Selina Concise Chemistry Class 9 ICSE Solutions Chemical Changes and Reactions
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- A chemical reaction is the process of breaking the chemical bonds of the reacting substances (reactants) and making new bonds to form new substances (products).
- Conditions necessary for a chemical change or reaction are
- Evolution of gas
- Change of colour
- Formation of precipitate
- Change of state
- A chemical bond is the force which holds the atoms of a molecule together as in a compound.
- Formation of gas bubbles in a liquid during a reaction is called effervescence.
- Chemical reactions which are characterised by the formation of insoluble solid substances are called precipitates.
- Silver nitrate solution is kept in brown bottles in the laboratory because it decomposes in the presence of light.
- Molybdenum increases the efficiency of the catalyst iron used in the manufacture of ammonia.
- This is because the blue colour of the copper sulphate solution fades and eventually turns into light green due to the formation of ferrous sulphate.
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- Double decomposition
- Accelerate, decelerate, unaffected
A reaction in which two or more substances combine together to form a single substance is called a synthesis or combination reaction.
A + B → AB
In the above reaction, substances A and B combine to give a molecule of a new substance, AB.
Carbon burns in oxygen to form a gaseous compound, carbon dioxide.
C + O2 → CO2
The reaction between an acid and a base which forms salt and water only is referred to as reaction of neutralisation.
Applications of neutralisation reactions:
- When someone is stung by a bee, formic acid enters the skin and causes pain, which can be relieved by rubbing the spot with slaked lime or baking soda, both of which are bases.
- Acid which accidentally spills on to our clothes can be neutralised with ammonia solution.
- If the soil is somewhat acidic and thus unfavourable for growing of certain crops, then slaked lime is added to neutralise the excess acid.
Hydrolysis is the process in which a salt and water react to form an acidic or basic solution.
In the process of hydrolysis, only those salts hydrolyse which are formed by the reaction of
- strong base and weak acid
- strong acid and weak base
This happens because a salt formed due to a strong base and a weak acid on dissolving in water will form a basic solution.
A basic solution turns red litmus blue.
Na2CO3 + 2H2O → 2NaOH + H2CO3
However, the salt formed due to a strong acid and a weak base on dissolving in water will make an acidic solution. Acidic solutions turn blue litmus red.
FeCl3 + 3H2O → Fe(OH)3 + 3HCl
Iron(III) chloride is a salt prepared from strong acid HCl and a weak base Fe(OH)3.
Fe(OH)3 + 3HCl→ FeCl3 + 3H2O
On the other hand, sodium carbonate is a salt prepared from a strong base NaOH and a weak acid H2CO3.
Fe(OH)3 + 3HCl → FeCl3 + 3H2O
Decomposition is the breaking up of a compound either into elements or simpler compounds such that these products do not combine to form the original compound.
Decomposition may occur in the presence of heat or light or by the passage of an electric current.
Example: Mercuric oxide when heated decomposes to form two elements-mercury and oxygen.
- Cl2 + 2KBr → 2KCl + Br2
- Fe + CuSO4 → FeSO4 + Cu
- 2H2O → 2Hg + O2
- PbO2 + SO2 → PbSO4
- AgNO3 + NaCl → AgCl + NaNO3
- 2KClO3 → 2KCl + 3O2
- 2H2O2 → 2H2O + O2
- KNO3 + H2SO4 → HNO3 + KHSO4
- CuO + H2 → Cu + H2O
- CaCO3 → CaO + CO2
- NH4Cl → NH3 + HCl
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Exothermic reactions are spontaneous and warm their surroundings with the release of heat energy.
Endothermic reactions absorb heat energy from their surroundings and cause their surroundings to cool down.
A chemical reaction is the process of breaking the chemical bonds of the reacting substances (reactants) and making new bonds to form new substances (products).
A chemical change or chemical reaction occurs when particles collide. Collisions occur when reactants are in close contact or by supply of energy.
- Lead nitrate decomposes on heating leaving a yellow residue lead monoxide, brown gas nitrogen dioxide and colourless gas oxygen.
- If chlorine water is exposed to sunlight, oxygen is evolved.
- Hydrogen peroxide breaks down to form water and oxygen gas along with heat energy.
- When hydrogen sulphide gas is passed through a blue solution of copper sulphate, a black precipitate of copper sulphide is obtained and the sulphuric acid so formed remains in the solution.
- A white insoluble precipitate of barium sulphate is formed.
- Sodium carbonate
- Sodium nitrate
- Zinc carbonate
- Lead nitrate
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