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CHEMISTRY [2 Mark Question Answer]

Chemical Changes and Reactions · ICSE STD 9 (ICSE - English Medium). 25 questions.

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[2 Mark Question Answer]

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25 questions · timed · auto-graded

Question 12 Marks
Define:
Electrochemical reaction
Give one example
Answer
It is a reaction which occurs with absorption of electrical energy.
Example:
Acidulated water breaks into hydrogen and oxygen.
$2 \mathrm{H}_2 \mathrm{O} \xrightarrow{\text { Electricity }} 2 \mathrm{H}_2+\mathrm{O}_2$
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Question 22 Marks
Define:
Photochemical reaction
Give one example
Answer
It is a reaction which occurs with absorption of light energy.
Example: Photosynthesis
$6 \mathrm{CO}_2+6 \mathrm{H}_2 \mathrm{O} \longrightarrow \mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6+6 \mathrm{O}_2$
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Question 32 Marks
State the effects of endothermic and exothermic reactions on the surroundings.
Answer
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.
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Question 42 Marks
Give an example of the following chemical changes.
A reaction where colour change is noticed.
Answer
Colour change
A few pieces of iron are added into a blue coloured copper sulphate solution; the blue colour of copper sulphate fades and eventually turns into light green due to the formation of ferrous sulphate.
$Fe + CuSO_4 \rightarrow FeSO_4 + Cu$
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Question 52 Marks
Give an example of the following chemical changes.
An exothermic and an endothermic reaction involving carbon as one of the reactants.
Answer
Exothermic reaction:
When carbon burns in oxygen to form carbon dioxide, a lot of heat is produced.
$ \mathrm{C}+\mathrm{O}_2 \rightarrow \mathrm{CO}_2+\text { Heat } $ Endothermic reaction:
When carbon is heated with sulphur at high temperature, liquid carbon disulphide is formed.
$ \mathrm{C}+2 \mathrm{~S} \xrightarrow{\Delta} \mathrm{CS}_2 $
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Question 62 Marks
Give an example of the following chemical changes.
A reaction involving
(i) Change of state
(ii) formation of precipitate
Answer
(i) Change of state
Ammonia gas reacts with $HCl$ gas to give solid ammonium chloride.
$NH_{3(g)} + HCl_{(g)} ⇋ NH_4Cl_{(s)}$​​​​​​​
(ii) Formation of precipitate
When a solution of silver nitrate is added to a solution of sodium chloride, a white insoluble substance, silver chloride, is formed.
$AgNO_{3(aq)}+ NaCl_{(aq)} \rightarrow AgCl_{(aq)} + NaNO_{3(aq)}​​​​​​​$​​​​​​​
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Question 72 Marks
State The main characteristic of chemical reactions. Give at least one example in each case.
Answer
Main characteristics of chemical reactions are as follows
1. Evolution of gas
In certain chemical reactions one of the products will be gas
Ex.
$\mathrm{Zn}+\mathrm{H}_2 \mathrm{SO}_4 \longrightarrow \mathrm{ZnSO}_4+\mathrm{H}_2$
2. Change of color
Some chemical reactions are characterized by change in color
Ex.
$\mathrm{Fe}+\underset{\text { (Blue solution) }}{\mathrm{CuSO}_4} \longrightarrow \underset{\text { (Green solution)(Brown/Red deposit) }}{\mathrm{FeSO}_4+\mathrm{Cu}}$
3.Formation of precipitate
Few chemical reactions give out insoluble solid substance called as precipitates.
Ex.
$\mathrm{AgNO}_3+\mathrm{NaCI} \longrightarrow \mathrm{AgCI}(\mathrm{ppt})+\mathrm{NaNO}_3$
4.Change of state
In many chemical reactions change of state is observed. Solid might turn to liquid and gases. Liquids and gases lead to formation of solids
Ex.
$\mathrm{NH}_3(\mathrm{~g})+\mathrm{HCI}(\mathrm{I}) \rightleftharpoons \mathrm{NH}_4 \mathrm{CI}(\mathrm{s})$
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Question 82 Marks
What do you understand by 'chemical reaction'?
Answer
