Q1. Why should a
Magnesium ribbon be cleaned before burning in air?
Solution: When left unused, the
outer layer of Magnesium metal reacts with the atmospheric oxygen and forms
Magnesium Oxide (MgO) layer which is a very stable compound thus preventing
further reactions with Oxygen. It is therefore necessary to clean the ribbon by
brushing it to remove the layer of MgO so that the reaction becomes feasible
with the atmospheric oxygen.
Q2. Write the balanced
equations for the following chemical reactions.
i) Hydrogen + Chlorine→ Hydrogen Chloride
ii) Barium Chloride + Aluminium Sulphate → Barium
Sulphate + Aluminium Chloride
iii) Sodium + Water
→ Sodium Hydroxide + Hydrogen
i) H2 + Cl2 → 2HCl
ii) 3BaCl2 + Al2(SO4)3 → 2AlCl3 + 3BaSO4
iii) 2Na + 2H2O → 2NaOH + H2
Q3. Write a balanced
chemical equation with state symbols for the following reactions.
i) Solutions of
Barium Chloride and Sodium Sulphate in water react to give insoluble Barium Sulphate
and solution of Sodium Chloride.
ii) Sodium Hydroxide
solution (in water) reacts with Hydrochloric Acid solution (in water) to
produce Sodium Chloride solution and water.
i) BaCl2 + Na2SO4 → BaSO4 + 2NaCl
ii) NaOH + HCl → NaCl + H2O
Q1. A solution of a
substance ‘X’ is used for white washing.
i) Name the substance
‘X’ and write its formula.
ii) Write the
reaction of the substance ‘X’ named in (i) above with water.
(i) The substance ‘X’
which is used in whitewashing is quick lime or Calcium Oxide and
its formula is CaO.
(ii) CaO + H2O → Ca(OH)2
Q2. Why is the amount of
gas collected in one of the test tubes in Activity 1.7 double of the amount
collected in the other? Name this gas.
The chemical reaction
involved in the electrolysis of water is
2H2O(l) 2H2(g) +
(H2O) contains two parts hydrogen and one part oxygen. Therefore,
the amount of hydrogen and oxygen produced during electrolysis of water is in a
2:1 ratio. During electrolysis, since hydrogen goes to one test tube and oxygen
goes to another, the amount of gas collected in one of the test tubes is double
of the amount collected in the other.
Q1. Why does the
colour of Copper Sulphate solution change when an iron nail is dipped in it?
When an iron nail is dipped
in the copper sulphate solution, the iron displaces copper from the copper
sulphate since iron is more reactive than copper. The colour of the copper
sulphate solution changes due to this displacement. The reaction is -
+ CuSO4 → FeSO4 + Cu
Q2. Give an example
of a double displacement reaction other than the one given in Activity 1.10.
Reactions in which
there is an exchange of ions between the reactants are called double
2KBr(aq) + BaI2(aq)
→ 2KI(aq) + BaBr2(aq)
Q3. Identify the
substances that are oxidized and the substances that are reduced in the following reactions.
(i) 4Na(s) +
(ii) CuO(s) +
(i) Sodium (Na) is
oxidized with the addition of Oxygen (O2)
(ii) Copper (Cu) is
reduced when Hydrogen (H2) is added to CuO.
Q1. Which of the
statements about the reaction below are incorrect?
(a) Lead is getting
(b) Carbon Dioxide is
(c) Carbon is getting
(d) Lead Oxide is
(i) (a) and (b)
(ii) (a) and (c)
(iii) (a), (b) and (c)
Solution: (i) (a) and (b)
Explanation: (a) because Oxygen
is being removed and (b) because the removed oxygen from Lead is added to the
The above reaction is
an example of a
(a) Combination reaction
(b) Double displacement reaction.
(c) Decomposition reaction.
(d) Displacement reaction.
Solution: (d) Displacement
Explanation: The Oxygen from the
Ferrous Oxide is getting displaced to the Aluminium metal to form Aluminium
Q3. What happens when
dilute hydrochloric acid is added to iron fillings? Tick the correct answer.
(a) Hydrogen gas and Iron chloride are
(b) Chlorine gas and Iron hydroxide are
(c) No reaction takes place.
(d) Iron salt and water are produced.
Hydrogen gas and Iron Chloride
Explanation: The Chlorine from
the Hydrogen Chloride is displaced due to addition of the Iron fillings
undergoing the following reaction.
2HCl + Fe → FeCl2 + H2
Q4. What is a
balanced chemical equation? Why should a chemical equation be balanced?
