1455780078science 10 - T 1 - (CH 1)
1455780078science 10 - T 1 - (CH 1)
1455780078science 10 - T 1 - (CH 1)
Chemical Reactions
and Equations
SYLLABUS
Chemical reactionsChemical equation, Balanced chemical equation, Implications of a
balanced chemical equation, Types of chemical reactions: combination, decomposition,
displacement, double displacement, precipitation, neutralization, oxidation and
reduction.
ACTIVITY 1.2
Ans. Lead nitrate reacts with potassium iodide to form lead iodide which is insoluble in
water and yellow in colour.
Pb(NO3)2(aq) + 2KI(aq) PbI2(s) + 2KNO3(aq)
ACTIVITY 1.3
Ans. When zinc metal reacts with dilute hydrochloric acid then zinc chloride is formed
and bubbles of hydrogen gas are observed. The conical flask becomes hot which
shows that the reaction is exothermic.
Zn(s) + 2HCl(dil) ZnCl2(aq) + H2(g) +
ACTIVITY 1.4
Ans. 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(aq) +
The reaction is combination in nature as calcium oxide reacts with water to form a
single product, calcium hydroxide.
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Science-X
ACTIVITY 1.5
Ans. The green colour of ferrous sulphate crystals changes to brownish-black ferric oxide
and smell of burning sulphur dioxide is observed.
2FeSO4(s) Fe2O3(s) + SO2(g) + SO3(g)
ACTIVITY 1.6
Ans. Pungent smelling, brown fumes are evolved due to NO2 gas and brownish residue of
lead oxide (PbO) is left.
2Pb(NO3)2(s) PbO(s) + 4NO2(g) + O2(g)
ACTIVITY 1.7
Ans. The volume of one of the gases i.e., hydrogen, is twice the volume of the other gas,
oxygen. One of the gases catches fire and burns with pop sound (hydrogen) whereas
in other gas, candle burns brightly (oxygen). The gas collected at anode is oxygen
and the gas collected at cathode is hydrogen.
ACTIVITY 1.8
Ans. White silver chloride turns grey in sunlight because silver metal is formed due to
photolytic reaction and chlorine gas is evolved.
sunlight
ACTIVITY 1.9
Ans. Copper sulphate solution is blue when an iron nail is dipped in it, its blue colour
changes. This happens due to the following displacement reaction. Iron being more
reactive than Cu, displaces it thereby forming a new product iron sulphate and copper
metal.
Fe(s) + CuSO4(aq) FeSO4(aq) + Cu(s)
The colour of copper sulphate in test tube A is dark blue and its colour in test tube
B is light blue.
The colour of iron nail becomes brownish when dipped in test tube B as copper
metal deposits on it.
The colour of the solution becomes green due to the formation of FeSO4.
ACTIVITY 1.10
Ans. When sodium sulphate reacts with barium chloride white precipitate of barium
sulphate is formed.
Na2SO4(aq) + BaCl2(aq) BaSO4(s) + 2NaCl(aq)
ACTIVITY 1.11
Ans. The surface of copper powder on heating becomes black. Copper when heated in air
reacts with oxygen to produce copper oxide.
2Cu + O2
2Cu
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12
Science-X
Q7. Give an example of a double displacement reaction other than the one given in Activity 1.10.
Ans.
Pb ( NO3 )2 +
(Lead nitrate)
2HCl
(Hydrochloric
acid)
PbCl2 ( s ) + 2HNO3
(Lead chloride)
(Nitric acid)
Q8. Identify the substance that is oxidised and the substances that are reduced in the
following reactions.
(i) 4Na(s) + O2(g) 2Na2O(s)
(ii) CuO(s) + H2(g) Cu(s) + H2O(l)
Ans. (i) Sodium metal is oxidised and oxygen is reduced to Na2O.
(ii) Copper oxide is reduced to copper and hydrogen is oxidised to H2O.
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Q4. What is balanced chemical equation? Why should chemical equations be balanced?
