Solubility Of Gases In Liquids | Notes for NCERT Chemistry Class 12
Solubility Of Gases In Liquids – Gases are defined as the substances that can expand freely and fills the space. It is one of the four fundamental states of matter (others are solid, liquid, and plasma). Gases do not have a fixed shape. A pure gas comprises of individual atoms (e.g. a noble gas like neon), elemental molecules i.e.having one type of atom (e.g. oxygen), or compound molecules made from a variety of atoms (e.g. carbon dioxide). A gas mixture contains a variety of pure gases much like the air.
Gas does not have any particular shape. Molecules in the gases are attached together by weak electrostatic forces or van der Waals forces. Gas molecules are very loosely packed as compared to solids and liquids. This means that molecules in gases are very far apart from each other, and are attached together by weak electrostatic forces or van der Waals forces.
SOLUBILITY OF GASES
The solubility is the maximum quantity of solute that can dissolve in a certain quantity of solvent or quantity of solution at a specified temperature or pressure (in the case of gaseous solutes).
Factors affecting the solubility of gases in liquids
- Nature of gas and the nature of solvent: The gases which are easily liquefiable are more soluble than dihydrogen and dioxygen. The gases which are capable of undergoing a chemical reaction with water are relatively more soluble in water than any other solvents. Some polar gases are ammonia (NH3), Hydrogen Sulphide (H2S) etc. Whereas, non-polar gases are diatomic gases like O2, H2, N2, CO2 etc.
Effect of Nature of Gas on Solubility
(i) Polar gases are more soluble in polar solvent like water.
(ii) Non-polar gases like O2 , have less solubility in polar solvent.
- Effect of Temperature: In the case of gases, solubility decreases as temperature increases. The reason for this is that when most gases dissolve in solution, the process is exothermic (accompanied by the release of heat). Increased temperature causes an increase in kinetic energy. The higher kinetic energy causes more motion in the gas molecules which break intermolecular bonds and re-enter the gas phase. Thus as the temperature increases, the solubility of a gas decreases.
Figures showing the effect of temperature on the solubility of gases
- Effect of Pressure: Liquids and solids do not exhibit a change in solubility with changes in pressure. Gases show a significant increase in solubility with an increase in pressure. Suppose, there is a piston tank with a gas above a saturated aqueous solution of the gas. At equilibrium, at a given pressure, the same number of gas molecules enters and leave the solution per unit time. If the piston is pushed down, this equilibrium is disturbed. The gas volume is decreased above the aqueous solution, but the gas pressure is increased. Gas particles collide the surface of the solution more often. Thus, more particles enter the solution per unit time. Now more gas dissolves to reduce this disturbance until the system re-establishes equilibrium.
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Henry’s Law states that the solubility (Sgas) of a gas in a liquid is directly proportional to the partial pressure (Pgas) of that gas above the surface of the solution. Thus, it expresses the quantitative relationship between gas pressure and solubility.
Sgas = kH X Pgas
kH is Henry’s law constant. It is specific for a given gas- solvent combination at a given temperature. Sgas is expressed in mol/L and Pgas in atm, the units of kH are mol/L/atm.
Graph showing the relationship between solubility (in mole fraction) and pressure of gases.
Application of Henery’s Law
(i) To increase the solubility of CO2 in soda water and soft drinks, the bottle is sealed under high pressure.
(ii) The tanks used by scuba divers are filled with air diluted with helium, to avoid the toxic effects of high concentration of nitrogen in the blood.
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