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Q 1. Zn + 2HCl → ZnCl2 +
H2 In the given equation, how will you depict that it is
accompanied by evolution of gas and heat?
Solution
We can show heat
produced as "+ Heat" on the product side and gas produced with an
upward arrow ↑. Zn + 2HCl → ZnCl2 + H2 ↑ +
Heat
Q 2. Give an example of an
endothermic and exothermic reaction.
Solution
Endothermic reaction:
The reaction between barium hydroxide and ammonium chloride is an example of an
endothermic reaction. Exothermic reactions: Burning of L.P.G in the presence of
air is an example of the exothermic reaction since it is accompanied by the
evolution of a large amount of heat.
Q 3. Why do we need to
clean magnesium ribbon before burning it? Why is it advised to burn magnesium
ribbon at a distance far from eyes?
Solution
We need to clean
magnesium ribbon before burning it to remove the protective layer of basic
magnesium carbonate which is formed by the reaction of magnesium with moist
air. The bright white light given out during the burning of magnesium ribbon is
harmful to the eyes so, the magnesium ribbon is burned by keeping it as far as
possible from the eyes.
Q 4. Why decomposition
reactions are called the opposite of combination reactions?
Solution
In combination
reactions, two substances combine to form one compound and in decomposition
reactions, a compound breaks down into two or more substances, so they are
opposite to each other.
Q 5. How does a chemical
reaction result in the formation of new products?
Solution
The rearrangement of
atoms results in the formation of new substances with properties, entirely
different from those of the reactants. During a chemical reaction, the already
existing bonds between reactants are broken and new bonds are formed leading to
the formation of products.
Q 6. Give two merits of
chemical equation.
Solution
The two merits of
chemical equations are: (i) Representation of a chemical reaction becomes easy
as equations save time and space. Since there is uniformity in symbols all over
the world, scientists can exchange data and understand each other’s work
easily. (ii) From a balanced equation, we can easily calculate the amount of
reactants and products.
Q 7. What is a skeletal
equation?
Solution
A chemical equation
expressed in symbols and formulae such that number of atoms of different
elements on the side of reactants is not equal to the number of atoms on
the side of products.
Q 8. Define oxidizing
agent and reducing agent.
Solution
In redox reactions, the
substance which gives oxygen or takes away hydrogen is called oxidizing agent.
On the other hand, the substance which takes away oxygen or gives hydrogen is
called reducing agent.
Q 9. Name and state the
law which is followed when we balance any chemical equation.
Solution
Chemical equations are
balanced to satisfy the Law of conservation of mass and this law states that,
mass can neither be created nor can be destroyed during a chemical reaction.
Q 10. How can we make
equations more informative?
Solution
The equations can be
made more informative in three ways: 1. by indicating the physical states of
the reactants and products. 2. by indicating the heat changes taking place in
the reaction. 3. By indicating the conditions under which the reaction takes
place like temperature, pressure, presence of catalyst etc.
Q 11. Which one is a
chemical change - rusting of iron or melting of iron? Explain.
Solution
Rusting of iron is a
chemical change, it involves the formation of a new product, rust. Rust cannot
be converted back to iron by any process.
Q 12. Give example of
reactions accompanied by: (a) Formation of precipitate. (b) Change in state.
Solution
(a) When a solution of
silver nitrate is added to a solution of sodium chloride, a white precipitate
of silver chloride is formed. AgNO3 + NaCl →
AgCl ↓ + NaNO3 (b) Ammonia gas reacts with
hydrogen chloride gas to form ammonium chloride, which is in a solid state. NH3(g)
+ HCl(g) 
NH4Cl(s)
NH4Cl(s)
Q 13. Define the term
rancidity.
Solution
The oils and fats react
with oxygen and become oxidized or turn rancid. This process is called
rancidity.
Q 14. Can we store iron
sulphate solution in a zinc container? Give reason for your answer.
Solution
Zinc being more reactive
than iron will displace it and form a colorless solution of zinc sulphate. This
could even lead to appearance of holes in zinc container. Hence, we cannot
store iron sulphate in a zinc container.
Q 15. What are double
displacement reactions? Give one special feature of such reactions.
Solution
Reactions in which two
compounds react with each other and exchange their ions to form two new
compounds, are called double displacement reaction. In these reactions, two
compounds swap components, in the format: AB + CD → AD + CB The reactants of
double displacement reaction are in aqueous form i.e. solution and are often
accompanied by precipitation. In other words, a solid product separates or
settles at the bottom of the solution.
