Class 10 Students Study Material
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Here is Science:-
CHEMICAL REACTIONS AND EQUATIONS
LIST 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 are known as the chemical equation.
3) Balanced Chemical equations – The chemical equation in which the no. of atoms of different
elements are the same on both sides of the arrow is called the 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
the new substance is called a 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
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 an electric current through it is known as
electrolysis.
The decomposition of a substance on heating is known as thermal decomposition.
The decomposition of a substance by absorbing light energy is called photochemical decomposition.
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 these reactions, an aqueous solution of two salts is mixed
whereby Some salts precipitate due to the mutual exchange of ions between the two salts.
For example
AgNO3 (aq) + NaCl (aq) -----> AgCl(s) + NaNO3.
ii) Neutralization reaction: In this type of reaction an acid reacts with a base to form salt
and water by the 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 are hence called redox reactions.
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
Corrosion – The 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 a red substance called rust.
Rancidity – The taste and odour of food materials containing fat and oil change when they are left exposed to air for a long time. This is called rancidity. It is caused due to the 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 an airtight container and flushing out air with nitrogen.
HOTS QUESTIONS (SOLVED) Chemical Reactions And Equations
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 milkiness disappeared when the gas was passed in excess.
Identify the substance ‗X‘. Write the chemical equations of the reactions involved.
Ans. The water-insoluble substance ‗X‘ is a 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 the air forms the white magnesium oxide. When 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 tool pure water for the electrolytic decomposition of water but did not see any bubbles near the electrodes. Explain why?
Q. 6 Rancidity is a process used for the spoiling of cooked food materials like vegetables, etc. When kept for a long time in the open. How can you prevent such a process to proceed? Give an example.
Q. 7 A substance ‗X‘ displaces ‗Y‘ from its solution in water. It is called a displacement reaction.
What other chemical names can be given to such types of reactions? Explain, giving an
example?
Q. 8 A grey-coloured metal ‗Z‘ (Atomic weight=65) is used in making a 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 l 0f gas?
Solve these questions to score 100 per cent from the Chemical Reactions and Equations.
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