There are many substances which dissolve in water quite appreciably while some substances either dissolve to a smaller extent or do not dissolve at all. Similarly, when we mix the aqueous solutions of AgNO3 and KCl we get the formation of white precipitate of AgCl. But we do not get any precipitation on mixing aqueous solutions of AgNO3 and K2SO4.
How can we predict the extent of dissolution of a substance or formation of a precipitate on mixing aqueous solution of the two substances? The answers to these basic questions comes from solubility and the concept of solubility product. The main focus in this unit is to study solubility of substances, concept of solubility product and its applications to predict the precipitation of ions from their aqueous solutions.
In general, solubility of a substance is its maximum amount that can dissolve in a specified amount of solvent under given set of conditions. Solubility depends on nature of solute, and solvent as well as the conditions of temperature and pressure. Let us, study the effect of these factors on solutions of solids in liquids.
SOLUBILITY OF SOLIDS IN LIQUIDS
When a solid solute is added to solvent, dissolution of solute occurs. The solute particles go into solution and acquire random movement. The concentration of the solution gradually increases as more and more solute dissolves. The dissolved solute particles also collide with solid solute and some of them separate out of solution and get deposited there. This process is called crystallisation. Soon a stage is reached when the two opposite processes; dissolution and crystallisation occur at the same rate and a state of dynamic equilibrium is established.
At this stage, the concentration of solution becomes constant at the given conditions because the number of solute particles going into solution will be equal to number of solute particles separating out of solution. such a solution at which no more solute can be dissolved at u given conditions of temperature and pressure is called saturated solution.
• The solution which can dissolve more of solute at given temperature is called unsaturated solution.
• The amount of solute that can be dissolved in 100 g of the solvent at a given temperature is called its solubility.
• Another common way of expressing solubility of a substance at a given temperature is the maximum quantity of solute in moles (or gram) that can dissolve in a solvent to form 1 dm3 of solution.
• The concentration of solute, its ions or molecules in a saturated solution is constant at a given temperature.
• The units commonly used for expressing solubility of a substance are mol dm-3 or g dm-3•
These are empirical generalisations about the solubility of ionic compounds in water. These generalisation have been made from the experimental observations regarding the solubility of ionic compounds.
These rules are very useful in predicting the formation of precipitate particularly in qualitative analysis. Of course the more logical explanation about the precipitation comes from the concept of solubility product. The solubility rules about the ionic compounds have been summed up in table 28.1. They refer to solubility of ionic compounds in water at room temperature.
Utility of Solubility Rules
The usefulness of the rule is clear from the. Following example. If a solution of unknown salt forms precipitate with aqueous solution of hydrochloric acid (containing Cl- ions), then the precipitate could be any of the insoluble chloride i.e., Hg2CI2 or PbCI2or AgCl. The implies that the cation of the unknown salt must be any of Hg2+ or Pb2+ or Ag+ ions.