Solutions

In our daily life, we rarely come across pure substances. Most of these are mixtures containing two or more substances. These mixtures are also called solutions. Depending upon their composition (particles size of the component), these are divided into true, solutions, suspension and colloids. Their utility or importance in life depends on their composition.

Solution or True Solution

A solution is a homogenous mixture of two or more substances in which at a constant temperature the relative amounts of components can change up to a certain definite limit. Lemonade, soda water, etc are the examples of solutions in our daily life.

Components of a Solution

A binary solution (a mixture of two substances) has two components; solvent and solute

1. Solvent. The component of the solution that is present in the largest quantity, is known as the solvent. The solvent which has greater dielectric constant is a better solvent. The dielectric constant of water is large (80), so it is called universal solvent. Solvents are used in making perfumes, drugs, in the processing of various foodstuff and in beverages. These are also used in dry cleaning.
2. Solute. One or more components present in the solution other than the solvent, are called solutes. In general, in a binary solution amount of solute is smaller than solvent.

e.g., a solution of iodine in alcohol (solvent) known as ‘tincture of iodine’, has iodine (solid) as a solute. Aerated drinks like soda water contain CO2 as solute and water as a solvent.

Properties of a Solution

1. A solution is a homogenous mixture (a mixture of uniform composition).
2. The particles of a solution are smaller than 1 nm (10-9 m) in diameter. So, they can not be seen by naked eye.
3. Because of very small particle size, they do not scatter a beam of light passing through the solution. So, the path of light is not visible in a solution. In other words, they do not exhibit Tyndal effect.
4. The components of a solution (i.e., solute and solvent) diffuse into each other in such a way that they can not be distinguished.
5. The solute particles cannot be separated from the mixture by the process of filtration because the size of solute particles is very tiny. The solute particles do not settle down when left undisturbed, i.e., a true solution is stable, permanent and transparent.

Types of Solutions

1. Depending upon the amount of solute in a given solvent, the solution can be classified into following types.
   1. Unsaturated Solution A solution in which more solute can be dissolved without increasing temperature is called unsaturated solution.
   2. Saturated solution A solution in which no solute can be dissolved further at a given temperature, is called saturated solution.
   3. Supersaturated Solution When a saturated solution is heated, its capacity to hold more solute increases and it is called supersaturated solution. The supersaturated solution contains an excess amount of dissolved solute in it which is beyond the capacity of the solution at a given temperature. If a small crystal of solute is added to it, the excess solute immediately crystallizes out.
2. On the basis of states of solute and solvent, the solution may be of the following types

Aqueous and Non-aqueous Solutions

When the solute is dissolved in water, it is known as an aqueous solution, e.g., ethanol in water. When the solute is dissolved in a solvent other than water, it is known as a non-aqueous solution. e.g., iodine in alcohol (tincture of iodine)

Acidic and Basic Solutions

Acidic solutions have more H+ ions than that of OH- ions while basic solutions have more OH- ions than that of H+ ions.

Neutral Solutions

They have equal concentration of H+ ions (hydrogen ions) and OH- ions (hydroxyl ions)

Concentration of Solution

It is defined as the amount of solute present in a given amount (mass or volume) of solution or solvent.

Solutions with relatively low concentration are called dilute solutions, while those with relatively high concentration are called concentrated solution.

Various ways of Expressing the Concentration of a Solution

1. Mass percentage of a component = Mass of the component/total mass of solution x100
2. Volume percentage of a component = Volume of Component/Total volume of Solution x 100
3. Mole fraction of a Component = Number of moles of the component/ Total number of moles of all the components
4. Pars per million (ppm) = Number of parts of the component/ Total number of parts of all components of solution x 10^6
5. Molarity (M) = Moles of solute/Volume of solutions in liter
6. Molality (m) = Mole of solute/ Mass of solvent in kg

Solubility

The maximum amount of a solute that can be dissolved in a given amount of solvent (generally 100g) at a given temperature and pressure, is known as its solubility at that temperature.

If at a given temperature w g of solute is dissolved in W g of solvent (water) then.

Solubility of solute in solvent = wx100/W

Factors affecting solubility of a Solute in a Solvent

1. Nature of Solute and Solvent (like dissolves like) Polar solute like sodium chloride dissolves in a polar solvent like water. Similarly, non-polar solute like cholesterol, bromine, etc. dissolves in a non-polar solvent like benzene, carbon tetrachloride (CCl4), etc.
2. Effect of Temperature Usually solubility of a solute increases with increase in temperature of solution i.e., their dissolution process is endothermic (proceeds with the substances such as calcium nitrate, calcium oxide, sodium sulphate, calcium hydroxide and calcium citrate decreases with increase in temperature i.e., their dissolution process is exothermic (involves evolution of energy or heat). The solubility of a gas in a liquid decrease with increase in temperature.
3. Effect of pressure Pressure has no effect on the solubility of solids in a liquid. But solubility of gases in liquids increases with increase in pressure.
4. Size of Substance Solubility decreases as the molecular mass of substance increases.