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.
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Question 92 Marks
Why energy is involved in a chemical change?
Answer
In every chemical change, change in energy is involved.
There is a difference between the chemical energies of the reactants and products. It involves the breaking up of chemical bonds between the atoms resulting in the absorption of energy in the form of heat and simultaneous formation of bonds with the release of energy.
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Question 102 Marks
What is a chemical change? Give two examples of chemical change?
Answer
A chemical change is a permanent change in which the chemical composition of a substance is changed and a new substance is formed.
Examples:
Heating of copper carbonate
Formation of curd from milk
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Question 112 Marks
Decomposition reactions can occur by sunlight.
Give two balanced reactions.
Answer
Sunlight
Decomposition of silver chloride or silver bromide when exposed to sunlight. Silver chloride decomposes to give silver and chlorine whereas silver bromide gives silver and bromine. These reactions are used in black and white photography.
$2 \mathrm{AgCI}(\mathrm{s}) \xrightarrow{\text { Sunlight }} 2 \mathrm{Ag}(\mathrm{s})+\mathrm{CI}_2(\mathrm{~g})$
$2 \mathrm{AgBr}(\mathrm{s}) \xrightarrow{\text { Sunlight }} 2 \mathrm{Ag}(\mathrm{s})+\mathrm{Br}_2(\mathrm{~g})$
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Question 122 Marks
Decomposition reactions can occur by electricity
Give balanced reactions.
Answer
Electricity
Electrolysis of water i.e. the decomposition of water into hydrogen and oxygen gas when an electric current is passed through water.
$2 \mathrm{H}_2 \mathrm{O} \longrightarrow 2 \mathrm{H}_2+\mathrm{O}_2$
Sunlight:
$6 \mathrm{CO}_2+12 \mathrm{H}_2 \mathrm{O} \longrightarrow \mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6+6 \mathrm{H}_2 \mathrm{O}+6 \mathrm{O}_2$
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Question 132 Marks
What is a decomposition reaction?
Answer
A decomposition reaction is a reaction in which a compound breaks down into two or more simpler substances. The general form of a decomposition reaction is:
AB → A + B
Most decomposition reactions require an input of energy in the form of heat, light, or electricity.
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Question 142 Marks
What is a double displacement reaction?
Answer
A double displacement reaction this is a type of chemical change in which two compounds in a solution react to form two new compounds by mutual exchange of radicals. Double decomposition reaction is also called a double displacement reaction.
$\mathrm{AB}+\mathrm{CD} \longrightarrow \mathrm{AD}+\mathrm{CB}$
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Question 152 Marks
Decomposition brought about by heat is known as thermal decomposition. What is the difference between thermal dissociation and thermal decomposition?
Answer
A decomposition reaction brought about by heat is known as thermal decomposition.
$
2 \mathrm{HgO}_{(s)} \xrightarrow{\Delta} 2 \mathrm{Hg}_{(s)}+\mathrm{O}_{2(g)}
$A simultaneous reversible decomposition reaction brought about only by heat is thermal dissociation.
$
\mathrm{NH}_4 \mathrm{Cl} \leftrightharpoons \mathrm{NH}_3+\mathrm{HCl}
$
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Question 162 Marks
(a) What is synthesis?
(b) What kind of chemical reaction is synthesis? Support your answer by an example.
Answer
(a) A reaction in which two or more substances combine together to form a single substance is called a synthesis or combination reaction.
$
A+B \rightarrow A B
$
In the above reaction, substances $A$ and $B$ combine to give a molecule of a new substance, $A B$.
(b) Carbon burns in oxygen to form a gaseous compound, carbon dioxide.
$
\mathrm{C}+\mathrm{O}_2 \longrightarrow \mathrm{CO}_2
$
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Question 172 Marks
