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Monday, 29 June 2015


      Key topics  OF THE LESSON

1 Chemical reaction— Chemical changes or chemical reactions are the changes in which one or more new substances are formed.

2 Chemical Equations – Representation of a chemical reaction in terms of symbols and formulae of the reactants and products is known as chemical equation.

3 Balanced Chemical equations The chemical equation in which the no. of atoms of different elements is same on both sides of the arrow is called balanced chemical equation.

4 The chemical reactions can be classified into different types such as—
a) Combination reaction The reactions in which two or more substances combine
to form a new substance are called combination reaction. For example,
2Mg(s) + O2 (g)  2 MgO (s)

b) Decomposition reaction - The reaction in which a single compound breaks up into
two or more simpler substances are called decomposition reactions. For example,
2Pb (NO3)2 (s)  2PbO (s) + 4NO2 (g) +O2 (g)
The decomposition of a substance by passing electric current through it is known as
The decomposition of a substance on heating is known as thermal decomposition.
The decomposition of a substance by absorbing light energy is called photochemical

c) Displacement reactions -The chemical reactions in which a more reactive element
displaces a less reactive element from a compound are known as displacement
reactions. For example,
i) Zn (s) + CuSO4 (aq)  ZnSO4 (aq) + Cu (s).
ii) Cu (s) + 2AgNO3 (aq)  Cu (NO3)2 (aq) +2Ag (s).

d) Double Displacement Reactions - The chemical reactions in which compounds  react to form two different compounds by mutual exchange of ions are called double displacement reactions.
These reactions take place in solution two common types of this reaction are
precipitation reactions and neutralization reactions

i) Precipitation reaction : In this reactions, aqueous solution of two salts are mixed
whereby Some salts precipitate due to mutual exchange of ions between the two salts. For
example AgNO3 (aq) + NaCI (aq) -----> AgCl(s) + NaNO3.

ii) Neutralization reaction: In this type of reaction an acid reacts with a base to
form salt and water by exchange of ions.
NaOH (aq) + HCl (aq)  NaCl (aq) + H2O.

e) Redox reaction: Chemical reaction which shows both oxidation and reduction reaction.
Oxidation: Reaction that involves the gain of oxygen or loss of hydrogen.
Reduction: Reaction that shows the loss of oxygen or gain of hydrogen.
Both oxidation and reduction take place simultaneously and hence called redox reaction.
ZnO + C  Zn + CO
ZnO reduce to Zn ---- reduction
C oxidize to CO ------oxidation

f) Exothermic reaction and endothermic reaction: On the basis of energy changes during
chemical reaction, they can be classified as

i) Exothermic reaction: A chemical reaction in which heat energy is produced.
C + O2  CO2 (g) + heat

ii) Endothermic reaction: A chemical reaction in which heat energy is absorbed.
CaCO3 + Heat  CaO + CO2

5 CorrosionThe process of slow conversion of metals into their undesirable
compounds due to their reaction with oxygen, water, acids, gases etc. present in the
atmosphere is called corrosion.

Rusting Iron when reacts with oxygen and moisture forms red substance called rust.

6 Rancidity – The taste and odour of food materials containing fat and oil changes when
they are left exposed to air for long time. This is called rancidity. It is caused due to oxidation
of fat and oil present in food material.
It can be prevented by using various methods such as by adding antioxidants to the food
materials, Storing food in air tight container and by flushing out air with nitrogen.
1. Burning of a magnesium ribbon in air and collection of magnesium oxide
2. Formation of hydrogen gas by the action of dilute sulphuric acid on zinc
3. i) Combination reaction Formation of slaked lime by the reaction of calcium oxide with water 
ii) Decomposition reaction Heating of lead nitrate and emission of nitrogen dioxide
Electrolysis of water
iii) Displacement reaction Iron nails dipped in copper sulphate solution
iv) Double displacement reaction Formation of barium sulphate and sodium chloride.
v) Oxidation reaction Oxidation of copper to copper oxide

Q.1. A water insoluble substance ‗X‘ on reacting with dilute H2SO4 released a colourless and odourless gas accompanied by brisk effervescence. When the gas was passed through water, the solution obtained turned blue litmus red. On bubbling the gas through lime water, it initially became milky and milkyness disappeared when the gas was passed in excess.
Identify the substance ‗X‘. Write its chemical equations of the reactions involved.

Ans. The water insoluble substance ‗X‘ is metal carbonate CaCO3.

CaCO3 (S) + H2SO4 (Aq)  CaSO4(Aq) + H2O (Aq) + O2 (G)
Ca(OH)2 + CO2 (G)CaCO3 (S) + H2O (l) (milky)
CaCO3 (S) + CO2 (G)+ H2O (Aq)Ca(OH)2 (milkiness)

Q.2. Ahmad took a magnesium ribbon (cleaned) and burned it on a flame. The white powder

formed was taken in a test tube and water was added to it. He then tested the solution formed with red and blue litmus paper. What change was seen? Why?

Ans. Red litmus paper turned blue. Blue litmus paper remained blue.

This is because the magnesium ribbon on burning in air forms the white magnesium
oxide. Which dissolved in water, it forms magnesium hydroxide, which is Basic in nature.

Q.3. Give one example of a combination reaction in which an element combines with a

compound to give you a new compound.
Ans. O2 + 2SO2 2SO3
8NH3 + 3Cl2 6NH4Cl 

Q.4. Marble statues often slowly get corroded when kept in open for a long time. Assign a

suitable explanation.
Q.5. Mohan took pure water for the electrolytic decomposition of water but did not see any
bubbles near the electrodes. Explain why?
Q . 6Rancidity is a process used for spoiling of cooked food materials like vegetables, etc. When kept for long time in open. How can you prevent such process to proceed? Give an
Q. 7 A substance ‗X‘ displaces ‗Y‘ from its solution in water. It is called displacement reaction. What other chemical name can be given to such type of reactions? Explain, giving an example?
Q. 8 A grey coloured metal ‗Z‘ (Atomic weight=65) is used in making dry cell. It reacts with dil. HCl to liberate a gas. What is the gas evolved? Calculate the minimum amount of ‗Z‘
required to produce 100 litres of gas?
Q. 9 Why is respiration considered an exothermic reaction? Explain.
Q. 10 Why are decomposition reactions called opposite of combination reactions? Write equations for these reactions.
Q. 11 A shiny brown colored element ‗X‘ on heating in air becomes a black coloured compound. Name the element ‗X‘& black the coloured compound formed. Also write the equation
Chemistry for class 10....

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