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Solids in Liquids Solubility

In general, solubility of a substance is its maximum amount that can in 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. We shall now, study the effect of these factors on solutions of solids, and gases 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 state and some of them separate out of solution and get deposited these. This process is called crystallization. Soon a stage is reached when the two opposite processes; dissolution and crystallization occur at the same rate and a state of dynamic equilibrium is established. Solid solute + Solvent <=> Solution

At this state, the concentration of solution becomes constant at the given conditions because the number of solute particles going into solution will be equal to the number of solute particles separating out of solution. Such a solution at which no more solute can be dissolved at a given conditions of temperature and pressure is called saturated solution, and the amount of solute dissolved in a given amount of solvent is called solubility. In general, solubility of solid is expressed as the amount of solid that can be dissolved in 100 g of the solvent at a given temperature. In fact, solubility refers to concentration of solute on a saturated solution at a given temperature. Solubility of a solute in liquid depends upon the following factors.

(i) Nature of solute: dissolution of solid solutes in liquids can be summed up in a phrase “like dissolves like”. This means that, polar solutes dissolve in polar solvents and non-polar solvents dissolve in non-polar solvents. For example, ionic substances such as NaCl, KCl, and KNO3 etc. have larger solubilities in polar solvents like water but they have poor solubilities in solvents like CCl, CS2 etc. on the other hand non-polar solids such as I2, S2 etc are more soluble in non-polar solvents like CCl4, CS2 but they are almost insoluble in polar solvents like water.

(ii) Temperature: saturated solution represents equilibrium between undissolved solute and dissolved solute.

Undissolved solute + solvent solution; ΔsolH = ±x.

If the value of ΔsolH < 0; i.e the solution is exothermic, then by Le-chatelier’s principle the solubility of such solute

will decrease with the rise in temperature. On the other hand, if ΔsolH > 0, i.e solution process is endothermic, then, solubility of such a solute will increase with rise in temperature.