The document discusses how the solubility of a substance is defined as the amount of solute that dissolves in 100g of solvent to form a saturated solution at a given temperature. It then explains that the energy of solution formation, or enthalpy of solution, equals the sum of the energy required to separate solvent and solute molecules, plus the energy released when they interact in solution. Solutions will only form when the energy of interaction between solvent and solute is greater than the sum of their individual interactions. The document also discusses how several factors, including temperature, molecular structure, particle size, and added substances can affect a substance's solubility.

1. “Solubility of a substance may be defined as the
amount of solute dissolved in 100gms of a solvent to
form a saturated solution at a given temperature”.

2. The Solution Formation Process
 Therefore, the energy of solution formation, the enthalpy of solution, equals
the sum of the three steps:
 ΔHsoln = ΔH1 + ΔH2 + ΔH3.
 ΔH1 and ΔH2 are both positive because it requires energy to pull molecules
away from each other. When the expanded form of the solvent and the solute
are combined to form a solution, energy is released, causing ΔH3 to be
negative. This makes sense because the solute and solvent can interact through
the various types of intermolecular forces.
 solutions will form only when the energy of interaction between the solvent and
solute is greater than the sum of the solvent-solvent and solute-solute
interactions.

3. Factors Affecting solubility
Crystal
Characteristics
Molecular
structure of
Solute
Effect of
Temperature
on Solubility
Effect of Added
Substances on
Solubility:
Particle size of
Solid

4. The solubility of solutes is dependent on temperature.
CASE I: Decrease in solubility with
temperature:
CASE II: Increase in solubility with
temperature:
 If the heat given off in the dissolving process is
greater than the heat required to break apart
the solid, the net dissolving reaction is
exothermic ΔH1negative value (energy given
off). The addition of more heat (increases
temperature) inhibits the dissolving reaction
since excess heat is already being produced by
the reaction.
 If the heat given off in the dissolving reaction
is less than the heat required to break apart
the solid, the net dissolving reaction is
endothermic ΔH1positive value (energy
required). The addition of more heat
facilitates the dissolving reaction by providing
energy to break bonds in the solid. This is the
most common situation where an increase in
temperature produces an increase in solubility
for solids.

5. The solubility of solutes is dependent on
temperature.
 The solubility of a solute in a solvent is dependent on temperature, nature of
solute and nature of solvent.
 Heat of solution ΔH represents the heat released or absorbed when a mole of
solute is dissolved in a large quantity of solvent.
 Most of the substances are endothermic, absorbing heat in the process of
dissolution. For this substances, an increase in temperature results in an
increase in solubility.
 Exothermic substances give off heat in the process of dissolution. The solubility
of such substances would decrease with increase in temperature.

6. The solubility of solutes is dependent on
temperature.
 Why does solubility change with temperature? Consider a beaker that contains
a saturated solution of table sugar. The bottom of the beaker is covered with
sugar crystals. When a tiny amount of sugar dissolves, heat is absorbed. When
a tiny amount of sugar crystallizes out of solution, heat is released. We can
write:
 heat + solid sugar + water = dissolved sugar
 The equation represents two processes: dissolution going left to right, and
crystallization going right to left. When the sugar crystals are dissolving at
exactly the same rate that sugar is crystallizing out of solution, the system is at
equilibrium. The balance between dissolution and crystallization
can be changed by changing the temperature of the solution.
Adding heat will favor dissolution. Cooling the solution will
favor crystallization.