Ans. Balanced chemical equations means total number of atoms of each element should
be equal on both sides of the reaction in reactants and products. The reaction should
be balanced because matter can neither be created nor be destroyed. The total mass
of reactants should be equal to total mass of products i.e., mass of reactants = mass
of product.
Q5. Translate the following statements into chemical equations and then balance them.
(a) Hydrogen gas combines with nitrogen to form ammonia.
(b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide.
(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a
precipitate of barium sulphate.
(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.
Ans. (a) 3H2(g) + N2(g) 2NH3(g)
(b) 2H2S(g) + 3O2(g) 2H2O(l) + 2SO2(g)
(c) 3BaCl2(aq) + Al2(SO4)3(aq) 2AlCl3(aq) + 3BaSO4(s)
(d) 2K(s) + 2H2O(l) 2KOH + H2(g)
Q6. Balance the following chemical equations.
(a) HNO3 + Ca(OH)2
Ca(NO3)2 + H2O
Na2SO4 + H2O
AgCl + NaNO3
BaSO4 + HCl
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Science-X
Q8. Write the balanced chemical equation for the following and identify the type of reaction
in each case.
(a) Potassium bromide(aq) + Barium iodide(aq) Potassium iodide(aq) +
Barium bromide(s)
(b) Zinc carbonate(s) Zinc oxide(s) + Carbon dioxide(g)
(c) Hydrogen(g) + Chlorine(g) Hydrogen chloride(g)
(d) Magnesium(s) + Hydrochloric acid(aq) Magnesium chloride (aq) + Hydrogen(g)
Ans. (a) 2KBr(aq) + BaI2(aq) 2KI(aq) + BaBr2(s)
(b) ZnCO3(s) ZnO(s) + CO2(g)
(c) H2(g) + Cl2(g) 2HCl(g)
(d) Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g)
Q9. What does one mean by exothermic and endothermic reactions? Give examples.
Ans. Exothermic: Those reactions in which heat is evolved are called exothermic
reaction. Heat symbol is shown at product side.
e.g., C(s) + O2(g) CO2(g) +
Endothermic: Those reactions in which heat is absorbed are called endothermic
reaction. Heat symbol is shown at reactant side.
CaCO3(s) + CaO(s) + CO2(g)
Q10. Why is respiration considered as exothermic reaction? Explain.
Ans. (1) In respiration glucose gets oxidized to form carbon dioxide, water and heat is
evolved. (2) As heat energy is released during respiration it is regarded as exothermic
reaction.
Q11. Why are the decomposition reactions called the opposite of combination reactions? Write
equations for these reactions.
Ans. In decomposition reaction, single compound is broken down into simpler compounds
or elements.
e.g., CaCO3(s) CaO(s) + CO2(g)
In combination reaction, two or more elements or compounds combine to form a single
new compound.
e.g., CO2(g) + CaO(s) CaCO3(s)
2SO2(g) + O2(g) 2SO3 (g)
Hence the decomposition and combination are opposite to each other.
Q12. Write one equation each for decomposition reactions where energy is supplied in the
form of heat, light and electricity.
Ans. In decomposition reaction, a compound is broken down into simpler compounds or
elements e.g.,
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heat
2H2(g) + O2(g)
Electricity 2H2O(l)
current
Q13. What is the difference between displacement and double displacement reactions? Write
equations for these reactions.
Ans. In displacement reaction more reactive metal can displace a less reactive metal from
its salt solution.
e.g., Zn(s) + CuCl2(aq) ZnCl2(aq) + Cu(s)
In double displacement reaction, two different compounds exchange their ions and
form new compounds.
e.g., NaOH + HCl NaCl + H2O
Q14. In the refining of silver, the recovery of silver from silver nitrate solution involved
displacement by copper metal. Write down the reaction involved.
Ans. Cu(s) + 2AgNO3(aq) Cu(NO3)2(aq) + 2Ag(s)
Q15. What do you mean by precipitation reaction? Explain it by giving examples.