A balanced chemical
equation is the one in which the number of atoms on both the sides of the
chemical equation (the reactant side and the product side) of a reaction are
equal. The chemical equation needs to be balanced so that it obeys the Law Of
Conservation of Mass.
Q5. Translate the
following statements into chemical equations and 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
(d) Potassium metal
reacts with water to give Potassium Hydroxide and Hydrogen gas.
(a) Unbalanced: H2 + N2 → NH3
Balanced: 3H2 + N2 → 2NH3
(b) Unbalanced: H2S + O2 → H2O + SO2
Balanced: 2H2S + 3O2 → 2H2O + 2SO2
(c) Unbalanced: BaCl2 + Al2(SO4)3 → AlCl3 + BaSO
Balanced: 3BaCl2 + Al2(SO4)3 → 2AlCl3 + 3BaSO4
(d) Unbalanced: K + H2O → KOH + H2
Balanced: 2K + 2H2O → 2KOH + H2
Q6. Balance the
following chemical equations.
HNO3 + Ca(OH)2 → Ca(NO3)2 + H2O
NaOH + H2SO4 → Na2SO4 + H2O
NaCl + AgNO3 → AgCl + NaNO3
BaCl2 + H2SO4 → BaSO4 + HCl
2HNO3 + 2Ca(OH)2 → 2Ca(NO3)2 + 2H2O
6NaOH + 3H2SO4 → H2SO4 + 6H2O
NaCl + AgNO3 → AgCl + NaNO3
BaCl2 + H2SO4 → BaSO4 + 2HCl
Q7. Write the
balanced chemical equation for the following reactions.
Hydroxide + Carbon Dioxide → Calcium Carbonate + Water
Silver Nitrate → Zinc Nitrate + Silver
+ Copper Chloride → Aluminium Chloride + Copper
Chloride + Potassium Sulphate → Barium Sulphate + Potassium Chloride
2Ca(OH)2 + 2CO2 → 2CaCO3 + 2H2O
Zn + 2AgNO3 → Zn(NO3)2 + 2Ag
2Al + 3CuCl3 → 2AlCl3 + 3Cu
BaCl2 + K2SO4 → BaSO4 + 2KCl
Q8. Write a balanced
chemical equation for the following and identify the type of reaction of each
Bromide(aq) + Barium Iodide(aq) → Potassium Iodide(aq) + Barium Bromide(s)
→ Zinc Oxide(s) + Carbon Dioxide(g)
+ Chlorine(g) → Hydrogen Chloride(g)
+ Hydrochloric Acid(aq) → Magnesium Chloride(aq) + Hydrogen(g)
2KBr + BaI2 → 2KI + BaBr2 (Double
ZnCO3 → ZnO + CO2 (Decomposition Reaction)
H2 + Cl → 2HCl (Combination
Mg + 2HCl → MgCl2 + H2 (Displacement Reaction)
Q9. What does one
mean by exothermic and endothermic reactions? Give examples.
An endothermic reaction
occurs when energy is absorbed from the surroundings in the form of heat (For
eg. photosynthesis, melting of ice, evaporation).
An exothermic reaction is
one in which energy is released from the system into the surroundings (For eg.
explosions, concrete setting, nuclear fission and fusion).
Q10. Why is
respiration considered an exothermic reaction? Explain.
For the survival of
life we require energy. We obtain this energy from the food we eat. The food molecules,
through the process of digestion are broken down into simpler molecule like
glucose. These substances come in contact with the Oxygen present in our body
cells to form Carbon Dioxide and water along with a certain amount of energy (Respiration
process). Since the energy is in the form of heat (that maintains our body
temperature) the respiration is considered to be an exothermic reaction.
The reaction taking
+ 6H2O + Energy
Q11. Why are
decomposition reactions called the opposite of combination reactions? Write
equations for these reactions.
is said to be the reaction between two or more molecules to form a larger
molecule whereas the decomposition reaction is defined as the splitting of a
larger molecules into two or more smaller molecules. That essentially explains
that the decomposition reaction is the opposite of the combination reaction.
In most of the cases
the decomposition reaction is endothermic since heat from the surrounding or
induced heat is used to break the bonds of the larger molecule.
Few examples of
decomposition reactions are:
CaCO3 + Energy → CaO + CO2
2HgO → 2Hg + O2
→ ZnO + CO2
Q12. Write one
equation each for decomposition reactions where energy is supplied in the form
of heat, light or electricity.
decomposition reaction (Thermolysis)
Decomposition of Potassium
heated strongly, potassium chlorate decomposes into potassium chloride and
oxygen. This reaction is used for the preparation of oxygen.
