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Exam Focus:
Chemical reaction observations, chemical equations,
balancing, combination, decomposition, displacement,
double displacement, precipitation, exothermic and
endothermic reactions, oxidation, reduction, corrosion
and rancidity.
A. Very Short Answer Questions
Question 1
What is a chemical reaction?
A chemical reaction is a process in which one or more
substances are converted into one or more new substances
having different properties.
Question 2
Write any two observations that may indicate that a chemical
reaction has occurred.
Examples include a change in colour, evolution of gas,
formation of a precipitate, change of state, or a change
in temperature.
Question 3
What are reactants?
The substances that take part in a chemical reaction are
called reactants.
Question 4
What are products?
The new substances formed during a chemical reaction are
called products.
Question 5
Why should a chemical equation be balanced?
A chemical equation is balanced to satisfy the law of
conservation of mass, so that the number of atoms of each
element is equal on both sides.
B. Chemical Equations & Balancing
Question 6
Balance the following equation:
Balanced equation:
2H₂ + O₂ → 2H₂O
Question 7
Why should the subscripts in a chemical formula not be
changed while balancing an equation?
Changing a subscript changes the chemical identity of the
substance. Balancing is done by changing only the
coefficients in front of chemical formulae.
Question 8
What information do the coefficients in a balanced chemical
equation provide?
They give the relative numbers of molecules or formula units,
and in stoichiometric interpretation they also represent
relative mole ratios of the substances involved.
C. Types of Chemical Reactions
Question 9
What is a combination reaction? Give one example.
A combination reaction is one in which two or more substances
combine to form one main product.
Question 10
What is a decomposition reaction?
A decomposition reaction is a reaction in which a single
compound breaks down into two or more simpler substances.
Question 11
Give one example of thermal decomposition.
Question 12
Give one example of photochemical decomposition.
Silver chloride decomposes in the presence of sunlight.
Question 13
What is a displacement reaction?
A displacement reaction is one in which a more reactive
element displaces a less reactive element from its compound.
Question 14
Identify the type of reaction:
It is a displacement reaction.
Question 15
What is a double displacement reaction?
A double displacement reaction involves exchange of ions
between two compounds to form two new compounds.
Question 16
What is a precipitation reaction?
A reaction in which an insoluble solid separates from a
solution is called a precipitation reaction.
Question 17
Identify the precipitate in the following reaction:
Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl
The precipitate is barium sulphate (BaSO₄).
D. Exothermic & Endothermic Reactions
Question 18
What is an exothermic reaction?
A reaction that releases energy, usually as heat, to the
surroundings is called an exothermic reaction.
Question 19
What is an endothermic reaction?
A reaction that absorbs energy from its surroundings is
called an endothermic reaction.
E. Oxidation & Reduction
Question 20
What is oxidation in terms of oxygen?
Oxidation can be described as the addition of oxygen to
a substance.
Question 21
What is reduction in terms of oxygen?
Reduction can be described as the removal of oxygen from
a substance.
Question 22
Identify oxidation and reduction:
CuO loses oxygen and is reduced to Cu.
H₂ gains oxygen and is oxidised to H₂O.
Question 23
What is a redox reaction?
A reaction in which oxidation and reduction occur
simultaneously is called a redox reaction.
F. Corrosion & Rancidity
Question 24
What is corrosion?
Corrosion is the gradual deterioration of a metal because
of chemical reactions with substances in its environment.
Question 25
Mention two conditions necessary for rusting of iron.
Question 26
Mention two methods used to prevent corrosion.
Painting, oiling, greasing, galvanisation and alloying are
common methods of protecting metals from corrosion.
Question 27
What is rancidity?
Rancidity is the oxidative spoilage of fats and oils that
causes unpleasant smell and taste.
Question 28
How can rancidity be prevented?
-
Store food in airtight containers.
-
Refrigerate suitable foods.
-
Use antioxidants where appropriate.
-
Use suitable inert-gas packaging in some packaged
foods.
G. Reasoning-Based Questions
Question 29
Why is magnesium ribbon cleaned before burning?
Magnesium develops a coating of magnesium oxide on its
surface. Cleaning removes this coating so that magnesium
can react more readily with oxygen.
Question 30
Why does a reaction involving a gas sometimes show visible
bubbles?
The bubbles indicate that a gaseous product is being formed
and released during the reaction.
Question 31
Why can a precipitate be used as evidence of a chemical
reaction?
Formation of a new insoluble substance indicates that the
original substances have undergone a chemical change.
H. Competency-Based Practice
Question 32
A student writes an equation for a reaction but the number
of oxygen atoms is different on the two sides. What should
the student do?
The equation should be balanced by changing suitable
coefficients in front of the formulae without changing
the chemical formulae themselves.
Question 33
Two salt solutions are mixed and a white insoluble solid
appears. Which type of reaction is most likely occurring?
It is most likely a double displacement reaction accompanied
by formation of a precipitate.
Question 34
A reaction requires heat continuously to proceed. What type
of energy change may be involved?
The reaction may be endothermic because it absorbs energy
from the surroundings.
Question 35
A metal is placed in a salt solution and another metal
separates from the solution. What does this indicate?
It indicates a displacement reaction in which the added
metal is more reactive than the metal displaced from its
compound.
I. Long Answer / Conceptual Questions
Question 36
Explain the different types of chemical reactions with
suitable examples.
Combination:
Two or more substances combine to form one main product.
Decomposition:
One compound breaks into simpler substances.
Displacement:
A more reactive element displaces a less reactive element.
Double displacement:
Ions are exchanged between two compounds.
Question 37
Explain oxidation, reduction and redox reactions using a
suitable chemical equation.
Consider:
Copper oxide loses oxygen and is reduced to copper.
Hydrogen gains oxygen and is oxidised to water.
Since oxidation and reduction occur together, the reaction
is a redox reaction.
J. Quick Revision Questions
Try answering these without looking at the answers:
-
Why is balancing a chemical equation necessary?
-
What is the difference between combination and
decomposition?
-
What is a displacement reaction?
-
What is the role of a precipitate in identifying a
chemical reaction?
-
Differentiate oxidation and reduction.
-
What are the conditions needed for rusting?
-
What is rancidity and how can it be controlled?