The key ways to prevent rancidity mentioned in the passage are: (A) Adding antioxidants, which are substances that prevent oxidation (B) Refrigeration (C) Storing the food in air-tight containers to prevent exposure to oxygen in the air The correct answer is A, B, and C - adding antioxidants, refrigeration, and storing in air-tight containers all help prevent oxidation and thus rancidity.

2. •A chemical reaction involves chemical change
in which substances react to
form new substances with entirely new
properties.
•During a chemical reaction, atoms of one
element don’t change in to those of another
element. Only a rearrangement of atoms takes
place in a chemical reactions.
•During a chemical reaction, there is a
breaking of bonds between atoms of the
reacting molecules to give products
•Matter can neither be created nor destroyed
in a chemical reaction.

3. A chemical reaction can be observed
with the help of any of the following
observations:-
a. Evolution of a gas
b. Change in temperature
c. Formation of a precipitate
d. Change in colour
e. Change of state

4. Physical change: If a change involves change
in colour or state but no new
substance is formed, then it is a physical change.
Chemical change: If a change involves
formation of new substances, it is a
chemical change.
Exothermic and endothermic reactions: If
heat is evolved during a reaction,
then such a reaction is known as Exothermic
reaction. If heat is absorbed from
the surroundings, then such a reaction is known as
endothermic reaction.

5. Chemical equation:-
The method of
representing a chemical
reaction with the help of
symbols and formulae of
the substances involved in
it is known as chemical
equation.

6.  Reactants – the substances that exist before a
chemical change (or reaction) takes place.
 Products – the new substance(s) that are formed
during the chemical changes.
 CHEMICAL EQUATION indicates the reactants and
products of a reaction.
REACTANTS  PRODUCTS

7. Features of a chemical
equation:-
a. The reactants are written on the left hand
side with a plus sign between
them.
b. The products are written on the right hand
side with a plus sign between
them.
c. An arrow separates the reactants from the
products. The arrow head points
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towards the products and indicates the
direction of the reaction.

8. “Yields”; indicates result of reaction
Used to indicate a reversible reaction
A reactant or product in the solid state;
also used to indicate a precipitate
(s)
Alternative to (s), but used only to indicate a precipitate
A reactant or product in the liquid state
(l) A reactant or product in an aqueous solution
(dissolved in water)
A reactant or product in the gaseous state
(aq)
(g) A reactant or product in the gaseous state

9. Unbalanced and Balanced Equations
H
Cl Cl
H H
Cl H
Cl Cl Cl
H
Cl
H
H
H2 + Cl2  HCl (unbalanced) H2 + Cl2  2 HCl (balanced)
reactants products reactants products
H 2 1 H 2 2
Cl 2 1 Cl 2 2

10. Balanced chemical equation: A balanced
equation is a chemical equation in
which number of atoms of each element is equal on both sides
of the equation
i.e. number of atoms of an element on reactant side = number
of atoms of
that element on the product side.
As per the law of conservation of mass, the total mass of the
elements present
in the products of a chemical reaction is equal to the total mass of
the
elements present in the reactants.
Unbalanced chemical equation:- An unbalanced
chemical equation has an unequal no. of atoms of one or more
elements in the reactants and products.
•An unbalanced equation has unequal masses of various
elements in reactants and products.

11. Examples:-
NO(g) + O2(g)  NO2(g)
NO(g) + O(g)  NO2(g)
NO(g) + ½ O2(g)  NO2(g)

13. The reactivity
series
•The reactivity series is
a list of metals
arranged in the order
of decreasing
reactivity. The most
reactive metal is
placed at the top and
the least reactive
metal is placed at the
bottom.

15. Combination reaction
Combination reaction is a reaction in which 2 or
more substances combine to
give a single product. Combination reaction can
be between 2 elements,
between an element and a compound or
between 2 compounds
2 Na + Cl2  2 NaCl
Na Cl
Na Cl
General form: A + B  AB
element or element or compound
compound compound

16. Photosynthesis
C6 2 +
O H2O6 C6H12O6 + O2 6
Formation of water
2 H2 + O2 H2O 2
Formation of salt
2 Na + Cl2 NaCl 2
General Form
A + B C

17. Decomposition reaction: -
In a decomposition reaction, a single reactant
decomposes to give 2 or more products. Decomposition
reactions require
energy in the form of heat, light or electricity
Types of decomposition reactions:
a. Decomposition reactions which require heat are
known as thermolytic decomposition reactions
b. Decomposition reactions which require light are
known as photolytic decomposition reactions
c. Decomposition reactions which require electricity are
known as electrolytic decomposition reactions

18. Decomposition reaction
2 H 2O 2 H2 + O2
H
O
H
+
H
O
H
General form: AB A + B
compound two or more elements
or compounds

19. Hydrogen Peroxide
2 H2O2 H2O2 + O2
Electrolysis of water
electricity
2 H2O H2 2
+ O2
Nitrogen triiodide
2 NI3 N2 + I2 3
General Form
AB A + B

20. Displacement
reaction:-
Those reaction in which one element
takes the place of another element in
a compound, are known as
displacement rection.
Single-replacement reaction
Mg + CuSO4  MgSO4 + Cu
General form:
A + BC  AC + B

21. Double displacement reaction:-
A chemical reaction in which there is
an exchange of ions between the
reactants to give new substances is
called double displacement reaction
Double-replacement reaction
CaCO3 + 2 HCl  CaCl2 + H2CO3
General form:
AB + CD  AD + CB

22. If a substance gains oxygen or loses hydrogen
during a reaction, it is said to be oxidised.

23. •A substance that loses oxygen or gains
hydrogen is known as an oxidising
agent.
•A substance that loses hydrogen or gains
oxygen is known as a reducing
agent.
•An oxidising agent gets reduced
whereas a reducing agent gets
oxidised
•The oxidation and reduction reaction
are also called redox reaction.

26. Rancidity can be prevented by:
(A) Adding antioxidants i.e. the substances which
prevent oxidation
(B) Refrigeration
(C) Storing the food in air-tight containers