
• heating increases velocities of particles
• particles can collide with enough energy to knock
off the ion barriers, thus allowing the colloid
particles to aggregate and precipitate out.
7. Colloids
• To destroy a colloid (coagulation)
– add an electrolyte
• the electrolyte will neutralize the ion layers
• Ex: deposition of clay where a river reaches the
ocean. The high salt content of the seawater causes
the suspension of clay particles to coagulate.
