Solutions of solids in solids (solid solutions)
This type of solution is formed by mixing two solid components. For example, brass, bronze, monel metal and steel.
Solid solutions are classified into two categories: substitutional solid solutions and interstitial solid solutions.
l Substitutional solid solutions are formed by placing atoms of one kind into the place of other substance in its crystal lattice. Brass is a common example of the substitutional solid solution of copper and zinc.
l Interstitial solid solutions are formed by placing atoms of one kind into the voids in the lattice of atoms of other substance. Tungsten carbide, an extremely hard substance, is a common example of interstitial solid solutions.
Solubility of solids in liquids (Saturated, unsaturated, supersaturated solutions)
A solution which contains as much solute as it can be dissolved at a given temperature is called a saturated solution.
Solubility - What does it mean?
The amount of solute dissolved in 100g of a solvent to form a saturated solution at a given temperature is termed as the solubility of the solute in the given solvent at that temperature. Each substance has a characteristic solubility in a given solvent at a definite temperature.
When a solid solute is added to the solvent, the particles from the solid solute diffuse into it. The solute and solvent molecules move constantly in the solution phase. Some of the particles of the solute return to the solid state due to collisions. Thus, two opposite processes operate simultaneously:
l Dissolution: Particles of solute leaving the solid and dissolving in the solvent
l Recrystallisation: Solute particles returning to the solid form.
When these two processes move with same speed, an equilibrium stage is reached,
Solute (Solid) ↔ Solute (dissolved)
Thus, a dynamic equilibrium exists in a saturated solution.
A solution which contains lesser solute than the amount of solute it can dissolve is called a unsaturated solution.
A solution which contains more solute than the amount of solute it can dissolve at a particular temperature is called a supersaturated solution.
When a saturated solution prepared at a higher temperature is cooled, it gives a solution which contains usually more of solute than required for the saturated solution at that temperature. Such a solution is referred to as a supersaturated solution.
Note: A super saturated solution is metastable. A metastable (super saturated) state is obtained by suddenly cooling the saturated solution as the rate of crystallization is slow in comparison to fall in temperature.
On cooling an unsaturated solution some dissolved solid may begin to separate from the solution and the solution may become saturated. On heating a saturated solution, the undissolved solute dissolves and the solution becomes unsaturated.
The state of a solution wrt saturation is determined by dissolving some substance. If on adding this, one observes that
(a) The substance dissolves, then it is unsaturated
(b) The substance does not dissolve, then it is saturated
(c) The substance precipitates, then it is super saturated
Following are the factors affecting the solubility of a solid solute in a liquid:
1. Nature of the solute and solvent:
The basic concept of solubility is based on the fact ‘like dissolves like’. Polar solvents like H2O, NH3, Liquid HF, etc., dissolve polar and ionic compounds whereas, non-polar solvents like C6H6, CS2, CCl4, etc., dissolve non-polar compounds.
The solubility of ionic compound is usually explained in terms of dielectric constant of solvent, hydration energy and lattice energy of solute. Ionic solids dissolve to a larger extent in a solvent having high dielectric constant as compared to solvents of low dielectric constants. The ionic substances having high lattice energy like BaSO4 are less soluble while those having less lattice energy have more solubility. The ion are solvated by the solvent molecules and in this process energy (known as hydration energy) is released. When the hydration energy is high, the ionic solid is more soluble.
The solubility of non-polar compounds is due to similar solute-solute, solute-solvent, and solvent-solvent interactions. Many non-ionic substances dissolve in polar solvents due to hydrogen bonding.
The extent to which one component dissolves in another component differs a great deal, so much so that the two components may be completely miscible or immiscible. During the solution formation for components having molecules of near equal size, the entropy factor is always favourable due to greater disorder in the solution compared to that in the pure components. However, to a large extent, the solubility depends upon the strength of interaction between two different components. Thus, if EA-A and EB-B are the interaction energies between the molecules of A and B, respectively, and EA-B is the energy of interaction between molecules of A and B, then the magnitude of 2(EA-B) - (EA-A + EB-B) determines whether the two components are miscible, partially miscible, or completely immiscible.
2. Effect of pressure: Pressure does not have any marked effect on the solubility of solid in
liquids. It is so because solids and liquids are highly incompressible and practically remain unaffected by change in pressure.
3. Effect of temperature on solubility:
Various substances can be classified into three categories based on the effect of temperature on solubility in water.
(a) The substances whose solubility increases continuously with increase of temperature:
Most of the substances such as NaNO3, KNO3, NaCl, KCl, NH4Cl etc., fall in this category. The reason for this behavior is that in case of all such substances, the process of dissolution is endothermic.
On applying Le Chatelier’s principle, as the temperature is increased, equilibrium will shift in a direction in which the heat is absorbed, i.e., in the forward direction. Consequently, more of the solute passes into the solution.
(b) The substances whose solubility decreases continuously with increase of temperature:
Most of the substances such as CaO, CaCO3, CaSO4 etc., fall in this category. The reason for this behavior is that in case of all such substances, the process of dissolution is exothermic.
(c) The substance whose solubility does not increase or decrease continuously:
These are substances which on heating change at a particular temperature from one polymorphic form to another or from one hydrated form to another. The temperature at which one form of the substance changes into another is called transition temperature.
At transition temperature there is equilibrium between sodium sulphate decahydrate (Na2SO4.10H2O) and anhydrous sodium sulphate (Na2SO4). Above this temperature only anhydrous sodium sulphate exists while below this temperature hydrated form exists.
Note: 1. Dissolution of NaOH is exothermic but its solubility increases with temperature.
2. Hydrated solutes, e.g., CuSO4.5H2O, CaCl2.6H2O etc., usually show endothermic dissolution.
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