Colloids

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.

3. Colloids • Type of colloid depends on the medium and the dispersed phase

4. Colloids

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.