Ans. Those reactions in which reactants react to form an insoluble compound precipitate
are called precipitation reactions.
e.g., AgNO3(aq)+ NaCl(aq) AgCl (white ppt.)(s) + NaNO3(aq)
Q16. Explain the following in terms of gain or loss of oxygen with two examples each.
(a) Oxidation
(b) Reduction
Ans. (a) Oxidation: It is a process in which gain of oxygen takes place or loss of
hydrogen takes place.
Burning
2MgO(s)
e.g., 2Mg(s) + O2(g)
Mg is oxidized to MgO.
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Science-X
Q19. Oil and fat containing food items are flushed with nitrogen. Why?
Ans. The food items containing oil and fat are flushed with nitrogen because oil and fat
become rancid on oxidation which has the bad taste and smell. The nitrogen flushing
prevents the oxidation of food so that it does not become rancid.
Note: (Food is kept in refrigerator so as to reduce the temperature which slows
down the rate of oxidation and preserve the food for longer time).
Q20. Explain the following terms with one example each.
(a) Corrosion
(b) Rancidity
Ans. (a) Corrosion: It is a process in which metal reacts with substances present in
atmosphere to form surface compounds. For example, iron reacts with oxygen in
the presence of moisture and forms rust. Silver turns black due to formation of
silver sulphide and copper turns green due to the formation of copper carbonate.
Rust: Fe O .2H O (Hydrated ferric oxide)
2
(b) Rancidity: It is a process in which food material gets spoiled when it comes in
contact with oxygen. It leads to the change in taste and smell of food materials.
For example, butter gets spoiled due to oxidation at room temperature if kept
for a longer time, it becomes sour in taste and gives a foul smell. The product
formed on oxidation of food is rancid and such a process is called rancidity.
(b) 45 g
(c) 10 g
(d) 18 g
(b) Pb(SO4)2
(c) PbSO4
(d) Pb 2(SO4)3
3. In the reaction, SO2 (g) + 2H2S (g) 2H2O(l) + S(s), the reducing agent is
(a) SO2
(b) H2O
(c) H2S
(d) S
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5. Chemically rust is
(a) hydrated ferrous oxide
(b) colourless
9. Which of the following gases can be used for storage of fresh sample of an oil for a
long time?
(a) Carbon dioxide or oxygen
(b) H2O
(b) 2 : 1 by volume
(c) 8 : 1 by mass
(d) 1 : 2 by mass
12. In the decomposition of lead (II) nitrate to give lead (II) oxide, nitrogen dioxide and
oxygen gas, the coefficient of nitrogen dioxide (in the balanced equation) is
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(a) 1
(b) 2
(c) 3
(d) 4
Science-X
13. Reddish-brown copper metal forms a black solid on combustion. Which of the
following statement is incorrect?
(a) Black solid is CuO.
(b) The reaction is a redox reaction.
(c) The reaction is a precipitation reaction.
(d) Copper is being oxidised.
14. Which of the following reaction is used in white washing of walls?
(a) 2Ca + O2 2CaO
(b) CaO + H2O Ca(OH)2 +
(c) Ca(OH)2 + CO2 CaCO3 + H2O
(b) corrosion
(c) reduction
(d) hydrogenation
17. Silver article turns black when kept in the open for a few days due to formation of
(a) H2S
(b) AgS
(c) AgSO4
(d) Ag2S
18. Based on the reaction given below, what is the correct increasing order of reactivity
of metals?
(i) Fe(s) + CuSO4(aq) FeSO4(aq) + Cu(s)
(ii) Cu(s) + FeSO4(aq) No reaction
(iii) Cu(s) + 2AgNO3(aq) Cu(NO3)2 (aq) + 2Ag(s)
(iv) 2Ag(s) + Cu(NO3)2(aq) No reaction
(a) Ag < Cu < Fe
19. When crystals of lead nitrate are heated strongly in a dry test tube
(a) crystals immediately melt
20. Which of the following will be required to identify the gas evolved when dilute
hydrochloric acid reacts with zinc metal?
