Revision: Water Chemistry (English Medium) ICSE Class 9 CISCE

Eutrophication: “Organic matter in sewage poured into water bodies generally results in excessive growth of algae – which deoxygenates water and produces deadening atmosphere there.”

The temperature at which a liquid turns into solid when cooled is known as the freezing point. The freezing point of water is 0°C.

The temperature at which water boils and changes to steam is called as boiling point. The boiling point of water is 100°C at atmospheric pressure.

The substance which dissolves or disappears in the solvent, i.e., liquid, to form a solution is called a solute. E.g., sodium chloride.

A solute is a substance that dissolves in a medium, which can be water or any other substance. A solute is in a smaller quantity in a solution. e.g., sodium chloride.

A solvent is a medium in which a solute dissolves. It is in large quantity in a solution. Water is the most common solvent. The other solvents are alcohol, carbon tetrachloride, etc.

Example: Water

A homogenous mixture of a solute in a solvent is called a solution.

Two or more solutions exerting the same osmotic pressure are called isotonic solutions.

When two solutions are separated by a semipermeable membrane and no osmosis occurs, i.e., there is no net flow of water on either side through the membrane, the solutions are said to be isotonic solutions. If the membrane is perfectly semipermeable, the two solutions possess the same osmotic pressure and are also referred to as iso-osmotic solutions.

It is characterised as a solution that adheres to Raoult's Law, with no interactions between the molecules and no volume or heat change during mixing.

For an ideal solution, Enthalpy of mixing of the pure components to form the solution is Δ_mix H = 0 and the volume of mixing is Δ_mix V = 0.

• Unsaturated solution: If the amount of solute contained in a solution is less than the saturation level, it is called an unsaturated solution. (till it is dissolving).

• Saturated solution: When no more solute can be dissolved in a solution at a given temperature, it is called a saturated solution.

• Solubility: The amount of the solute present in the saturated solution at this temperature is called its solubility.

Water of crystallization refers to “the fixed number of water molecules which enter into a loose chemical combination with the substance when the substance is crystallized from its hot saturated solution, is called water of crystallization.” 

Crystallisation is a process by which crystals of a substance are obtained by cooling a hot saturated solution.

Water obtained from various sources does not easily form a lather with soap, rather it forms a white sticky scum or a precipitate. This water is called hard water.

The water present in different natural sources has different substances dissolved in it. The water drawn from certain sources forms a lather with soap rather easily. Such water is called soft water.

It is the phenomenon by which substances absorb moisture from air, but only sufficiently so as to become wet.

It is the fixed amount of water that is present in a crystal as an integral part of its constitution. Hydrated salts are salts having water of crystallisation.

Salt is a compound formed by the partial or total replacement of the ionisable hydrogen atoms of an acid by a metallic ion or an ammonium ion.

It is the phenomenon by which hydrated salts, on exposure to dry air, lose their water of crystallization and crumble to powder.

Henry’s law states that the solubility of a gas in a liquid is directly proportional to the pressure of the gas over the solution.