Solutions True Solutions: A true solution is a homogeneous solution in which the solute particles have diameters between 0.1 nm to 1 nm i.e., the solute particles are of molecular dimensions. Such dispersed particles dissolve in solution to form a homogenous system. These do not settle down when the solution is left standing. The particles are invisible even under powerful microscopes and cannot be separated through filter paper ,parchment paper or animal membranes. For example, sodium chloride in water is a true solution. Most ionic compounds form true solutions in water. Organic compounds like sugar and urea also form true solutions in water.

Suspensions Suspensions consist of particles of a solid suspended in a liquid medium. Suspensions are systems with two distinct phases. The particles in suspensions are bigger than 100 nm to 200 nm across. The particles of a suspension may not be visible to the naked eye but are visible under a microscope. Suspensions are heterogeneous systems. They stay only for a limited period i.e. these are not stable as the particles have a tendency to settle down under the influence of gravity. The particles of a suspension can neither pass through ordinary filter paper nor through animal membranes. Examples of suspensions are sodium chloride in benzene, silver chloride, barium sulphate or sand in water.

Colloidal solution or colloidal state or colloidal dispersion They represent an intermediate kind of a mixture between true solution and suspension. The size of a colloidal particle lies roughly between 1-100 nm. Colloids are also a two-phase heterogeneous system consisting of the dispersed phase and dispersion medium.

Colloidal solution or colloidal state or colloidal dispersion Since the dispersed phase in a colloidal system is uniformly distributed in the dispersion medium, the colloidal state appears homogenous to the naked eye or even an ordinary microscope (due to particles being invisible). However it is a heterogeneous dispersion of two immiscible phases and this is proved by viewing it under an ultra-microscope, where the light reflected by colloidal particles can be seen. Colloidal particles do not settle down under gravity: a colloidal solution of gold prepared by Faraday over 125 years ago continues to be in excellent condition even today. Colloids can pass through ordinary filter paper but do not pass through animal membranes.  

Difference Between True Solutions, Suspensions and Colloidal Solutions

solutions colloids suspensions < 1 nm > 100 nm transparent http://academic.pgcc.edu/~ssinex/Solutions.ppt < 1 nm > 100 nm solutions colloids suspensions transparent (clear) transparent with Tyndall effect translucent (cloudy) Brownian motion- colloidal particles moved by solvent molecular motion movement by gravity never settle coagulation – can settle

Colloids Lecture 1

colloid (kŏl'oid) [Gr.,=glue like], a mixture in which one substance is divided into minute particles (called colloidal particles) and dispersed throughout a second substance. The mixture is also called a colloidal system, colloidal solution, or colloidal dispersion. Familiar colloids include fog, smoke, homogenized milk.

Disperse systems A system in which one substance (the disperse phase), is dispersed as particles throughout another (the dispersion medium).

Classification of dispersed systems Particle diameter < 10-9 m 10-9 - 10-6 m > 10-6 m Homogenous Colloid Non homogenous mixture mixtures Particles are not resolved by ordinary microscope although they may be detected under ultra-microscope Visible in the electron microscope Pass through filter paper but don’t pass through semi-permeable membrane Diffuse very slowly

Lyophobic sols: these sols are "solvent hating", Colloid system can be classified as: Lyophobic sols: these sols are "solvent hating", are thermodynamically unstable, the particles tend to aggregate, they are irreversible systems. Unstable in presence of even small conc. of electrolytes leading to aggregation, depends on type, valency and concentration of counter ion of electrolyte Stability controlled by charge of particles Little interaction between particles and dispersion medium. Viscosity of the dispersion medium is not affected by the presence of lyophobic colloidal particles.  Ex, water-insoluble drugs, kaolin and oils form lyophobic dispersions

Stability is controlled by the charge of particles

Lyophilic sols: these sols are "solvent loving", are thermodynamically stable, they are reversible systems. Stable generally in presence of electrolytes, may be salted out by high concentration of very soluble electrolytes Molecules of the dispersed phase are solvated Stability controlled by charge and solvation of particles Molecules disperse spontaneously to form colloidal solution Viscosity of the dispersion medium increased by the presence of dispersed phase Ex, proteins, tragacanth and methylcellulose in water  

Surfactant molecules Dispersion phase consists of aggregates (micelles) of small ions whose size individually is below the colloidal range. Hydrophilic or lipophilic portion of the molecule is solvated , depending on the dispersion medium Because of their affinity for water and tendency to form micelles which are of colloidal dimension, they form hydrophilic colloidal sol in water but are usually classified separately as Association colloids Colloidal aggregates are formed spontaneously when concentration of amphiphile exceeds the critical micelle concentration (cmc) Viscosity of the system increases as the conc. of micelles increase. In aqueous solutions, the cmc is reduced by the addition of electrolytes

Properties of Colloids Heterogeneity A colloidal solution is heterogeneous system consisting of two phases of dispersed phase (colloidal particles of a solid) and the aqueous dispersion medium. Often a colloidal sol appears to be homogeneous as the particles are small in size and not visible to the naked eye. However, this is disproved when it is viewed under electron microscope.

2-Stable nature Colloidal solutions are quite stable. The colloidal particles do not settle at the bottom under the influence of gravity. This is because of the constant motion of colloidal particles. 3-Filterability Colloidal particles do not pass through ultrafilter papers, animal and vegetable membranes. The large pore size of ordinary filter paper enables colloidal particles to pass through.