Q 16. When magnesium metal
is reacted with sulphuric acid, magnesium sulphate and hydrogen gas are formed
with the evolution of heat. Mention the observations that will help to
determine that reaction has taken place.
Solution
The chemical reaction
between magnesium and sulphuric acid can be observed by: (a) Evolution of
hydrogen gas (b) Change in temperature i.e., there is evolution of heat during
the reaction.
Q 17. How do we represent
a chemical reaction?
Solution
The chemical reactions
are represented with the help of symbols or formulae of various reactants and products,
with a (+) sign between them. The reactants are written on the left side and
products on right side. The transformation of reactants into products is shown
by an arrow (→).
Q 18. Why is electrolysis
of water an endothermic reaction?
Solution
Electrolysis of water to
form oxygen and hydrogen is an endothermic reaction because electrical energy
is absorbed during this reaction.
Q 19. What are
Decomposition reactions? How do they take place?
Solution
Decomposition reactions
are those reactions in which a compound breaks or splits into two or more
simpler substances. In such reactions, one reactant breaks up to form many i.e.
at least two products. These reactions require energy to break the already existing
bonds, therefore they take place in presence of heat, light or electricity.
Q 20. FeSO4(aq) +
2NaOH(aq) → Fe(OH)2 ↓ + Na2SO4(aq) What
information is conveyed from the above equation?
Solution
An aqueous solution of
ferrous sulphate reacts with aqueous solution of sodium hydroxide to form a
precipitate of ferrous hydroxide and sodium sulphate remains in the solution.
Q 21. (a) Oil and fat
containing food items are flushed with nitrogen gas while packing them, why?
(b) What is rust? What is its formula?
Solution
(a) Oil and fat
containing food items are flushed or surrounded with an inert gas such as
nitrogen while packing them to prevent their contact with oxygen present in the
air. This is done to avoid rancidity of fats and oils. (b) When iron objects are
exposed to moisture, they combine with oxygen from the air in the presence of
water or moisture to form a red-brown flaky substance called rust. This process
is commonly called rusting. Its formula is Fe2O3.xH2O.
Q 22. State one basic
difference between a physical change and a chemical change.
Solution
Physical change: No new
substance is formed. Chemical change: New substance is formed.
Q 23. Name two
anti-oxidants which are usually added to fats and oils containing foods to
prevent rancidity.
Solution
BHA and BHT are the
common anti-oxidants used in foods to prevent rancidity.
Q 24. Identify the
substance that is oxidised and reduced in the reaction: CuO(s) +
Zn(s) → Cu(s) + ZnO(s)
Solution
CuO is reduced and Zn is
oxidized.
Q 25. Why is there a
change in temperature in a chemical reaction? What happens when quicklime
is added to water filled in a bucket?
Solution
Some chemical reactions
are characterised by a change in temperature. When a chemical reaction produces
heat energy, then the temperature of reaction mixture rises and it becomes hot.
However, many a times, chemical reactions absorb heat energy, then the
temperature of reaction mixture falls and it becomes cold. So, a change in
temperature in a chemical reaction can be 'a rise in temperature' or 'fall in
temperature'. When quicklime reacts with water filled in a bucket, slaked lime
is formed and a lot of heat is produced. The bucket feels to be quite hot on
touching.
Q 26. X + YSO4 →
XSO4+ Y Y + XSO4 → No reaction Explain the reason
for the above:
Solution
'X' is more reactive
than 'Y' hence 'X' is able to displace 'Y' from its salt whereas 'Y' is not
able to displace 'X' from its salt because it is less reactive than 'X'.
Q 27. Mention two chemical
reactions that are characterised by the evolution of gas.
Solution
1. When zinc granules
react with dilute sulphuric acid, then bubbles of hydrogen gas are produced.
2. The chemical reaction
between sodium carbonate and dilute hydrochloric acid is characterised by the
evolution of carbon dioxide gas. 
2. The chemical reaction
between sodium carbonate and dilute hydrochloric acid is characterised by the
evolution of carbon dioxide gas.
Q 28. (a) What are
anti-oxidants? State two examples. (b) How is rancidity prevented using
anti-oxidants?
Solution
(a) Anti-oxidant is a
substance which prevents oxidation. These are reducing agents. Examples: BHA
and BHT. (b) When anti-oxidants are added to foods, then the fats and oils
present in them do not get oxidised easily and hence do not turn rancid. So,
the food remains good to eat for a much longer time.
Q 29. State the main
difference between an endothermic reaction and an exothermic reaction.
