Revision Notes for Water Class 9 Chemistry ICSE

ICSE Revision Notes for Water Class 9 Chemistry

Chapter Name	Water
Topics Covered	Chemical Composition of Water Physical Properties of Water Chemical Properties of water Activity Series of Metals Solutions Common Solvents Concentration of a Solution Solubility Crystals and Crystallisation Water Pollution
Related Study	Selina Chemistry ICSE Solutions for Class 9 Frank ICSE Solutions for Class 9 Chemistry

Chemical Composition of Water 

• The molecular formula of water is H₂O.
• Mass ratio of elements: H₂O
  H:O, 2 × 1:16 × 1 = 1:8
  (Atomic mass of H = 1; atomic mass of O = 16) 
• Molecular formula of the water molecule is H₂O. 
• Chemical name of water is dihydrogen oxide. 
• Molecular mass of water
  = 2 (atomic mass of H) + 1 (atomic mass of O)
  = 2 × (1) + 1 (16)
  = 2 + 16
  = 18 amu 
• Hydrogen to oxygen to hydrogen bond angle in the water molecule is 104.5°. 

Physical Properties of Water

Nature	Pure water is a colourless, transparent, clear liquid at room temperature. It is colourless and tasteless.
Boiling point	Pure water boils at 100 °C at normal pressure.
Freezing point	Pure water freezes at 0°C at normal pressure (1 atm).
Effect of pressure	If the pressure is increased, then the boiling point increases. If the pressure is decreased, then the boiling point decreases.
Density	Maximum 1 g/cm3 or 1000 kg/m3 at 4°C.
Anomalous expansion of water	When water is cooled, it first contracts in volume just like other liquids upto 4°C. On further cooling, it expands instead of contracting. This expansion takes place upto 0°C. Thus, at 0°C, water has maximum volume and minimum density. At 0°C, it becomes ice and has a density of 0.92 g/cm3 and floats on water.
Conductivity	Pure water is a non-conductor of electricity because it does not form ions. Water can be decomposed by the passing of electric current. This process is called electrolysis.
Solvent properties	Water is a universal solvent and can dissolve many substances as compared to other solvents because of its polar covalent nature. Water has a high dielectric constant of 80.10 at 20°C. Because of this large value of the dielectric constant, water can dissolve a large number of ionic compounds.
Latent heat of fusion of ice	The amount of heat energy required by ice to change into water is called the latent heat of fusion of ice. The latent heat of fusion of ice is 336 J/g or 80 cal/g. In the reverse process, 336 joules of heat is released when 1 g of water solidifies to form 1 g of ice at 0°C.
Specific heat capacity	The specific heat or specific heat capacity of a substance is the amount of heat required to raise the temperature of a unit mass of that substance through 1°C. The specific heat of water is 4.2 joules or 1 calorie.

Chemical Properties of water

Nature	Water in its pure form is neutral to litmus. There is no change in the colour of blue or red litmus solution.
Stability	Water is a stable compound, i.e., it does not decompose on heating. However, at very high temperature, between 2000°C and 3500°C, it decomposes slightly to form hydrogen gas and oxygen gas.
Catalytic activity	Water acts as a catalyst in the synthesis f hydrogen chloride in the presence of moisture. Combustion of white phosphorous to phosphorous pentoxide takes place in the presence of moisture.
Reaction with non-metals	Reaction with carbon When steam is passed over red hot coke, water gas is formed. Water gas is an important industrial fuel.  Reaction with chlorine When chlorine gas is bubbled through water in the presence of diffused sunlight, hydrochloric acid and hypochlorous acid are formed.
Reaction with metallic oxides	Metallic oxides dissolve in water to form corresponding alkalis. Reaction with sodium oxide
	Reaction with potassium oxide
	Reaction with calcium oxide
Reaction with non-metallic oxides	Non-metallic oxides dissolve water to form corresponding acidic solutions. Reaction with carbon dioxide
	Reaction with sulphur dioxide
	Reaction with sulphur trioxide
	Reaction with nitrogen dioxide
Reaction with carbides	Reaction with aluminium carbide
	Reaction with calcium carbide
Reaction with metal nitrides	Boiling water reacts with metal nitrides to form their respective hydroxides and ammonia gas. Reaction with calcium nitride
	Reaction with magnesium nitride
	Reaction with alumimium nitride
Noble metals	Noble metals, such as silver, gold and platinum, are virtually inactive to water.

Activity Series of Metals 

• The series of metals arranged in the decreasing order of their reactivity is called an activity or reactivity series. 
• Hydrogen is a non-metal. It has been included in this series because it can form a positive ion. It would occupy the position based on its formation of a positive ion. 
• Metals above hydrogen may displace hydrogen from water and dilute acids, but the metals below hydrogen cannot displace hydrogen.
  

