CDS EXAM
Study Material For Chemistry
CHEMICAL REACTIONS AND EQUATIONS
● A complete chemical equation represents the reactants, products and their physical state
symbolically.
● Following observations helps us to determine whether a chemical reaction has taken place:
o Change in state
o Change in colour
o Evolution of a gas
o Change in temperature.
● Some of the examples of chemical reactions in everyday life:
o Photosynthesis
o Aerobic Cellular Respiration
o Combustion of wood
o Rusting of iron
o Metathesis
o Digestion
o Cooking an egg
o Souring of milk
o Rotting bananas
● Exothermic reactions are reactions or processes that release energy, usually in the form of
heat or light.
● Reactions in which energy is absorbed are known as endothermic reactions.
Balanced Chemical Equations
● Mass can neither be created nor destroyed in a chemical reaction. That is, the total mass of
the elements present in the products of a chemical reaction has to be equal to the total
mass of the elements present in the reactants.
● The number of atoms of each element remains the same, before and after a chemical
reaction. Some of the examples of balanced equations:
Zn + H2SO4 → ZnSO4 + H2
3Fe + 4H2O → Fe3O4 + 4H2
Types Of Chemical Reactions
Combination Reaction
● In a combination reaction two or more substances combine to form a new single Substance.
Example of combination reaction:
Calcium oxide reacts vigorously with water to produce slaked lime (calcium hydroxide)
Releasing a large amount of heat
CaO (s) + H2O (l) → Ca (OH)2 + Heat
● A solution of slaked lime produced by the above reaction is used for white washing wall.
NOTE: Calcium hydroxide reacts slowly with the carbon dioxide in air to form a thin layer of calcium
carbonate on the walls. Calcium carbonate is formed after two to three days of whitewashing and
gives a shiny finish to the walls. It is interesting to note that the chemical formula for marble is also
CaCO3.
Decomposition Reaction
● Decomposition reactions are opposite to combination reactions. In a decomposition
reaction, a single substance decomposes to give two or more substances.
● In this reaction, you can observe that a single reactant breaks down to give simpler products.
This is a decomposition reaction.
2FeSO4(s) Heat → Fe2O3(s) + SO2(g) + SO3(g)
● Decomposition of Silver bromide into silver and chlorine by light.
● Silver bromide used in black and white photography.
● Decomposition of calcium carbonate to calcium oxide and carbon dioxide on heating is an
important decomposition reaction used in various industries. Calcium oxide is called lime or
quick lime. It has many uses – one is in the manufacture of cement. When a decomposition
reaction is carried out by heating, it is called thermal decomposition.
Displacement Reaction
● It is a reaction between an element and a compound. When they react, one of the elements
of the compound- reactant is replaced by the element-reactant to form a new compound
and an element.
Fe + CuSO4 → FeSO4 + Cu
● In this reaction, iron has displaced or removed another element copper from copper
sulphate solution. This reaction is known as displacement reaction.
Double Displacement Reactions
● When two compounds react, if their ions are interchanged, then the reaction is called
double displacement reaction. The ion of one compound is replaced by the ion of another
compound.
Na2SO4 (aq) + BaCl2 (aq) → BaSO4 (s) + 2NaCl (aq)
Precipitation Reactions
● When aqueous solutions of two compounds are mixed, if they react to form an insoluble
compound and a soluble compound, then it is called precipitation reaction. Because the
insoluble compound, formed as one of the products, is a precipitate and hence the reaction
is so called.
● Precipitation reactions produce insoluble salts.
Neutralisation Reaction
● When an acid and a base react together to form salt and water, it is known as neutralization
reaction.
HCl + NaOH → H2O + NaCl
Oxidation and Reduction
Oxidation
● The chemical reaction which involves addition of oxygen or removal of hydrogen or loss of
electrons is called oxidation.
2 Mg + O2 → 2MgO (addition of oxygen)
CaH2 → Ca + H2 (removal of hydrogen)
Fe2+ → Fe3+ + e− (loss of electron)
Reduction
● The chemical reaction which involves addition of hydrogen or removal of oxygen or gain of
electrons is called reduction.
