CBSE Class 10 Chemistry Solubility and Electrolysis Notes Set 19

Solubility

The solubility of a substance is the number of grams of it that saturate 100 g of the solvent at a given temperature. A saturated solution holds as much solute as it can at that temperature; an unsaturated solution can still dissolve more; a supersaturated solution holds more than normal and crystallises on seeding.
1. Most solids dissolve more with rise in temperature (KNO₃ strongly, NaCl slightly). Ca(OH)₂ and some salts are less soluble when hot.
2. The solubility of gases decreases with rise in temperature and increases with pressure (Henry's law).
3. Like dissolves like: ionic compounds dissolve in water, non-polar substances in organic solvents.

Expressing Concentration

1. Molarity M = moles of solute per litre of solution.
2. Molality m = moles of solute per kg of solvent; it does not change with temperature.
3. Percentage by mass = (mass of solute / mass of solution) × 100.
Example: 40 g of NaCl (58.5 g/mol) in 2 L of solution is 0.684 mol / 2 L = 0.342 M.

Electrolytes

Electrolytes conduct electricity in the molten state or in solution because they give free ions. Strong electrolytes (HCl, NaOH, NaCl) are almost fully ionised; weak electrolytes (acetic acid, ammonium hydroxide) are only partly ionised. Non-electrolytes (sugar, urea, alcohol, benzene) give no ions. Dilution increases the degree of ionisation.

Electrolysis

Electrolysis is the chemical decomposition of an electrolyte by an electric current. Positive ions (cations) move to the cathode and gain electrons; negative ions (anions) move to the anode and lose electrons.
1. Fused NaCl: sodium at the cathode, chlorine at the anode.
2. Concentrated aqueous NaCl: hydrogen at the cathode, chlorine at the anode, NaOH left in solution.
3. Dilute H₂SO₄ or aqueous NaOH: hydrogen at the cathode and oxygen at the anode in the ratio 2 : 1 by volume (water is decomposed).
4. Copper sulphate with platinum electrodes: copper at the cathode and oxygen at the anode; with copper electrodes the anode dissolves and copper is deposited on the cathode (refining and electroplating).