Solubility :
The maximum soluble mass that can be dissolved in a given amount of solvent (usually 100 g) at a given temperature is called the solubility at that temperature.
Solubility of a Solid in a Liquid:
Not all solids are dissolved in this liquid. The basic concept of solubility is based on the following facts: "like dissolves like. Polar solvents like water, NH, liquid HF etc. dissolve polar and ionic compounds whereas, non polar solvents like benzene, carbon tetra chloride dissolve non polar compounds. In general, a solute dissolves in a solvent if the inter molecular interaction are similar in the two. On the basis of solubility of solid (solute) in a liquid (solvent), solution may be of the following types.
(i) Unsaturated Solution-
A solution is unsaturated if it can still dissolve more solute at the same temperature
(ii) Saturated Solution-
If a solution can dissolve no more of the solute at the same temperature, it is called saturated solution.
(iii) Super Saturated Solution-
A solution is super saturated if it contain more dissolved solute than they should at a particular temperature. It is obtained by cooling a hot concentrated solution of a solid in a liquid in the absence of traces of solids
The supersaturated solution is not in equilibrium with the solid. Thus, if a small crystal of solute (say sodium thiosulphate) is added to its supersaturated solution, the excess immediately crystallises out, which is usually quite fast (and dramatic too). Solubility of a solid in a liquid in significantly affected by temperature changes. In general, if the dissolution process is endothermic (AH(solution) > 0), the solubility increases with rise in temperature and if it is exothermic (AH solution) <0), the solubility decreases.
Solubility of Liquid in Liquid:
A liquid may or may not be soluble in another liquid. Depending upon the relative solubility of a liquid in another liquid, the following three cases are feasible.
(1) Miscible-
When two substances dissolve in each other, they are known to be completely miscible, e.g. water and alcohol, benzene and toluene.
(2) Immiscible-
Two chemically different liquids like water and oil, water and mercury do not dissolve in each other.
(3) Partially miscible-
Certain liquids such as water and phenol, water and ether which dissolve up to a limited extent only.
Solubility of Gas in Liquid:
All the gases are soluble in water as well as in other liquids to a smaller or larger extent. The solubility of a gas in liquids depends upon the following factors
(i) Nature of the Gas
Gases which can be liquefied easily are more soluble in water than the other gases which are not liquefied easily. NH, and CO2 are easily liquefied hence, soluble in water.
(ii) Nature of Solvent
Gases which form ion in the solvent, are soluble in that solvent. e.g. NH, is soluble in H₂O due to formation of NH and OH ions. H-bonding also plays greater role in making NH, soluble in H₂O. CO₂ is soluble in H₂O due to formation of ions. H₂O + CO₂ HCO3 + H NH, and CO2 both are insoluble in benzene in which ion formation does not take place.
(iii) Temperature
In general, solubility of gas in a liquid decreases with increase of temperature. When a solution of a gas is heated, the gas is usually expelled. Dissolution of a gas in liquid is an exothermic process Gas + Solvent Solution + Heat. According to Le chatelier's principle, the solubility would decrease with increase of temperature as increase of temperature would shift the equilibrium in the backward direction. Thus, aquatic species are more comfortable in cold water rather then in warm water.
(iv) Pressure
Solubility of gases in liquids is greatly affected by pressure. Henry was the first to give a quantitative relation between pressure and solubility of a gas in a solvent which is known as Henry's Law.
Henry's law & its Application:
According to Henry's law
"At a constant temperature, the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas present above the surface of a liquid or solution."
Henry further concluded that the mole fraction of a gas in the solution as a measure of its solubility. Thus, it can be said that the mole fraction of gas in the solution is proportional to the partial pressure of the gas over the solution. Hence, the most X ( commonly used form of Henry's law states that, "the partial pressure of the gas in vapour phase (P) is directly proportional to the mole fraction of the gas (x) in the solution." where, Ky is Henry's law constant. Value of K depends on the nature of the gas at constant temperature. If Henry's law is true then graph between partial pressure and mole fraction is a straight line of slope K.
Different gases have different K values at the given temperature values of K (Henry's law constant) have been given for some gases in water.
Gas Temp.(K) KH (kbar)
He 293 145.0
H₂ 293 69.0
N₂ 293 76.5
N₂ 303 88.8
O2 293 35.0
O2 303 47.0
From the above values, it is clear that Kh values for both N2 and O2 increases with increases of temperature and thus, solubility of gases decreases with increase in temperature. Thus, aquatic species are more comfortable in cold water than in warm waters. Thus, from Henry's law it can be concluded that Higher the value of k at a given pressure, the lower is the solubility of the gas in the liquid.
Application of Henry's law:
Some of the important applications of Henry's law are:
(a) To increase the solubility of CO2 in soft drinks and soda water, the bottles are sealed under high pressure when the bottle is opened to air, the partial pressure of CO2 above the solution decreases. As a result solubility decreases and hence CO2 fizzes out.
(b) To minimise the painful effects accompanying the decompression of deep sea divers, O2 diluted with less soluble He gas is used as breathing gas.
(c) In lungs, where oxygen is present in air with high partial pressure, hemoglobin combines with oxygen to form oxyhaeomoglobin. In tissues, when partial pressure of oxygen is low, oxyhaemoglobin releases O, for utilization in cellular activities.