Explain, giving one example for each of the following chemical changes:
Displacement
Answer
Displacement
It is a chemical change in which a more active element displaces a less active element from its salt solution.
$CuSO_4 + Zn \rightarrow ZnSO_4 + Cu$
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Question 182 Marks
Explain,giving one example of the following chemical changes:
Reversible reaction
Answer
Reversible reaction
A chemical reaction in which the direction of a chemical change can be reversed by changing the conditions under which the reaction is taking place is called a reversible reaction.
$CuSO_4.5H_2O_{(s)} ⇋ CuSO_{4(s)} + 5H_2O_{(g)}$
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Question 192 Marks
Explain, giving one example of the following chemical changes:
Double decomposition
Answer
Double decomposition is a type of chemical change in which two compounds in a solution react to form two new compounds by mutual exchange of radicals. Double decomposition reaction is also called a double displacement reaction.
$
\mathrm{AB}+\mathrm{CD} \longrightarrow \mathrm{AD}+\mathrm{CB}
$
Example -
$
\mathrm{AgNO}_3+\mathrm{NaCl} \longrightarrow \mathrm{AgCl} \downarrow+\mathrm{NaNO}_3
$
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Question 202 Marks
When hydrogen burns in oxygen, water is formed; when electricity is passed through water, hydrogen and oxygen are given out. Name the type of chemical change involved in the two cases.
Answer
When hydrogen burns in oxygen, water is formed - Combination Reaction.
When electricity is passed through water, hydrogen and oxygen are given out - Decomposition Reaction.
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Question 212 Marks
Give an example of each of the following chemical changes:
A reaction involving
(i) blue solution
(ii) formation of dirty green precipitate
Answer
(i)
$
\mathrm{Fe}+\underset{\text { Blue solution }}{\mathrm{CuSO}_{4(a q)}} \longrightarrow \underset{\text { Green solution }}{\mathrm{FeSO}_4}+\mathrm{Cu}
$
(ii)
$
\mathrm{FeSO}_4+2 \mathrm{NaOH} \longrightarrow \underset{\text { Dirty green ppt. }}{\mathrm{Fe}(\mathrm{OH})_2}+\mathrm{Na}_2 \mathrm{SSO}_{4(a q)}
$
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Question 222 Marks
Give an example of each of the following chemical changes:
A photochemical reaction involving
(i) silver salt (ii) water
Answer
(i) $ 2 \mathrm{AgNO}_3+\xrightarrow{\text { Sunlight }} 2 \mathrm{Ag}+2 \mathrm{NO}_2+\mathrm{O}_2 $
(ii) $ 6 \mathrm{CO}_2+6 \mathrm{H}_2 \mathrm{O} \longrightarrow \mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6+6 \mathrm{O}_2 $
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Question 232 Marks
Define Electrochemical reaction
Give an example

Answer
It is a reaction which occurs with absorption of electrical energy.
$
2 \mathrm{H}_2 \mathrm{O}+\xrightarrow{\text { Electricity }} 2 \mathrm{H}_2+\mathrm{O}_2
$
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Question 242 Marks
Define Photochemical reaction
Give an example
Answer
It is a reaction which occurs with absorption of light energy.
$
6 \mathrm{CO}_2+6 \mathrm{H}_2 \mathrm{O} \longrightarrow \mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6+6 \mathrm{O}_2
$
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Question 252 Marks
State the conditions necessary for a chemical change or reaction.
Answer
  1. close contact: for a chemical reaction to take place the reactants must come into close contact i.e., they should be mixed
  2. light: some reactions take place in the presence of light example- photosynthesis Carbon dioxide + water chlorophyll kn presence of sunlight glucose + oxygen
  3. Solution form: some substances react with each other when they r mixed in the solution form .
  4. catalyst: some reactions need a catalyst for speed up and down the reaction of e.g.manganese dioxide.
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[2 Mark Question Answer] - CHEMISTRY STD 9 Questions - Vidyadip