properties of colloids (Science Module 9 Assignment 2 - 3rd year)

 Tyndall effect

The term Tyndall effect is usually applied to the effect of light scattering on particles in colloid systems, such as suspensions or emulsions. It is named after the Irish scientist John Tyndall. The Tyndall effect is used to tell the difference between the different types of mixtures, namely solution, colloid, and suspension. For example, the Tyndall effect is noticeable when car headlamps are used in fog. The light with shorter wavelengths scatters better, thus the color of scattered light has a bluish tint. This is also the reason why the sky looks blue: the light from the sun is scattered and we see the blue light because it scatters better.   This effect occurs because short wavelengths of light towards the blue end of the spectrum hit the air molecules in the earth's atmosphere and are reflected down to the earth's surface. Longer wavelengths towards the red end of the spectrum are not affected by the particles and pass on through the earth's atmosphere. This causes blue light to be reflected down to the earth's surface which makes the sky appear blue. Brownian movement or motion Brownian movement or motion, zigzag, irregular motion exhibited by minute particles of matter when suspended in a fluid. The effect has been observed in all types of colloidal suspensions solid-in-liquid, liquid-in-liquid, gas-in-liquid, solid-in-gas, and liquid-in-gas. It is named for the botanist Robert Brown who observed  the movement of plant spores floating in water. The effect, being independent of all external factors, is ascribed to the thermal motion of the molecules of the fluid. These molecules are in constant irregular motion with a velocity proportional to the square root of the temperature. Small particles of matter suspended in the fluid are buffeted about by the molecules of the fluid. Brownian motion is observed for particles about 0.001 mm in diameter; these are small enough to share in the thermal motion, yet large enough to be seen with a microscope or ultramicroscope. The first satisfactory theoretical treatment of Brownian motion was made by Albert Einstein in 1905. Jean Perrin made a quantitative experimental study of the dependence of Brownian motion on temperature and particle size that provided verification for Einstein's mathematical formulation. Perrin's work is regarded as one of the most direct verifications of the kinetic-molecular theory of gas Colligative Property Colligative properties depend upon the number of moles of solute present in given mass of solvent. It does not depend on nature of solute particles. Colloidal particles are not simple molecules, they are aggregations of molecules. Hence, all colloidal dispersions give low value of osmotic pressure, depression in freezing point and elevation in boiling point. Electrophoresis The movement of colloidal particles under the influence of electric field is called Electrophoresis. When an electric field is applied across the colloidal solution, the colloidal particles migrate to oppositely charged electrode where they get neutralized. This phenomenon is known as electrophoresis. Electro Osmosis When electrophoresis of dispersed particle in a colloidal system is prevented by some suitable means, it is observed that dispersion medium itself begins to move in an electric field. This phenomenon is known as electro osmosis.