Lecture 1 B

Solutions, Suspension and Colloidal System

Solutions

It is a common observation that when a little sugars or salt is dissolved in water a homogeneous and stable mixture of the two components is obtained which is called as solution.

Of these two components, the one (here sugar or the salt) which is present in small quantity is called as solute while the other (here water) present in large quantity is called as solvent.

Solutions

The solution is homogeneous because the molecules or the solute become evenly distributed throughout the solvent.

It is a stable system because the molecules or the ions do not settle down.

Sometimes more than one molecule may be dissolved in a solvent to form a stable and homogeneous mixture.

the particles are not visible even under highest power of the microscope.

Solutions

Thus, true solution may be defined as :

Homogeneous and stable mixture of two or more chemical substances.

Solutions

Particle sizes distinguish one homogeneous mixture from another.

Solutions are mixtures with particle sizes at the molecule or ion level.

The particles have dimensions between 0.1 to 2 nanometers.

Typically solutions are transparent. Light can usually pass through the solution.

If the solute is able to absorb visible light then the solution will have a color.

A solution cannot be filtered but can be separated using the process of distillation.

Suspensions

A suspension is a mixture between two substances, one of which is finely divided and dispersed in the other.

Common suspensions include sand in water, dust in air, and droplets of oil in air.

Particles in a suspension will settle out if the suspension is allowed to stand undisturbed.

Suspensions

Many particles of a suspension can be separated through a filter.

Suspensions are homogeneous mixtures with particles that have diameters greater than 1000 nm.

The size of the particles is great enough so they are visible to the naked eye.

They do not transmit light.

Colloidal System

A colloid is intermediate between a solution and a suspension.

The size of the colloidal particles is in between the size of particles of true solutions and suspension.

While a suspension will separate out, a colloid will not.

Colloidal System

There are 8 different kinds of colloidal system, one of them (solid + liquid type) .

This type of colloidal in which the solid forms the dispersed phase and the liquid dispersion medium, has a fluid like consistency and is called as colloidal solution or sol.

The particles of the dispersed phase in colloidal solution are called as colloidal particles or sol particles or micelles.

Colloidal System

The colloidal particles are not visible under

microscope but can be observed under an

ultramicroscope.

Types of Colloidal solutions :

a) lyophilic (solvent loving) called emulsions,

there is an affinity between the particles of

the two phases, When water is the

dispersion medium it is called hydrophilic

b) lyophobic (solvent hating) called

suspensions, When water is the dispersion

medium it is called hydrophobic.

Colloidal System

Colloids are mixtures with particle sizes that consist of clumps of molecules.

The particles have dimensions between 2 to 1000 nanometers.

The colloid looks homogeneous to the naked eye. milk is an example of colloids.

Solution Suspension Colloid

Appearance Clear, transparent and homogeneous

Cloudy, heterogeneous, at least two substances visible

Cloudy but uniform and homogeneous

Particle Size molecule in size (10-7-10-8 cm)

larger than 10,000 Angstroms (10-3-10-5 cm)

10-1000 Angstroms (10-3-10-7 cm)

Effect of Light Tyndall Effect

none -- light passes through, particles do not reflect light

variable light is dispersed by colloidal particles

Effect of Sedimentation

None

particles will Eventually settle out

None

Visibility Particles non visible even under The ultramicroscope

Particles visible even with naked eye

Particles visible under ultramicroscope

Important Properties of Colloidal Solution

1. Filterability

The colloidal particles are unable to pass through parchment membrane.

Important Properties of Colloidal Solution

2. Adsorption and Increased Surface Area

The colloidal particles have a tendency to attract and retain at their surface other particles with which they come in contact. This is called as adsorption.

The adsorption is increased if the surface area of the same mass of an adsorbent is also increased.

Important Properties of Colloidal Solution

2. Adsorption and Increased Surface Area

In a colloidal solution the little mass of dispersed phase is present in the form of a large number of small particles, thus increasing its total surface area.

The adsorption and the large surface area offered by the colloidal particles help to carry on many complex biochemical reaction in the protoplasm.

Important Properties of Colloidal Solution

3.Tyndal Effect

If a strong beam of light is passed through a colloidal solution and viewed from the side a bluish light cone illuminates the path of the beam.

This phenomenon is called as Tyndall effect and results due to the scattering or diffraction of light by the colloidal particles.

Important Properties of Colloidal Solution

4. Brownian Movement Brownian movement may be used to

distinguish between solutions and colloids.

Brownian motion is the random movement of colloidal particles suspended in a liquid or gas, caused by interference with molecules of the surrounding medium.

However colloid particles are large enough to be observed and are small enough to still be affect by the random molecular motion.

Important Properties of Colloidal Solution

5. Electric Properties

The colloidal particles constituting the dispersal phase carry an electric charge probably due to the preferential adsorption of ions present in the dispersion medium.

All these colloidal particles in a particular colloidal system carry electric charge of the same sign.

As a result, they repel each other and remain dispersed in the dispersion medium, and if the colloidal solution is placed under an electric field, all these particles move towards the oppositely charged pole.

Important Properties of Colloidal Solution

5. Electric Properties

This phenomenon is called as cataphoresis or electrophoresis. Gums, Starch etc., are some of the examples of negative sols.

While basic dyes such as methylene blue, metallic hydroxides such as Al(OH)3 are examples of positive sols.

However, a colloidal solution is electrically neutral as a whole because the particles of the dispersion medium have equal electric charge of opposite sign.

Important Properties of Colloidal Solution

6. Coagulation or Flocculation

The precipitation of the colloidal particles constituting the dispersed phase of the colloidal solution by the addition of an electrolyte is called as coagulation or flocculation.

Important Properties of Colloidal Solution

6. Coagulation or Flocculation

It is because the electric charge carried by the particles of dispersed phase is neutralized by the electrically charged ions resulting from the dissociation of the electrolyte in colloidal solution.

Now these colloidal particles can no longer repel each other.

They come close to each other due to Brownian movements and soon settle down to gravity.

Important Properties of Colloidal Solution

7. Osmotic Pressure

The osmotic pressure of the colloidal solution is usually very small.

Important Properties of Colloidal Solution

8. Dialysis

The colloidal particles are unable to pass through parchment membrane or collodion but true solution can pass them.

Thus a true solution if mixed with a colloidal solution can be separated from the latter by filtration through such membranes.

This method of purifying a colloidal solution or separating a colloid from a crystalloid by filtration through a membrane was called as dialysis .

Colloidal Nurture of Protoplasm

Although a large number of chemical substances are found in the protoplasm with water constituting the major portion, the protoplasm is not a true solution.

Most of the particle phase of the protoplasm is colloidal in nature.

Again, it is not a simple colloidal system but is considered as complex colloidal system of many phases and shows many properties of the colloidal system. The consistency of the protoplasm is both of a sol and a gel type.

The cell membranes seem to be more gel-like in nature.

However, both these forms are not static but constantly changing.

Colloidal Nurture of Protoplasm

The colloidal nature of the protoplasm is chiefly due to the presence of protein molecules which often reach colloidal dimensions and are distributed through it.

In fact, these macromolecules of proteins constitute next major category of chemical substances after water in protoplasm.

Moreover, all the enzymes are essentially proteins which provide large surfaces due to their large and often colloidal dimensions to catalyse most of the biochemical reactions in the protoplasm so important for life to exit.