Solution
Exothermic reaction:
reactions involving the release of heat energy. Endothermic reaction: reactions
involving absorption of heat energy.
Q 30. Identify the substance
oxidized, substance reduced, oxidizing agent and reducing agent in the
following reaction: CuO + H2 → Cu + H2O
Solution
Oxidised: H2 Reduced
: CuO Oxidising agent : CuO Reducing agent : H2
Q 31. Cu + AgNO3 →
CuNO3 + Ag Fe + CuSO4 → FeSO4 +
Cu Zn + FeSO4 → ZnSO4 + Fe From above
information arrange the given metals in increasing order of reactivity. Give
reason for your choice.
Solution
Since zinc displaces
iron, it is more reactive than iron but iron displaces copper therefore iron
comes above copper in reactivity series. Similarly, copper displaces silver
that is silver is least reactive. Ag < Cu < Fe < Zn
Q 32. How will you
incorporate the following information into an equation? (i) Presence of a
catalyst (ii) Temperature and pressure conditions (iii) Evolution of a gas
Solution
(i) Catalyst is written
above the arrow. (ii) The temperature and pressure conditions are
written above or below the arrow. (iii) Evolution of a gas is depicted by an
upward arrow ↑.
Q 33. Giving one example
of each, define the following terms: (i) Corrosion (ii) Rancidity
Solution
(i) The eating up of
metals by the action of air, moisture or a chemical (such as an acid) on their
surface is called corrosion. For example: When an iron object is left in damp
air (or water) for a considerable time, it gets covered with a red-brown flaky
substance called rust. This is called rusting of iron. (ii) The condition
produced by aerial oxidation of fats and oils in foods marked by unpleasant
smell and taste is called rancidity. It spoils the food materials prepared in
fats and oils which have been kept for a considerable time and makes them unfit
for eating. For example: Potato chips are made in oil. On keeping them exposed
for a long time, these chips start giving unpleasant smell and taste. They turn
rancid.
Q 34. What are
displacement reactions? Explain with an example.
Solution
Displacement reactions
are those reactions in which one element takes the place of another element in
a compound. The more reactive element displaces the less reactive element from
its compound. AB + C → AC + B Example: Zn + CuSO4 → ZnSO4 +
Cu Here Zinc displaces Copper from copper sulphate solution to form zinc
sulphate and Copper.
Q 35. What is meant by a
chemical reaction?
Solution
A process in which two
or more substances react to form some other new substances with new set of
properties is called a chemical reaction.
Q 36. Give three uses of
decomposition reactions.
Solution
Uses of decomposition
reactions are as follows: 1. Electrolytic decomposition is used to
obtain metals from their chloride or oxide ores. 2. In digestion, the
process of the breakup of starch and other carbohydrates into simple sugars
such as glucose in the presence of enzymes is a catalytic decomposition
reaction. 3. Decomposition reactions are used in photography. The photographic
film has a coating of silver bromide which on exposure to light splits into
silver and bromine.
Q 37. Hydrogen being a
highly inflammable gas and oxygen being a supporter of combustion, yet water
which is a compound made up of hydrogen and oxygen is used to extinguish fire.
Why?
Solution
During a chemical
combination, the product formed has its own set of properties and the reactants
fail to retain their properties. So, water has different properties from its
reactants and is hence used for extinguishing fires.
Q 38. Why do we balance a
chemical equation?
Solution
As per law of
conservation of mass, "matter cannot be created or destroyed".
Therefore, total mass of reactants should be equal to the total mass of
products. To have that, we must have equal number of atoms on reactant side and
product side. In this way, we balance a chemical equation.
Q 39. Describe three
reactions to show that chemical reactions are characterised by a change in
colour.
Solution
a. The chemical reaction
between citric acid and purple coloured potassium permanganate solution is
characterised by a change in colour from purple to colourless. b. The chemical
reaction between sulphur dioxide gas and acidified potassium dichromate
solution is characterised by a change in colour from orange to green. c. The
chemical reaction between iron and blue coloured copper sulphate is
characterised by a change in colour from blue to light green.
Q 40. On heating a colourless powder, it forms
yellow crystals and brown gas is evolved. Identify the reactants and products.
Give a balanced equation to represent the reaction.
Solution
The colourless reactant
is lead nitrate which on heating decomposes into yellow precipitate of lead
monoxide and brown fumes of nitrogen dioxide gas is evolved. Colourless and
odourless oxygen gas is also produced. It is a decomposition reaction. 2Pb(NO)3 →
2PbO + 4NO2 ↑ + O2 ↑
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