(a) Lime water
(d) pH paper
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21. Dilute hydrochloric acid is added to granulated zinc taken in a test tube. The
following observations are recorded. Point out the correct observation.
(a) The surface of metal becomes shining
(b) The reaction mixture turns milky
(c) Odour of a pungent smelling gas is recorded
(d) A colourless and odourless gas is evolved
22. When carbon dioxide is passed through lime water,
(a) calcium hydroxide is formed
23. In which of the following chemical equations, the abbreviation represent the correct
states of the reactants and products involved at reaction temperature?
(a) 2H2(g) + O2(g) 2H2O(l)
(b) 2H2(g) + O2(l) 2H2O(l)
(c) 2H2(g) + O2(g) 2H2O(g)
(d) 2H2(l) + O2(l) 2H2O(g)
24. Identify the following type of reaction:
Heat
(b) white
(c) yellow
(d) pink
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Science-X
33. On immersing an iron nail in CuSO4 solution for few minutes, you will observe
(a) no reaction takes place
(b) the colour of solution fades away
(c) the surface of iron nails acquire a black coating
(d) the colour of solution changes to green
34. An element X on exposure to moist air turns reddish-brown and a new compound Y
is formed. The substance X and Y are
(a) X = Fe, Y = Fe2O3
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Answers
1. (d)
2. (c)
3. (c)
4. (d)
5. (c)
6. (d)
7. (b)
8. (b)
9. (b)
10. (c)
11. (b)
12. (d)
13. (a)
14. (d)
15. (a)
16. (c)
17. (d)
18. (b)
19. (b)
20. (c)
21. (d)
22. (c)
23. (a)
24. (d)
25. (c)
26. (b)
27. (c)
28. (d)
29. (c)
30. (a)
31. (d)
32. (c)
33. (d)
34. (a)
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Science-X
Q13. Name the compound used to test the evolution of carbon dioxide gas.
Ans. Calcium hydroxide solution (freshly prepared), also called lime water, is used to
test CO2 gas.
Q14. Name the gas evolved when lead nitrate is heated.
Ans. Nitrogen dioxide gas is brown in colour.
Q15. Write the formula for two oxides of sulphur.
Ans. SO2 and SO3
Q16. Give the examples of exothermic reaction.
Ans. Respiration and water added to lime
Q17. Give one example of decomposition reaction that occurs in nature.
Ans. Rotting of fruits and vegetables
Q18. Name the type of reaction in which two or more than two reactants form a single
compound.
Ans. Combination reaction
Q19. What is breaking and making of bonds in chemicals called?
Ans. Chemical reaction
Q20. Name the chemical used in black and white photography.
Ans. Silver bromide
Q21. Name the ions present in barium sulphate.
Ans. Ba
2+
and SO4
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Q2. How can you chemically remove the black coating of copper oxide?
Ans. The black coating of copper oxide can be removed chemically by passing hydrogen
gas over heated copper oxide. The black coating turns brown as oxygen is removed
by hydrogen.
Q3. Product formed when A and B react together are zinc chloride and hydrogen gas. Find
reactant A and B.
Ans. The reactant A is zinc metal and the reactant B is hydrochloric acid.
Zn + 2HCl ZnCl2 + H2
Q4. Name the product obtained and type of reaction given below:
Na2SO4 + BaCl2
Q9. Explain why most of the metal articles become dull when kept exposed to air?
Ans. Metal articles react with the gases present in the air and their surface gets coated
with the layer of compound it forms making them look dull and the lustre is lost. For
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Science-X
example, aluminium metal reacts readily with oxygen to form aluminium oxide and
its surface becomes dull.
Q10. What is rancidity? What is the general name of chemicals which are added to fat and oil
containing food so as to prevent the rancidity?
Ans. The oil and fat containing food when left exposed to air reacts with oxygen and gets
oxidized and become rancid, this process is called rancidity. In this process their
smell and taste changes. The general name of the chemicals that are added to prevent
this oxidation are called as antioxidants. For example, nitrogen gas is an antioxidant.
