Solution is defined as a homogeneous mixture of two or more chemical substances. The state of matter of a solution may be solid, liquid or gas. For example: common salt in water (liquid solution), air (gaseous solution), alloys (solid solution), etc. The components of a solution may be classified as:
- Solute
- SolventAn intimate mixture of solute and solvent is called a solution.
Difference Between Solute and Solvent
| Solute | Solvent |
|---|---|
| Present in lesser proportion in the solution | Present in a greater proportion in the solution |
| Dispersed phase | Medium of dispersion of solute |
| Solution may or may not be in the same of matter as the solute | Solution will be in the same state of matter as the solvent |
Aqueous Solutions and Non-aqueous Solutions
Aqueous Solutions
Solutions that contain water as the solvent are called aqueous solutions.For example: sugar in water, carbon dioxide in water, etc.Non-aqueous Solutions
Solutions that contain a solvent other than water are called non-aqueous solutions. Ether, benzene, petrol, carbon tetrachloride etc., are some common solvents. For example: sulphur in carbon disulphide, naphthalene in benzene, etc.Concentrated Solutions and Dilute Solutions
Between two solutions, the solute quantity may be relatively more or less. The solution that has a greater proportion of solute is said to be concentrated more than the other that has a lesser proportion. If the proportion of solute is less, the solution is said to be dilute. The concentration of a solution is expressed as the amount of solute present in a given amount (mass or volume) of the solution; or the amount of solute dissolved in a given mass or volume of a solvent. It is expressed as a percentage of these quantities.Saturated and Unsaturated Solutions
Saturated Solution
A solution in which no more solute can be dissolved at a given temperature is called a saturated solution.Unsaturated Solution
A solution in which more solute can be dissolved at a given temperature is called an unsaturated solution.Remember, a given solution that is saturated at a particular temperature may become unsaturated when the temperature is increased.
Solubility of a Solute
Solubility is defined as the number of grams of a solute that dissolves in 100g of a solvent to form a saturated solution at a given temperature and pressure. Solubility is the maximum weight of a solute that can be dissolved in 100g of a solvent at a given temperature and pressure.
Factors Affecting the Solubility of a Solid Solute in Water
Temperature
Increase in temperature increases the solubility. For example, it is easier to dissolve sugar in hot milk than in cold milk.Size of Solute Particles
Smaller the size of the particles greater is the solubility. For example, it is easier to dissolve powdered sugar than granules of sugar.Mechanical Stirring
Mechanical stirring increases solubility. For example, sugar dissolves faster on stirring with a spoon.Factors Affecting the Solubility of a Gas in Water
Temperature
Increase in temperature, decreases the solubility of the solute. On heating, the gases dissolved in milk escape making the milk bubble and boil over.Pressure
An increase in pressure increases the solubility of a gas. For example, aerated water bottles contain carbon dioxide gas under pressure.Solutions and Suspensions
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, turmeric in water, silver chloride, barium sulphate or sand in water.Colloids
Colloidal solution or colloidal state or colloidal dispersion, 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. However, colloidal particles present in small amount as the dispersed phase component behave like a solute in a solution when suspended in a solvent phase or dispersing medium, because of their small size.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
| Property | True Solutions | Suspensions | Colloidal Solutions |
|---|---|---|---|
| Particle size | Less than 10 - 7 cm | Greater than 10 -5cm | Between 10 - 5 and 10 - 7 cm |
| Visibility of particles | Invisible to naked eye not visible under powerful microscope | Easily visible | Invisible to naked eye. Visible under powerful microscope. |
| Sedimentation of particles | Do not settle down | Settle down due to gravity | Settle down under high centrifugation |
| Filtration through filter power | No residue is formed | Residue is formed | No residue is formed |
Comparative Sizes of Particles in Solutions
Classification of Colloids Based on Type of Phases
Each of the two phases of a heterogeneous colloidal system i.e., the dispersed phase and dispersion medium, can be in any one of the three physical states of matter. We have eight different types of colloidal solutions (not nine), since a colloidal state of gas-in-gas forms a single phase (gases consist of molecules, and with molecules of both gases mixing in all proportions, two separate phases do not form).
Types of Colloidal Solutions
| Dispersed phase | Dispersed medium | Colloidal system | Examples |
|---|---|---|---|
| Solid | Solid | Solid sols | Coloured glues, gem stones, pearls, some alloys |
| Solid | Liquid | Sols | Paints, gold sol, sulphur sol, starch, proteins |
| Liquid | Solid | Gels | Jellies, cheese butter, hoot polish |
| Liquid | Liquid | Emulsion | Milk, hair cream, emulsified oils, medicines |
| Solid | Gas | Aerosols of solids | Smoke, dust in air, smog |
| Liquid | Gas | Aerosols of Liquids | Mist,Fog,clouds,insecticide sprays |
| Gas | Solid | Solid foam | Foam, pumice stone,ice-cream, rubber |
| Gas | Liquid | Foam, Froth | Soda water, whipped cream froth, etc. |
Properties of Colloids
Heterogeneity
A colloidal solution is heterogeneous system consisting of the two phases of the 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.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.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. If ordinary filter paper is suitably impregnated with collodion, the size of the pores adjusts accordingly to disallow filtration.Colligative properties
In colloidal systems the number of colloidal particles per litre of the sol is relatively much smaller than solute particles in a true solution. Colloidal particles are aggregates of simple molecules and colligative properties such elevation in boiling point, depression in freezing point and lowering of vapour pressure depend upon the number of colloid particles present in system and not on the nature of the particle. The values of colligative properties are consequently much smaller as compared to true solutions.However, the osmotic pressure of colloidal solutions, though smaller than true solutions is measurable and gives information regarding the number of particles present. It is used for the calculation of molecular weights of polymers like proteins.
Mechanical Properties (Brownian movement)
When colloidal solutions have been observed through ultra microscope, the colloidal particles are seen in constant and rapid zigzag motion called Brownian movement. Sir Robert Brown first observed the phenomenon in 1827. Suspensions and true solutions do not exhibit Brownian movement.Optical Properties (Tyndall Effect)
When a strong beam of light is passed through a colloidal solution, the path of the light becomes visible when viewed from a direction at right angle to that of the incident light. This occurs because the colloidal particles absorb light energy and then scatter it in all directions. The phenomenon of scattering of light by sol particles to form illuminated beam or cone is called Tyndall effect or Tyndall beam or Tyndall cone.Tyndall effect is not shown by true solutions because the ions or solute molecules are of such minute sizes that they cannot reflect light. The Tyndall effect can therefore be used to distinguish between a true solution and a colloidal solution. The hazy illumination of the light beams from the headlights of a car on a dusty road is a familiar example of Tyndall effect. Blue colour of sky and seawater, twinkling of stars and visibility of tails of comets are also due to scattering of light of Tyndall effect.
