Solubility

Solubility

Solubility is the ability of a substance, either a solid, liquid, or gas, to dissolve in a solvent to form a homogeneous mixture. The solubility of a substance in a particular solvent depends on several factors such as temperature, pressure, and the nature of the solvent and solute.

Solubility of a Solid in a Liquid

The solubility of a solid in a liquid is defined as the maximum amount of solute that can dissolve in a given amount of solvent at a particular temperature and pressure. The solubility of a solid in a liquid is affected by temperature, pressure, and the nature of the solvent and solute.

Effect of temperature on solubility

The solubility of most solids in liquids increases with an increase in temperature. This is because an increase in temperature provides more thermal energy to the solvent molecules, which increases their kinetic energy and helps to break down the intermolecular forces that hold the solute particles together. However, there are some exceptions, such as the solubility of calcium sulfate in water, which decreases with an increase in temperature.

Effect of pressure on solubility

The effect of pressure on the solubility of a solid in a liquid is generally small and can be neglected for most systems. However, for gases dissolved in liquids, the solubility increases with an increase in pressure. This is because an increase in pressure increases the partial pressure of the gas above the liquid, which provides a greater driving force for the gas to dissolve in the liquid.

Solubility of a Gas in a Liquid

The solubility of a gas in a liquid is defined as the maximum amount of gas that can dissolve in a given amount of liquid at a particular temperature and pressure. The solubility of a gas in a liquid is affected by temperature, pressure, and the nature of the solvent and gas.

The solubility of gases in liquids generally decreases with an increase in temperature. This is because an increase in temperature provides more thermal energy to the gas molecules, which increases their kinetic energy and helps them to escape from the liquid. Additionally, the solubility of gases in liquids increases with an increase in pressure. This is because an increase in pressure increases the concentration of gas molecules above the liquid, which provides a greater driving force for the gas to dissolve in the liquid.

Henry’s law

·         Henry’s law states that “the partial pressure of the gas in vapour phase (p) is proportional to the mole fraction of the gas (x) in the solution” and is expressed as-

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·         Here KH is the Henry’s law constant.

·         This law is applicable to ideal gases only.

·         This law is not applicable to gases which react chemically with the solvent.

Applications-

·         To increase the solubility of CO2 in soft drinks and soda water, the bottle is sealed under high pressure.

·         Scuba divers must cope with high concentrations of dissolved gases while breathing air at high pressure underwater. Increased pressure increases the solubility of atmospheric gases in blood. When the divers come towards surface, the pressure gradually decreases. This releases the dissolved gases and leads to the formation of bubbles of nitrogen in the blood. This blocks capillaries and creates a medical condition known as bends, which are painful and dangerous to life.

·         To avoid bends, as well as, the toxic effects of high concentrations of nitrogen in the blood, the tanks used by scuba divers are filled with air diluted with helium (11.7% helium, 56.2% nitrogen and 32.1% oxygen).

·         At high altitudes the partial pressure of oxygen is less than that at the ground level. This leads to low concentrations of oxygen in the blood and tissues of people living at high altitudes or climbers. Low blood oxygen causes climbers to become weak and unable to think clearly, symptoms of a condition known as anoxia.

·         Solubility increases with decrease of temperature. It is due to this reason that aquatic species are more comfortable in cold waters rather than in warm waters.

Effect of Temperature

The effect of temperature on Henry's law can be explained by the fact that the solubility of gases generally decreases with increasing temperature. This is because the kinetic energy of the gas molecules increases with temperature, making them less likely to be captured by the liquid molecules. As a result, the value of kH decreases with increasing temperature.