Q11. Define displacement reaction. Give one example of it, how is it different from double
displacement reaction?
Ans. The reaction in which more reactive metal displaces (takes place of) the less reactive
metal from its compound, is called as displacement reaction.
e.g., Fe(s) + CuSO4 (aq) FeSO4 (aq) + Cu (s)
It is different from double displacement reaction because in double displacement
reaction two different elements or atoms or ions are exchanged for each other.
e.g., Na2SO4 + BaCl2 BaSO4 + 2NaCl
Q12. Give differences between the exothermic and endothermic reactions.
Ans. Exothermic reaction: In this reaction heat is given out during the reaction.
Endothermic reaction: In this reaction heat is taken in to break the bonds and to
form new compounds.
Q13. Explain and name the type of reaction seen when iron reacts with hydrochloric acid.
Ans. Iron reacts with HCl to release hydrogen gas and iron chloride is formed. This
reaction is called as displacement reaction.
Q14. Why is photosynthesis considered as endothermic reaction?
Ans. Photosynthesis is a reaction in which energy is required to form glucose from carbon
dioxide and water. Energy in the form of sunlight is also required to break the bonds
of hydrogen and oxygen. Hence it is termed as endothermic reaction.
Q15. What is electrolytic decomposition? Give two uses of electrolytic decomposition reaction.
Ans. When electricity is passed through a molten compound which is ionic in nature, then
the ions of the compound separate into its components thereby decomposing the
compound.
It is used in separating hydrogen and oxygen gas from water. It is also used to
separate gases or metals (elements) from the compound.
Q16. Give one example for each of the following reactions:
(a) Combination reaction
25
2Mg + O2 2MgO
heat
(b) Decomposition reaction CaCO3
CaO + CO2
(c) Displacement reaction
Zn + CuSO4 ZnSO4 + Cu
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Science-X
Q22. Give equations to show the chemical reactions of zinc and lead where it displaces copper
from its compound.
Ans. Zn(s) + CuSO4(aq) ZnSO4(aq) + Cu(s)
Copper sulphate
Zinc sulphate
Lead chloride
(CBSE 2008)
Ans. Quick lime is CaO. On adding water to it large amount of heat is released, hissing
sound is produced and bubbles are formed.
Q25. How will you test for the gas which is liberated when hydrochloric acid reacts with an
active metal?
(CBSE 2008)
Ans. The gas liberated is hydrogen and it can be tested by taking a burning match stick
near the mouth of the test tube in which reaction is taking place. The match stick
will burn with a pop sound.
Q26. What is an oxidation reaction? Identify in the following reactions:
(i) the substance oxidized and (ii) the substance reduced.
ZnO + C Zn + CO
(CBSE 2008)
Ans. The reaction in which a substance looses hydrogen or gains oxygen, is called
oxidation reaction. In the given reaction C is oxidized as it gains oxygen and ZnO is
reduced as it looses oxygen.
Q27. Give an example of photochemical reaction.
(CBSE 2008)
Ans. In black and white photography AgBr gets decomposed due to light and forms Ag
and bromine gas.
Q28. Give an example of a decomposition reaction. Describe an activity to illustrate such a
reaction by heating.
(AI CBSE 2008)
Ans. Example of decomposition reaction:
heat
CaCO3
CaO + CO2
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Activity:
Take a test tube add CaCO3 in it and heat. Gas is evolved allow it to pass through
lime water which turns milky.
Q29. Balance the following chemical equation:
Fe(s) + H2O(g) Fe3O4(s) + H2(g)
heat
Ans. 2FeSO4
Fe2O3 + SO2 + SO3
(CBSE 2008 F)
Ans. The number of atoms on left hand side of the reaction (i.e., reactants) is equal to the
number of atoms on the right hand side of the equation (i.e., product).
Law of Conservation of Mass: Mass of atoms in chemical reaction can neither be created
nor be destroyed, it remains the same.
