Factors affecting solubility of Solids in liquid

Factors affecting solubility of Solids in liquid

The maximum amount of solute that can be dissolved by the solvent at a particular temperature is called its solubility. [The solubility is defined as the maximum amount of the solute in grams which can dissolves in 100 g of the solvent at a given temperature to form a saturated solution.]

When a solid solute is added continuously to a liquid solvent, the solute keeps on dissolving and the concentration of the solution keeps on increasing. This process is called dissolution. Ultimately, a stage is reached when no more solute dissolves at the given temperature. The solution at this stage is said to be saturated solution.

At this stage rate of dissolution becomes equal to rate of crystallization.

Crystallization is the process when soluble solute particles in solution collides on the surface of solid solute particles and get precipitated out.

Nature of the solute and the solvent

Every solid does not dissolve in a given liquid. In general, a solid dissolve in a liquid (solvent) if the intermolecular interactions are similar in solute and solvent. This is expressed by saying “Like dissolves like”. This means that, non-polar substances are more likely to be soluble in non-polar solvents while ionic (or polar) substances are more likely to be soluble in polar solvents.

Compounds like NaCl and sugar dissolve more readily in polar solvents (like water) but are very little soluble or almost insoluble in non-polar solvents (like benzene, CS2 , CCl4 , ether etc.). Similarly, non-polar compounds like naphthalene and anthracene are soluble in non-polar solvents (like benzene, ether, CCl4 , CS2 etc. ) but very little soluble in water.

[NOTE  :  The solubilities of NaCl and sugar in water are 5.3 and 3.8 moles per litre respectively.]

In case of ionic compounds, the ions are being attracted to the oppositely charged poles of the solvent molecules by electrostatic attraction. The ion thus moving freely in the solution are said to hydrated. During the hydration of an ion a significant amount of energy is released (called hydration enthalpy) which splits ionic compounds into ions. The energy required for splitting is called lattice enthalpy.

A substance dissolves if hydration enthalpy is greater than the lattice enthalpy.

Effect of Temperature

The solubility may increase or decrease or show irregular behaviour with increase in temperature. We observe following three behaviours:

The solubility of solids increases with increase in temperature

The solubility of most of substances such as NaNO3 , NH4Cl , KCl , AgNO3 , NaCl, KI and sugar etc. increases with rise in temperature. This is because the dissolution process for these substances is endothermic solH > 0). Therefore, according to Le-Chatelier’s principle, an increase in temperature will result in an increase in the solubility of a gas if the process is endothermic.  [or equilibrium will shift to a direction in which the heat is absorbed]

Solvent + Solute + Heat   ⇌   Solution

The solubility of solids decreases with increase in temperature

The solubility of some substances like Li2SO4 , Ce2(SO4)3 , Na2CO3 and CaO etc. decrease with rise in temperature. This is because dissolution process is exothermic (ΔsolH < 0).

The solubility shows irregular behavior with increase in temperature

The solubility of some substances (like Na2SO4) increases up to a certain temperature and then decreases as the temperature is further raised. The temperature at which solubility starts decrease is called transition temperature.

For example, the solubility curve of Na2SO4 shows a sharp break at 32.8 ˚C. This is due to change in one solid form into another solid form. Below transition temperature, only hydrated Na2SO4.10H2O exists while above this temperature, anhydrous Na2SO4 exists.

Effect of Pressure

The effect of pressure on the solubility of solids in liquid is very small or insignificant. This is because solids and liquids are incompressible and practically remain unaffected by changes in pressure. 

Example:  Change of 500 atm in pressure increase the solubility of NaCl in water only by 2.3 %. 

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