SS2 Chemistry Lesson Note on Types of Solution

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TYPES OF SOLUTION

A solution is a homogenous mixture which is formed when a solute is completely dissolved in a solvent. A solution can be saturated, unsaturated or supersaturated.

1. A saturated solution is one which contains, at a given temperature, as much solute as it can hold in the presence of the dissolving substance. That is, a given solute dissolves in a given solvent, such that no more of the solute will dissolve in the solvent at the temperature of dissolution.
2. An unsaturated solution is one which contains less solute than it can hold at the given temperature.
3. A supersaturated solution is one which contains more of the dissolves substance than it can hold at a given temperature, when the saturated solution is in contact with the solid solution. Super saturated solution are unstable. The excess solute can be separated out by slightly shaking the solution or creating centres of crystallization for the excess solute.

Relationship Between solubility and crystallization

The different solubilities of substances are utilized in their purification by the process of crystallization. In manufacturing KNO₃, solutions of KCl and NaNO₃ are mixed. The resulting solution is then concentrated at boiling point and the NaCl is then deposited. The equilibrium in the equation is displaced to the right.

K⁺ + Cl^– +  Na⁺ +  NO₃^–      →        K⁺ +  NO₃^– + NaCl

The solution is filtered hot and later cooled. KNO₃, with the lowest solubility, crystallizes out first and is purified by recrystallization :

KCl  +  NaNO₃    ⇌   KNO₃   +   Na⁺ + Cl^–

A saturated solution is in dynamic equilibrium with the excess solid present in the solution. The dissolution equilibrium can be expressed in terms of an equilibrium constant, e.g. in the case of silver chloride

AgCl_(s)     ⇌     Ag⁺_(aq)  + Cl^–_(aq)

EVALUATION

1. Define the term saturated solution.
2. Distinguish between saturated and unsaturated solutions.

PERIOD 4.  FACTORS AFFECTING SOLUBILITY OF SOLIDS IN LIQUIDS

 The four factors that influence or affect the solubility of substances are:

1. Nature of solvent: Generally, ionic compounds are more soluble in polar solvent (e.g. water), than in non- polar increases the solubility of the solvent e.g. NaCl is more soluble in water than in ethanol.
2. Nature of the solute: Ionic compounds are more soluble than covalent compounds and vice-versa. For instance, NaCl is more soluble in water than sugar at room temperature.
3. Temperature: In most cases solubility increases with increase in temperature. In endothermic reactions solubility increases with increasing temperature and vice versa e.g. calcium oxide.
4. Common ion effect: The solubility of an ionic compound in water is affected by the presence of another compound if both compounds have the same cation or anions. For example, the solubility of NaCl in water is much higher than its solubility in dilute HCl because there is a common chloride in NaCl and HCl.

Uses of Solubility curves

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