Q34. State any two observations in an activity, which may suggest that a chemical reaction
has taken place. Given examples to support your answer.
(CBSE 2008 F)
Ans. The following observations in an activity may suggest that a chemical reaction has
taken place:
Change in state
Change in colour
Evolution of a gas
Change in temperature
(CBSE 2008 C)
28
Science-X
(CBSE 2008 C)
(CBSE 2008 C)
(CBSE 2009)
heat
Ans. 2Pb(NO3)2
2PbO(s) + 4NO2(g) + O2(g)
Q41. Name a reducing agent that may be used to obtain manganese from manganese dioxide.
(CBSE 2009)
Ans. Coke or carbon is used as reducing agent.
Q42. In electrolysis of water, why is volume of gas collected over one electrode double that of
gas collected over the other electrode?
(AI CBSE 2009)
Ans. The volume of gas collected over one electrode is double than that of the gas
collected over the other electrode as the ratio of H : O in water is 2 : 1.
Q43. What change in colour is observed when silver chloride is left exposed to sunlight? What
type of chemical reaction is this?
(CBSE 2009 F)
Ans. Silver turns black, the reaction is decomposition.
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(c) Displacement reaction: The reaction in which more reactive metal displaces the
less reactive metal is called displacement reaction.
e.g., Fe + CuSO4 FeSO4 + Cu
(d) Double displacement reaction: The reaction in which two different atoms or groups
of (substances) atoms exchange for each other is called double displacement
reaction.
e.g., Na2SO4(aq) + BaCl2(aq) BaSO4(s) + 2NaCl(aq)
(e) Oxidation-reduction reaction: The reaction in which oxygen is added/hydrogen is
removed is called oxidation reaction.
The reaction in which hydrogen is added/oxygen is removed is called reduction
reaction.
e.g., Oxidation,
heat
2Cu + O2
2CuO
Reduction
reduction
heat
CuO + H2 Cu + H2O
Q2. Give an activity to prove that water contains H : O in the ratio of 2 : 1.
Ans.
Take a plastic mug. Drill two holes at its base and fit rubber stoppers in these
holes. Insert carbon electrodes in these rubber stoppers as shown in figure.
Connect these electrodes to a 6 volt battery.
Fill the mug with water such that the electrodes are immersed. Add a few drops
of dilute sulphuric acid to the water.
Take two test tubes filled with water and invert them over the two carbon
electrodes.
Switch on the current and leave the apparatus undisturbed for some time.
You will observe the formation of bubbles at both the electrodes. These bubbles
displace water in the test tubes.
The gas collected in test tube attached to cathode is twice in volume than the
gas collected at anode.
Once the test tubes are filled with the respective gases, remove them carefully.
The burning match stick or candle when brought near to the test tube containing
gas obtained at cathode, burns with a pop sound indicating the presence of
hydrogen gas.
30
Science-X
Plastic mug
Hydrogen
Oxygen
Test tube
Bubbles of gas
Water
Graphite rod
Rubber stopper
Anode
Cathode
6V
Battery
Switch
Electrolysis of water
Q3.
Ans. (i) Rancidity: Fat and oil containing food when kept open in air gets oxidised and
become rancid due to which the taste and smell of food changes.
(ii) Two methods to reduce rancidity are keep the food in closed containers and use
antioxidants.
(iii) Corrosion is seen in all metals, when kept exposed. Forms a layer of compound by
reaction of metal with moisture, acid or gases present in it.
Rusting is the process in which iron metal reacts with air and moisture to form
brownish powder called rust.
Q4. What is meant by exothermic and endothermic reaction? Give examples to explain the same.
Ans. Exothermic reactions: Those reactions in which heat is released out during chemical
reactions.
e.g., burning of fuel
The natural gas (methane) burns in air to form carbon dioxide and water and releases
large amount of heat.
Endothermic reaction: Those reactions in which heat is required, is called endothermic
reaction.
heat
CaCO3(s)
CaO(s) + CO2(g)
When CaCO3 is heated it forms CaO and CO2.
e.g.,
31
Q5. Solid calcium oxide was taken in a container and water was added slowly to it.
