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Solubility is the property of a solute(solid, liquid, or gas) to dissolve in a solvent(solid, liquid, or gas) and form a solution. When a solute dissolves in a solvent, the mixture obtained is a solution, which is homogeneous, and exists in a single phase, i.e. as a gas, liquid, or solid Air, sweat, and steel are gaseous, liquid, and solid solutions respectively. A solution is a homogeneous mixture of two or more substances.

Saturated and Unsaturated Solutions

An Unsaturated solution is a solution that will dissolve more of the solute at a given temperature while a saturated solution is one that will not dissolve more of the solute in the presence of excess solute, at a given temperature.

In a saturated solution, there exists a dynamic equilibrium between the solution and the undissolved solute. At saturation point, the quantity of the excess solute remains constant; hence the concentration of the solution is constant. Therefore, a solution is saturated when the solute is in dynamic equilibrium with the solution at a particular temperature.

Factors That Influence Solubility Of Solids In Liquids

There are four factors that influence or affect the solubility of solids in liquids and some of the common ones include the following:

  • Nature(Polarity) Of The Solute: Generally, Ionic compounds are soluble in polar solvents, like water, but insoluble in non-polar solvents like petrol; while covalent compounds are soluble in non-polar solvents like petrol, but insoluble in polar solvents like water.
  • Nature(Polarity) Of The Solvent: Generally, a polar solvent will dissolve an ionic compound, while a non-polar solvent will dissolve a covalent compound.
  • Temperature Changes: There is no general rule to describe the variation of solubility of solids with a change in temperature. Reason: When a solid dissolves in a liquid, heat may be given out(Exothermic) or absorbed(endothermic)., or there may be no significant temperature change.
  • Common Ion Effect: The solubility of an ionic compound in water is not affected by the presence of another compound already in the solution, except if both compounds have the same cations or anions. For instance, The solubility of NaCl in water is the same as the solubility in a dilute sugar solution, or in dilute H2SO4. This is because there is no cation or anion common to NaCl and H2SO4

There are ways to increase the dissolution rate of a solid in a liquid and some of the common ways include the following:

  • By stirring the mixture
  • By grinding the solid into powder
  • By heating the mixture

Effects Of Solute On The Solvent In A Solution- Colligative Properties

When a solid dissolves in a liquid to form a solution, the liquid particles become heavier; hence, its density increases, i.e a solution is denser than its pure solvent. The following physical properties of the pure liquid are affected:

  • Lowering of Vapour Pressure: The rate of evaporation of the liquid particles is decreased. Hence the vapour pressure of a solution is less than that of its pure solvent at a given temperature.
  • Elevation of boiling Point: A solution would boil at a higher temperature than the pure solvent. This is because more heat energy must be absorbed by a solution in order to overcome the attractive forces between the liquid and the solid particles.
  • Lowering of the freezing point: A solution freezes at a lower temperature than the pure solvent. This is because more heat energy must be released in order to freeze a solution.

Solubility Curve

The Solubility curve of salt represents the variation of its solubility with temperature. It is obtained by determining the solubility of a salt in a solvent, water, at various temperatures. A solubility curve is applied to separate and purify mixtures of solids, to identify the most suitable solvents for the extraction of solutes from natural solvents, and to determine the amounts of solid drugs in a solution or mixture of drugs.

Concentrating A Solution & Supersaturated Solutions

There are two physical methods that can be used to increase the concentration of a given solution at a particular temperature and they include: By increasing the quantity of solute in the solution, and partial evaporation of the solvent, such that the solution is still unsaturated at the given temperature.

A solution is said to be supersaturated if it contains more solute than it can dissolve at a particular temperature in the presence of the undissolved solute. Three methods to crystallize a supersaturated solution include: seeding, swirling, and scratching.

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