Solution

7.1 SOLUTION

A solution is defined as a homogeneous mixture of two or more substances. The relative amounts of the components can be gradually changed within certain limits. The component of solution present in smaller amount is called solute, whereas the component present in greater quantity is called solvent. For example in a 5% glucose solution in water, solute is glucose and solvent is water. When water is solvent the solution is called aqueous solution.

Types of Solution:

Since matter exists in three states i.e. solid, liquid and gas, on mixing they produce nine types of solutions, which are listed as following:

Table

No.

Solute

Solvent

Examples

1.

Gas

Gas

Air (Mixture of 78% N2> 21% 02 and 1% other gases)

2.

Gas

Liquid

Carbonated soft, drinks such as coca-cola, Bubble-up etc. NH3gas in water, and air dissolved in water.

3.

Gas

Solid

H, gas adsorbed over palladium metal

4. "

Liquid

Gas

Cloud (water vapours in air), steam.

5.

Liquid

Liquid

Alcohol in water, water in milk, milk in tea, vinegar (acetic acid in water)

6.

Liquid

Solid

Amalgam (e.g. Mercury in sodium), and water in jelly powder.

7.

Solid

Gas

Smoke (carbon particles in air),

8.

Solid

Liquid

Sugar in water, sea water

9.

Solid

Solid

Alloys such as brass (copper and zinc) bronze (copper and tin), steel (carbon and iron), glass.                            

 

7.2 FACTORS AFFECTING SOLUBILITY

Many factors affect the solubility of a solute in a solvent. These factors may be.

Temperature. Pressure (For gases). Nature of solute. Nature of solvent

Solubility and Temperature:

Solubility of a solid in liquid or solubility of partially miscible liquids increases with increase in temperature. Thus solubility of sugar in water at 0°C is 179 g/100ml whereas at 100°C it is 487 g / 100 ml.

But the solubility of gases in a liquid decreases with the increase in temperature. For this reason when a glass of cold water is warmed, bubbles of air are seen on the inside of the glass.

Solubility and Pressure:

The solubilities of solids and liquids are not affected by pressure. But *e solubility’ of a gas in a liquid is directly proportional to the pressure of gas. This is called Henry's law i.e.

M∞P or m=KP

Where V is the amount of gas dissolved.

This effect is used in manufacture of bottled soft-drinks as coca-cola; 7-up etc. These are bottled under a CO2 pressure slightly greater than 1 atm. When the bottles are opened, pressure decreases, so solubility of CO2 also decreases, hence bubbles of CO2 come out of solution.

Solubility and Nature of Solute and Solvent:

Solute and solvent may be polar (as H2O; Alcohol) and non-polar (as benzene, carbon tetrachloride). Polar and ionic solutes easily dissolve in polar solvents whereas non-polar solutes easily dissolve in non-polar solvents. Hence it is said that like dissolves like. For example common salt (NaCl) being an ionic compound easily dissolves in polar solvent like water but is insoluble in non-polar solvent like benzene or petrol.

7.3 UNSATURATED, SATURATED AND SUPER SATURATED SOLUTIONS:

An unsaturated solution is that one which contains less solute than it has the capacity to dissolve. If more solute is added, at least some of it will dissolve.

Preparation of unsaturated solution:

Take a few crystals of sugar and dissolve them m a glass of water. This results in an unsaturated solution, because the solution has a capacity to dissolve more crystals of sugar (solute) at a given temperature.

A saturated solution is that one which contains maximum amount of solute in a given solvent at room temperature. No more solute can dissolve in it and there is a dynamic equilibrium between the undissolved and dissolved solute. This type of solution is called saturated solution.

The amount in grams of a solute required to be dissolved in 100 grams or 100 ml of a solvent to prepare a saturated solution at room temperature is called solubility of that solute, it is expressed in g/100 ml or g/100 g at that specific temperature.

Preparation of a Saturated Solution:

Take some water, in a beaker. With constant stirring, add crystals of sugar, till they do not dissolve and start to settle down. The solution thus obtained is saturated solution of sugar, at room temperature.

A super saturated solution is that which contains greater amount of dissolved solute than that present in a saturated solution; at the given temperature. Preparation of super saturated solution is required for crystallization. Crystallization is simple and general technique for purification of impure compounds.

 

 
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