# Solubility Chemistry Notes

## Solubility Chemistry Notes

“Solubility is that maximum amount of a substance which is dissolved in certain amount of solvent at cerin temperature.” It depends on nature of solute and solvent as well as on temperature and pressure.

Solubility of Solids in Liquids :

• All solids are soluble in all liquid solvents in less or more amount. A solid has different solubilities in different solvents.
• Solubility : Solubility is the maximum amount of dissolved solid in 100 g solvent at certain temperature. In this stage, the solution is called saturated solution.

Factors Affecting the Solubility of solids :

→ Nature of Solid and Solvent Ionic solids are soluble in polar solvents and covalent solids are soluble in non-polar solvents. The crystalline solids whose nature is ionic, are soluble in polar solvents. Its main reason is ion-dipole interaction. The solubility of various ionic solids in water is different. The reasons of solubility are lattice energy and hydration energy.

→ Note-Lattice Energy: The energy required to remove ions of one mole ionic solid is called lattice energy.
Hydration Energy : The energy released by hydration of ions present in one mole solid is called hydration energy, If value of hydration energy is more than lattice energy, then ionic solid will be soluble in water.

→ If the value of hydration energy is less than lattice energy then ionic solid does not soluble in water. Non-polar solids like I2, S8 etc are soluble in non-polar solvents.

Effect of Temperature :

Following two processes take place on dissolving a solid in solvent :

→ Exothermic Process : Some salts like lithium carbonate, cerium sulphate, sodium carbonate (Na2CO3. H2O) release heat on dissolving in water. So, this process is exothermic. According to Le-Chatelier’s principle, if temperature of an exothermic process increases then solubility of solid decreases in solvent.

→ Endothermic Process : Most of the crystalline salts like NaCl, KCI, NH4Cl, NaNO3 ete represent endothermic reaction on dissolving in water. Here, energy is used to separate ions of solute before dissolving in solvent.

→ According to Le Chatelier’s principle, if temperature of an endothermic process increases, then solubility of solid increases in solvent. So, following equilibrium is present between insoluble solid and solution in saturated solution.

• Insoluble solid + solvent ↔ Solution : ∆H = +ve
• Some salts represent abnormal behaviour.

→ Example-The solubility of glaubar salt (Na2SO4. 10H2O) increases up to 32.40°C, but solubility decreases at higher temperature. This is due to separation of crystalline water from salt to make it anhydrous. The temperature at which solubility changes, is called transition temperature.

→ Effect of Pressure : There is no effect of pressure on solubility of solids in liquids because solids and liquids are highly non-compressible and unaffected by change in pressure.

Solubility of Gases in Liquids :

The solubility of gas in liquids at certain temperature takes place at certain limit. Gas solution is obtained by absorption of gas through liquid. It is also called absorption of gas. The solubility of gases in liquid is expressed by absorption coefficient.

Absorption coefficient :

The volume of gas dissolved in unit volume of solvent at NTP i.e., at certain temperature and one atmospheric pressure is called its absorption coefficient (α). For determining solubility of gas in mol L-1 at a certain temperature and one atomspheric pressure, its absorption coefficient is divided by 22.4.

Factors Affecting the Solubility of Gas :

The solubility of gas in a solvent depends on following factors:

→ Nature of the Gas : Gas which reacts with solvent or ionized in solution is very much soluble For example : NH3, HCl and SO2 are more soluble in water. They form NH4OH, HCl(l) and H2SO4 compounds by dissolving in water. Oxygen is more soluble in blood because it reacts from haemoglobin of blood. N2, O2 and H2 etc gases are relatively less soluble but more soluble in ethyl alcohol.

→ The solubility of gases also depends on absorption coefficient. Those gases which have more absorption coefficient also possess more solubility. The decreasing order of absorption coefficient of various gases in 1cm water is given as below:
NH3 > HCI > SO2 > H2S > CO2 > C2H2 > O2 > N2

→ Nature of the Solvent: It is seen in reference of effect of nature of solvent on solubility of gas that those gases which have polar molecules, are more soluble in polar solvents than non-polar solvents.
Example HCl gas is more soluble in water as compared to benzene.

→ Effect of Temperature : According to Le-Chatelier’s principle, at constant presure, the solubility of gases decreases with increase in temperature. Since the kinetic energy of molecules of gas in liquid increases with increase in temperature due to which the tendency of escaping gas increases. Example Heat is absorbed on dissolving H2 and He, so the solubility of these gases increase, if temperature increases.

→ Effect of Pressure-Henry’s Law: We know that the nature of gas is highly effected by pressure. The solubility of gases is also affected by pressure as other properties. First of all, William Henry (1803) studied the effect of pressure on solubility of various gases and he proposed a quantitative relation, which is called Henry’s law. According to this law, “At constant temperature, the amount of gas dissolved in unit volume of solvent is directly proportional to pressure created by gas in equilibrium at the surface of solvent”.
m α p
or m = KHP.
Where
m = Amount of Gas
P = Pressure of gas at equilibrium
KH = Henry constant

→ If we consider mole fraction of gas in solution as its solubility, then according to Henry’s law “The partial pressure (P) in vapour state of a gas is proportional to mole fraction (x) of a gas in that solution”.
P = KH.x
Where
KH = Henry constant
x = Mole fraction of Gas
P = Pressure of Gas

→ Let m g gas with molecular weight M is present at T temperature and the volume is V. A certain volume of gas is soluble in equlibrium pressure P then ideal gas equation will be

PV = n RT
or PV = $$\frac{m}{M}$$RT
then V = $$\frac{m}{P}$$.$$\frac{RT}{M}$$
At constant temperature, $$\frac{m}{P}$$and V are constant.

Applications of Henry’s Law :

There are various applications of Henry law in industries as well as helpful in understanding of some biological phenomenon. Its some improtant applications are given of below:

→ Bottles are closed at high pressure to increase the solubility of Co2 in soda water and cold drinks.

→ The divers can face more solubility of gases at high pressure during breathing in deep sea. The solubility of atmospheric gases taken by breathing is more in blood due to more external presure decreases slowly. Due to this, dissolved gases evolve outside and bubbles of nitrogen are formed in blood.

→ They create a therapautic condition which is called bends. It is very painful and fatal. To prevent from harmful effect of more nitrogen in blood and from bends, air diluted with helium is filled in tanks used for breathing by divers. (11.7% Helium, 56.2% Nitrogen and 32.1 % Dioxygen)

→ The partial pressure of oxygen at high altitudes is less than surface. So low concentration of oxygen is found in blood and tissues of peoples living in these places and tourists. Due to this. tourists become weak and do not think clearly. These characters are called anoxia.

Limitations of Henry’s Law :

There are following limitations of Henry’s law :

• Temperature should not be very low.
• The value of pressure should not be more.
• The solubility of gas should be low in a solvent.
• The molecular state of gas should be same in both liquid and gaseous states i.e, gas should neither associated nor dissociated.