(AI CBSE 2008 C)
Water
Slaked lime
(CBSE 2009)
Ans. The reaction in which oxygen is added or hydrogen is removed is called oxidation
reaction.
heat
e.g., 2Cu + O2
2CuO
It is an endothermic reaction.
Q7. Define a combination reaction. Give one example of a combination reaction which is also
exothermic.
(CBSE 2009)
Ans. Combination reaction: It is a reaction in which two elements combine together to
form a product.
e.g. CaO + H2O Ca(OH)2 + heat
(Quick
lime)
(Water)
(Slaked
lime)
(i) What is observed when a solution of potassium iodide is added to a solution of lead
nitrate taken in a test tube?
(ii) What type of reaction is this?
(iii) Write a balanced chemical equation to represent the above reaction. (AI CBSE 2009)
Ans. (i) Yellow coloured precipitate is formed due to formation of lead iodide.
(ii) It is a double displacement reaction.
(iii) 2KI + Pb(NO3)2 2KNO3 + PbI2
Q8.
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Science-X
Q9. What is a redox reaction? When a magnesium ribbon burns in air with a dazzling flame
and forms a white ash, is magnesium oxidised or reduced? Why?
Ans. The reaction which shows both oxidation and reduction reaction in it is called redox
reaction.
2Mg + O2 2MgO
In this case magnesium is oxidised as oxygen combines with it.
Q10. Distinguish between an exothermic and an endothermic reaction. Amongst the following
reactions, identify the exothermic and the endothermic reaction.
(i) Heating coal in air to form carbon dioxide.
(ii) Heating lime-stone in a lime kiln to form quick lime.
Ans.
Exothermic Reaction
(CBSE 2009 F)
Endothermic Reaction
The reaction in which heat is released. The reaction in which heat is absorbed.
(i) Exothermic
(ii) Endothermic
Q11. Distinguish between a displacement reaction and a double displacement reaction. Identify
the displacement and the double displacement reaction from the following reactions.
(i) HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)
(ii) Fe(s) + CuSO4(aq) FeSO4(aq) + Cu(s)
Ans.
Displacement Reaction
(CBSE 2009 F)
33
Q4. 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 and why?
Ans. Red litmus turned blue.
Blue litmus remained blue.
This is because the magnesium ribbon on burning in air, forms the white magnesium
oxide. When this is dissolved in water, it forms magnesium hydroxide, which is basic
in nature.
Q5. 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
Q6. Deepa added magnesium into a test tube containing dilute hydrochloric acid. She saw
some gas coming out of it. She took a burning match stick near the mouth of the test
tube and she heard a popping sound while the match stick extinguished. Deepa concluded
that the gas evolved in hydrogen and it is not combustible. Find the error in her
conclusion and support your answer with one valid reason.
Ans. The conclusion that hydrogen gas is not combustible is wrong, because hydrogen gas
is highly combustible and burns very fast, to produce large amount of heat.
Q7. Arnav took magnesium and reacted it with dil. HCl to record the observation. Then
Deepak took the same piece of magnesium and reacted it with conc. HNO3 and dil.
H2SO4 but did not see a reaction. Explain this behavior.
Ans. Arnav had already reacted Mg with dil. HCl to form MgCl2 and gas. The piece of
metal that Deepak used was not magnesium but magnesium chloride (MgCl2), hence
it did not react with the given acids.
Q8. Name the type of reaction seen in the following cases:
(i) Garbage producing foul smell
(ii) Black and white photograph film when exposed to sunlight
(iii) Carbon dioxide gas passed through lime water
Ans. (i) decomposition reaction
(ii) decomposition
(iii) Combination.
Q9. Four beakers with chemicals are shown below. Name the beakers which will show
exothermic reaction and those which will be endothermic in nature.
Ans.