+ Heat → 2KCl + 3O2
decomposition reaction (Electrolysis)
Decomposition of Sodium
passing electricity through molten sodium chloride, it decomposes into sodium
2NaCl 2Na + Cl2
Decomposition of Hydrogen
the presence of light, hydrogen peroxide decomposes into water and oxygen.
2H2O2 2H2O + O2
Q13. What is the
difference between displacement and double displacement reactions? Write
equations for these reactions.
A displacement reaction is the one when a more
reactive substance displaces a less reactive one from its salt solution whereas
a double displacement reaction is the one where a mutual exchange of ions
happens between two compounds.
In a displacement reaction only a single displacement
takes place whereas in the double displacement reaction, as the name suggests
two displacements takes place between the molecules.
reaction: Mg + 2HCl → MgCl2 + H2
displacement reaction: 2KBr + BaI2 → 2KI + BaBr2
Q14. In the refining
of silver, the recovery of silver from Silver Nitrate solution involved
displacement by Copper metal. Write down the reaction involved.
Cu(s) + 2AgNO3(aq) →
Cu(NO3)2(aq) + 2Ag(s)
Q15. What do you mean
by precipitation reaction? Explain by giving examples.
When two solutions
containing soluble salts are combined, a double displacement reaction takes
place in which the ions are exchanged between the compounds. When one of such
compound formed is in solid form (that is insoluble in aqua) then it settles
down at the bottom of the container. This solid is known as the precipitate and
the respective reaction is termed as the precipitation reaction. Few examples
of precipitation reactions are:
CdSO4(aq) + K2S(aq) → CdS(s) + K2SO4(aq)
2NaOH(aq) + MgCl2(aq) →
2NaCl(aq) + Mg(OH)2(s)
Q16. Explain the
following in terms of gain or loss of oxygen with two examples each.
(a) In a chemical
reaction, when the oxygen is added to the element to form its respective oxide
it is the element being oxidised.
4Na(s) + O2(g) → 2Na2O(s)
H2S + O2 → H2O + SO2
(b) In a chemical
reaction, when the oxygen is being removed from the compound then it is said to
CuO(s) + H2(g) → Cu(s) + H2O(l)
2HgO → 2Hg + O2
Q17. A shiny brown
coloured element ‘X’ on heating in air becomes black in colour. Name the
element ‘X’ and the black coloured compound formed.
The shiny brown
coloured element is the Copper metal (Cu). When Copper is heated in air, it
reacts with the atmospheric oxygen to form copper oxide. Hence the black
coloured compound formed is copper oxide.
2Cu(s) + O2(g) → 2CuO(s)
Q18. Why do we apply
paint on iron articles?
When Iron articles
are exposed to moisture present in air, a layer of iron oxide is formed over
them. Iron oxide is brown in colour and commonly known as rust. Rust gradually
degrades and finally destroys the iron articles.
Hence, to prevent
iron articles from getting in contact with moisture paint is applied over them.
Painting the surface prevents Iron articles to come in contact with moisture
and air, thus preventing rusting.
Q19. Oil and fat
containing food items are flushed with Nitrogen. Why?
Food containing oil
and fat when come in contact with oxygen present in air, oxidation of fat and
oil takes place, because of which food starts giving bad taste and smell, i.e.
food becomes rancid. This process of oxidation of oil and fat present in food
items is called rancidity. Therefore by flushing Nitrogen, an unreactive
surrounding is created thus preventing rancidity.
Q20. Explain the
following terms with one example each.
(a) Corrosion is a
process where a refined metal is oxidised by the atmospheric oxygen to form a
more stable compound such as oxides. The metal gradually degrades during the
Metals react with oxygen, sulphide,
moisture, etc. present in environment and layer of different colour of metals
oxides, sulphides or hydroxide get deposited over them. Because of formation of
these layers; metals first lose their lustre and gradually get destroyed.
Rusting of iron, tarnishing of copper
and silver, etc. are some common examples of corrosion.
(b) The spoiling of
food items especially made of using oils because of hydrolysis and oxidation of
fats present in food is called rancidity.
When a food items
exposed to air, because of oxidation of fats present in food items food gets
rancid. When a food gets rancid, it gives bad taste and starts giving bad
smells. Rancidity also degrades the nutrients value of food. Eating of rancid
food may lead to health problems.