Water

Water of crystallisation — When a crystal is separated or deposited from a hot saturated solution on gentle cooling of the solution, it generally unites with a definite number of water molecules and enters into a loose chemical combination with them. This definite number of water molecules is called water of crystallisation.

The geometric shape and colour of the crystal is correlated with water of crystallization.

Blue vitriol [CuSO₄.5H₂O] is a pentahydrate.

Fused calcium chloride is deliquescent in nature, absorbs moisture and hence used as a desiccating agent.

(a) Differences between Hard and soft water:

(b) Differences between temporary and permanent hard water:

(a) CaCO₃ + H₂O + CO₂ ⟶ Ca(HCO₃)₂
MgCO₃ + H₂O + CO₂ ⟶ Mg(HCO₃)₂

(b) Ca(HCO₃)₂ $\xrightarrow{\text{boil}}$ CaCO₃ ↓ + H₂O + CO₂ [g]
Mg(HCO₃)₂ $\xrightarrow{\text{boil}}$ MgCO₃ ↓ + H₂O + CO₂ [g]

(c) $\underset{\text{ washing soda} }{\text{Na}_2\text{CO}_3} + \underset{\text{ calcium sulphate}}{\text{CaSO}_4} \longrightarrow \text{CaCO}_3 ↓ + \text{Na}_2\text{SO}_4$

Rain water is the purest form of natural water and river and lake water are impure form of natural water.

1. C: Solvent

2. A: Colloidal

3. E: Washing soda

4. B: Fused calcium chloride

5. D: Suspension

1. $\underset{\text{Washing soda} }{\text{Na}_2\text{CO}_3} + \underset{\text{ calcium sulphate}}{\text{CaSO}_4} \longrightarrow \text{CaCO}_3 ↓ + \text{Na}_2\text{SO}_4$

2. 3Fe + 4H₂O ⇌ Fe₃O₄ + 4H₂ ↑

3. SO₂ + H₂O ⟶ H₂SO₃

4. K₂O + H₂O ⟶ 2KOH

5. Mg(HCO₃)₂ $\xrightarrow{\text{boil}}$ MgCO₃ ↓ + H₂O + CO₂ [g]

1. Solubility of most solids increases with rise in temperature.

2. Kerosene & water form a immiscible mixture.

3. Solubility of a solute is the maximum amount of solute that will saturate 100 g. of water at t°C.

4. Hygroscopic substance absorb moisture from the atmosphere & do not change their original state.

5. The ratio of hydrogen & oxygen in water is 2:1.

1. As solute dissolves in the solvent and forms a uniform composition throughout hence, all solutions are homogenous mixtures of a solute in a solvent.

2. On boiling, the calcium and magnesium bicarbonates responsible for temporary hardness in water, decompose into insoluble carbonates and carbon dioxide which are filtered out making water soft.
   The chlorides and sulphates of calcium and magnesium responsible for permanent hardness in water, are stable to heat hence, permanent hardness cannot be removed by boiling.

3. Colloidal solutions exhibit Brownian movement due to the constant bombardment of molecules of the dispersion medium on the colloidal particles.

4. Ordinary air contains 21% oxygen. As oxygen is more soluble in water as compared to air. Hence, air in water contains 33% oxygen which is more than that in ordinary air.

5. On adding washing soda to water containing calcium & magnesium chlorides, sulphates & bicarbonates an insoluble precipitate of calcium & magnesium carbonate is formed which is filtered out. The remaining soln. contains soluble sodium salts which do not cause hardness in water & hence the residual water is rendered soft. Hence, washing soda can be used to remove both temporary and permanent hardness in water.

Potable water is water which is fit for human consumption & for drinking purposes.

Characteristics of potable water which make it fit for human consumption :

1. Potable water should be clear, colourless & odourless.
2. It should be free from harmful bacteria, germs, suspended impurities & harmful salts such as nitrates, nitrites, cyanides & urea.
3. It should contain small amounts of dissolved gases & minerals which give it a good taste. Dissolved air & salts of sodium, calcium, magnesium etc. impart taste to the water & are useful for metabolic functions of the human body.

Depending upon the reactivity of metals with water, they are arranged in a series called - 'activity series of metals' in a way such that the most reactive metal is at the top of the series, and the least reactive at the bottom.

(a) Potassium (K)

(b) Magnesium (Mg)

(c) Iron (Fe)

(a) As alcohol is soluble in water hence they form a miscible mixture whereas oil is not soluble in water hence they both form an immiscible mixture.

(b) As soluble gases in water contribute to the taste of water and on boiling, the temperature of the water increases, thereby the solubility of the dissolved gases decreases and the dissolved gases are expelled out. Hence, boiled water tastes flat.

(c) Soda bottles contains carbon dioxide dissolved in water under pressure. On opening the bottle, the pressure decreases hence solubility of dissolved carbon dioxide decreases & the gas escapes out with a 'fizz'.

(a) Dissolved minerals & salts in water — Provides minerals for living bodies for their growth & imparts taste to water.

(b) Dissolved air [O₂, CO₂, & NO₂] in water Solutions — like :

• Dissolved O₂ is useful for respiration in aquatic plants and animals.
• Dissolved CO₂ for photosynthesis
• Dissolved NO₂ is converted into nitrogenous compounds by bacteria and provides nourishment for plants.

(a) Proportion of solute & solvent — The solute must be in a small amount compared to the solvent. To dissolve the solute, the solvent must be in a much greater amount.

(b) Particle size of the solute — The solute must be finely powdered and not be present as large crystals. This initiates quicker & complete dissolution of the solute in the solvent.

(c) Temperature of the solvent — The solvent may be slightly heated since the solute may not be completely soluble in the solvent at ordinary temperatures.

Solubility of a solute in water is defined as the maximum amount of solute in grams that will saturate 100g. of water at t°C.

No, 10g is not the solubility of the solute as the solution is unsaturated. Solubility will be attained when the solution is saturated.

Solutions are generally classified as:

(a) True solutions

(b) Colloidal solutions

(c) Suspensions

The basis of above classification is size of the dissolved or dispersed particles in the medium.

Examples of true solution

Examples of colloidal solution

Examples of suspension

In a true solution, the particles are neither visible to the naked eye nor under an ultra microscope. Hence, they cannot reflect light and cannot show 'Tyndall effect'.

As more of solute can be dissolved at that temperature, it is an unsaturated solution. A saturated solution cannot dissolve more of the solute at that particular temperature.

A saturated solution is one which cannot dissolve more of the solute at a given temperature.

Two methods to convert a saturated solution to an unsaturated solution are:

1. By heating the saturated solution slowly it can dissolve more of the solute at a higher temperature.
2. By adding more solvent to the saturated solution. Increased amount of solvent can dissolve more of the solute at that temperature.

A saturated solution of potassium chloride is prepared in boiling water. If this solution is cooled excess potassium chloride separates out from the hot saturated solution. The hot solution therefore contains more of the solute dissolved in than it can hold at that given temperature and is thus called a supersaturated solution.

Water is called a 'universal solvent' because it has the ability to dissolve a wide variety of substances. On the other hand, an alkali is not a universal solvent because it can only dissolve certain types of substances, specifically acids and some other specific compounds.

Solute — potassium chloride

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

Solvent — water

The liquid or medium of dissolution which allows the solute to dissolve in it so as to form a solution is called a solvent.

Solution — potassium chloride solution.

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