2 Na + H2 → 2 NaH (addition of hydrogen)
CuO + H2 → Cu + H2O (removal of oxygen)
Fe3+ + e− → Fe2+ (gain of electron)
Redox Reactions
● Generally, the oxidation and reduction occurs in the same reaction (simultaneously). If one
reactant gets oxidized, the other gets reduced. Such reactions are called oxidation-reduction
reactions or Redox reactions.
2 PbO + C → 2 Pb + CO2
Zn + CuSO4 → Cu + ZnSO4
Oxidation Reduction
Addition of oxygen Removal of oxygen
Removal of hydrogen Addition of hydrogen
Loss of electron Gain of electron
Oxidation and Reduction Agents
● Substance that loses oxygen or gains hydrogen is known as an oxidizing agent.
● Substance that loses hydrogen or gains oxygen is known as a reducing agent.
● Compounds with oxygen atom are called oxidizing agent and compounds with hydrogen
atom are called reducing agent.
● Some compounds can act as either oxidizing agents or reducing agents. One example is
hydrogen gas, which acts as an oxidizing agent when it combines with metals and As a
reducing agent when it reacts with non- metals.
Oxidation reactions in daily life
● The shining surface of metals tarnishes due to the formation of respective metal oxides on
their surfaces. This is called corrosion.
● The freshly cut surfaces of vegetables and fruits turn brown after some time because of the
oxidation of compounds present in them.
Rancidity
● When oils and fats or foods containing oils and fats are exposed to air, they get oxidized due
to which the food becomes stale and gives a bad taste or smell. This is called Rancidity.
● Following ways to preventing rancidity:
o Adding antioxidants
o Refrigerating
o Storing food in airtight containers with nitrogen gas
Common Name and Formula of Chemical Compounds
Chemical Compounds Chemical formula Common names
Calcium oxide Cao Quick lime
Calcium hydroxide Ca(OH)2 Slaked lime
Calcium carbonate CaCO3 Limestone
Trichloro Methane CHCl3 Chloroform
Calcium Oxychloride CaOCl2 Bleaching powder
Sodium hydrogencarbonate NaHCO3 Baking soda
Sodium carbonate Na2CO3 Washing soda
Calcium sulphate hemihydrate CaSO4 .1/2H2O Plaster of paris
calcium sulfate dihydrate CaSO .2H2O Gypsum
Acetic acid CH3COOH Vinegar
Silicon Oxide SiO2 Sand
Methane CH4 Marsh Gas
Nitrous oxide N2O Laughing Gas
Deuterium Oxide D2O Heavy water
Solid Carbondioxide CO2 Dry ice
Calcium Carbonate CaCo3 Chalk
Sulphuric Acid H2SO4 Oil of vitriol
Zinc sulphate ZnSO4 White Vitriol
Copper sulphate CuSO4.5H2O Blue Vitriol
Sodium hydroxide NaOH Caustic Soda
Potassium carbonate K2CO3 Potash Ash
Mercurous chloride Hg2Cl2 Calomel
Sucrose C12H22O11 Sugar
Silver nitrate AgNO3 Lunar caustic
Ethyl Alcohol C2H6O Alcohol
Hydrochloric acid HCl Muriatic acid
Chemical Compounds and Formula
Chemical Compounds Chemical formula
Sodium chloride NaCl
Zinc sulphate ZnSO4
Glucose C6H12O6
Ferric oxide Fe2O3
Ferrous sulphate FeSO4
Lead oxide PbO
Lead nitrate Pb(NO3)2
silver chloride AgCl
Silver bromide AgBr
Sodium sulphate Na2SO4
Chemical Bonding
Attraction between atoms, ions or molecules that enables the formation of chemical compounds is
called chemical bonding.
Types Of Chemical Bonding
● Ionic bond - Chemical bond formed between two atoms due to transfer of electron from one
atom to the other atom.
● Covalent bond - A covalent bond is a chemical bond that involves the sharing of electron
between two atoms.
● Metallic bond - Metallic bond is the force of attraction between metal ions to a number of
electrons within its sphere of influence.