Solutions 

• Solution: A homogeneous mixture of two or more substances, the components of which cannot be seen separately.
  Solute: A substance which dissolves in a solvent to form a solution.
  Solvent: A medium in which the solute dissolves.
  Solution = Solute + Solvent
• True solution: A homogeneous mixture of two or more substances, the composition of which is not fixed and may be varied within certain limits. 
• Dilute solution: A solution in which the amount of solute is relatively small as compared to the amount of solvent. 
• Concentrated solution: A solution in which the amount of solute is relatively large as compared to the amount of solvent. 
• Saturated solution: A solution which cannot dissolve any more quantity of solute in a given amount of solvent at a given temperature. 
• Unsaturated solution: A solution which can dissolve more of the solute in a given amount of solvent at a given temperature. 
• Supersaturated solution: A solution which contains more of the solute than what is present in its saturated solution at a particular temperature.
• Aqueous solution: A solution in which water has been used as a solvent.
  Example: Solution of common salt or sugar in water.
• Non-aqueous solution: A solution in which the solvent used is other than water.
  Example: Sulphur dissolved in carbon disulphide
  Non-aqueous solvents are alcohol, benzene, ether and acetone. 

Common Solvents 

Solvents	Solutes dissolved
Turpentine	Paint, paraffin wax
Carbon disulphide	Sulphur and phosphorous
Acetone	Nail polish
Methylated spirit	Chlorophyll
Petrol	Grease
Oxalic acid	Rust
Benzene	Rubber

Concentration of a Solution 

Concentration of a solution is the amount of solute dissolved in a given quantity of solution.

Mass Percent: The mass of a solid solute in gram present in 100 gram of solution.

Concentration of solution = (Mass of solute)/{Mass of solution(Solute + Solvent)} × 100

Volume Percent: The volume of a solute in millilitre present in 100 millilitre solution.

Concentration of solution (in terms of volume %) = (Volume of solute)/(Volume of solute + Volume of solvent) × 100

Solubility 

• Solubility of a solute in a particular solvent at a particular temperature is the maximum amount of a solute in gram which can be dissolved in 100 gram of a solvent at that temperature.
• Determination of the solubility of a solute at a particular temperature can be calculated by
• Solubility = (Mass of solute)/(Mass of solvent) × 100
  = (M₂ – M)/(M₁ – M) – (M₂ – M) × 100

Solubility Curve 

A solubility curve is a line graph which shows changes in the solubility of a solute in a given solvent with a change in temperature. 

Inferences from Solubility Curves

• Decrease in solubility of substances with rise in temperature.
  Example: Calcium sulphate (CaSO₄)
• Increase in solubility of substances with rise in temperature.
  Examples: Sodium nitrate (NaNO₃), potassium nitrate (KNO₃), potassium bromide (KBr₃) 
• Slight increase in solubility with increase in temperature.
  Example: Sodium chloride (NaCl)
• Anomalous solubility.
  Example: Sodium sulphate (Na₂SO₄.10H₂O) 

Crystals and Crystallisation 

• Crystal: A crystal is a homogeneous solid, arranged symmetrically, meeting at sharp edges at definite angles to one another and having a definite geometrical shape. 
• Crystallisation: A process by which the crystals are obtained from a hot saturated solution by cooling.
• Water of crystallisation: The fixed amount of water which is associated with crystals and which form an integral part of the crystal is called water of crystallisation. 
• Decrepitation: The heating of some crystals which produce a crackling sound is called decrepitation. Example: Sodium chloride crystals 
• Hydrated salt: A salt which contains a fixed number of water molecules, as water of crystallisation, with loose chemical bond is called a hydrated salt. 
• Anhydrous salt: A salt which does not contain any fixed number of water molecules, as water of crystallisation, with loose chemical bond is called an anhydrous salt. 
• Efflorescence: Crystalline hydrated salts which on exposure to the atmosphere lose their moisture (water of crystallisation) partly or completely to the atmosphere and change into the amorphous state.
  Examples: Washing soda (Na₂CO₃.10H₂O), Glauber salt (Na₂SO₄.10H₂O)
• Deliquescence: Water-soluble salts absorb moisture from the atmosphere and dissolve in it to form a saturated solution. The substance is called a deliquescent substance and the phenomenon is called deliquescence.
  Examples: Caustic soda (NaOH), caustic potash (KOH), magnesium chloride (MgCl₂), zinc chloride (ZnCl₂), ferric chloride (FeCl₂) 
• Hygroscopy: When a substance can absorb moisture from the air without changing its state (solid/liquid), the substance is called hygroscopic, and the phenomenon is known as hygroscopy.
  Examples: Copper oxide (CuO), calcium oxide (CaO), copper sulphate (CuSO₄), concentrated sulphuric acid (H₂SO₄) 
• Desiccants: Substances which can readily absorb or remove moisture from other substances are called desiccants. Most of the hygroscopic substances are desiccants (drying agents).
  Examples: Fused calcium chloride (CaCl₂), fused phosphorus pentoxide (P₂O₅), anhydrous calcium chloride CaCl₂), quick lime (CaO), concentrated sulphuric acid (H₂SO₄) 

Water Pollution 

Pollution is an undesirable change in the natural environment brought about by physical, chemical and biological factors in the atmosphere, water or land. 

Pollutants are physical, chemical and biological agents or foreign substances introduced into the environment in quantities which have an undesirable effect on human health and environment. 

Water pollution is defined as an undesirable change in the physical, chemical and biological conditions of water due the presence of foreign substances in water. 

Causes of Water Pollution 

• Household detergents
• Industrial waste 
• Domestic sewage 
• Offshore oil drilling
• Agricultural wastes
• Thermal pollution

Treatment of Water Pollution 

• Collection and disposal of domestic sewage, mainly sewage and municipal garbage.
• Treatment of industrial waste to yield safe effluents.