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H2O + K2SO4
H2O + NH4NO3
H 2O +
CuSO4(anhydrous)
H2O +
CuSO4(crystals)
Science-X
Q10. Give two examples in which reactants react to show combination reaction and a new
product is formed. This new product decomposes to give the initial reactants.
e.g., X + Y
combination
decomposition
X+Y
Ans. The reactions in which reactants combine to form products and then again
decomposes to give initial reactants are called reversible reactions.
(i) N2 + 3H2 2NH3
NH3 N2 + 3H2
Q11. Do all combination reactions get decomposed to produce the same reactants?
Ans. No.
Q12. Why are certain reagents like silver bromide stored in dark bottles in the labs?
Ans. Reagents/chemicals like silver bromide decompose when exposed to light. Hence,
they are kept in dark bottles in labs, to prevent exposure to light and their
decomposition.
Q13. Give six uses of decomposition reaction.
Ans. (i) It helps all the living matter to return back to nature, after death.
(ii) Management of garbage
(iii) Photosynthesis decomposition of water
(iv) Photography decomposition of silver bromide
(v) Used in chemical industry to obtain elements from complex compounds.
(vi) Decomposition of agricultural waste leads to formation of compost.
Q14. Draw a labeled diagram to show the decomposition of water.
Ans.
Electrolysis of water
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Q15. Mohan took pure water for the electrolytic decomposition of water but did not see any
bubbles near the electrodes. Explain why?
Ans. Pure water has covalent bond and therefore, does not allow the electricity to flow
through it and does not get decomposed. On adding a few drops of dil. acid in it, the
free ions are obtained and electricity flows through water to dissociate hydrogen and
oxygen.
Q16. A teacher took few crystals of sugar in a dry test tube and heated the test tube over
flame. The colour of sugar turned black. Explain why?
Ans. Sugar is a complex compound which on heating undergoes decomposition. Water from
sugar gets evaporated thereby leaving behind only black carbon in the test tube.
Q17. Blue crystals of copper sulphate on heating in a dry test tube become colourless. Give reasons.
Ans. The blue colour of copper sulphate is due to its crystalline nature which holds
5 water molecules (water of crystallization). On heating the water molecules
disappear and anhydrous copper sulphate (white in colour) is left back.
Q18. FeSO4 .7H2O, green colour crystals on heating, changes colour. Why?
Ans. The green colour of ferrous sulphate crystals is due to the presence of 7 water
molecules (water of crystallization). It loses the water of crystallization on heating,
thus leading to change in colour.
(i) Pickles react chemically with the metals like copper, aluminium, etc. Hence, it
should not be stored in metal containers.
(ii) Pickles contain acids which react with metals.
(iii) Nidhi showed the value of awareness and of a responsible citizen.
Q2. Sadiks younger sister was very upset as her silver jewellery had turned black and
lost its shine. Sadik washed and cleaned the jewellery with toothpaste, brought the
shine back.
(i) Why do silver jewellery turn black when kept open?
(ii) How had the toothpaste got the shine of silver back?
(iii) What value of Sadik is seen in the above act?
Ans.
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(i) Silver jewellery turns black because it reacts with gases and moisture present in
air due to which it corrodes.
Science-X
(ii) Toothpaste reacts with black coating (of silver sulphide) on silver and
removes it.
(iii) Sadik showed the value of responsible behaviour and helpful nature.
Q3. Gautam visited a government hospital to meet his cousin. He saw the medicine in dark
bottles were not stored properly. They were not kept away from light and heat. Gautam
immediately reported the issue to the medical superintendent and ensured that all
medicines are stored properly.
(i) Why are some medicines stored in cool places in dark bottles?
(ii) Why do some medicines need refrigeration?
(iii) What value of Gautam is seen in the above act?
Ans.
(i) Some medicines react when exposed to sunlight and high temperature. Their
composition changes completely.
(ii) Some medicines if not stored at appropriate temperature shows chemical
reaction and hence can become dangerous/toxic for use.
(iii) Gautam showed the value of a responsible and aware citizen.
ppp
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