CLASS 10 CBSE FREE STUDY MATERIAL
SCIENCE STUDY MATERIAL | CHAPTER NOTES CLASS 10 | SHORT NOTES |
CHAPTER 1 - CHEMICAL REACTIONS AND EQUATIONS
1. The changes which take place in substances can be broadly
classified as physical changes and chemical changes.
classified as physical changes and chemical changes.
2. During a physical change, only the state of the substance
changes, but not its chemical composition. Furthermore, a
physical change is a temporary change, which can be reversed
by changing the physical experimental conditions. At the same
time no net energy is absorbed or given out.
changes, but not its chemical composition. Furthermore, a
physical change is a temporary change, which can be reversed
by changing the physical experimental conditions. At the same
time no net energy is absorbed or given out.
3. During a chemical change, the state as well as the chemical
composition of a substance changes. Furthermore, a chemical
change is a permanent change, which cannot be reversed by
changing the physical experimental conditions. At the same
time, either energy is absorbed or given out.
composition of a substance changes. Furthermore, a chemical
change is a permanent change, which cannot be reversed by
changing the physical experimental conditions. At the same
time, either energy is absorbed or given out.
4. Whenever, a chemical change occurs, the process which takes
place, is called a chemical reaction.
place, is called a chemical reaction.
5. During a chemical reaction one or more of the following
changes occur : (i) Energy (generally in the form of heat or light) is
either absorbed or given out. (ii) New substances are formed. (iii)
The state of the reacting substances changes. (iv) There may be a
change in colour of the reacting substances. (v) The temperature
of the reacting substances changes. (vi) A gas may be evolved
during the chemical reaction. (vii) The substances formed during
a chemical reaction do not change back to the original substance,
when the physical experimental conditions are altered.
changes occur : (i) Energy (generally in the form of heat or light) is
either absorbed or given out. (ii) New substances are formed. (iii)
The state of the reacting substances changes. (iv) There may be a
change in colour of the reacting substances. (v) The temperature
of the reacting substances changes. (vi) A gas may be evolved
during the chemical reaction. (vii) The substances formed during
a chemical reaction do not change back to the original substance,
when the physical experimental conditions are altered.
6. A chemical equation which represents a chemical reaction
briefly in words is called a word equation.
briefly in words is called a word equation.
7. The substance/substances which take part in a chemical
reaction are called reactants.
reaction are called reactants.
8. The new substance/substances formed as a result of
chemical reactions, are called products. 9. A plus sign is
put in between reactants or products, if their number is two
or more.
chemical reactions, are called products. 9. A plus sign is
put in between reactants or products, if their number is two
or more.
10. An arrow ( ) is put between reactants and products.
This sign is read as “to yield” or “to form”.
This sign is read as “to yield” or “to form”.
11. The direction of the arrow points in the direction in which
the reaction proceeds.
the reaction proceeds.
12. A chemical equation is a statement that describes a
chemical reaction in terms of symbols and formulae.
chemical reaction in terms of symbols and formulae.
13. A chemical equation expressed in symbols and formulae,
such that the number of atoms of different elements towards
the side of the reactants is not equal to number of atoms of
the same elements towards the side of the products, is called
a skeletal equation or an unbalanced equation.
such that the number of atoms of different elements towards
the side of the reactants is not equal to number of atoms of
the same elements towards the side of the products, is called
a skeletal equation or an unbalanced equation.
14. An unbalanced equation is unacceptable or is no equation,
because, it goes against the law of conservation of mass.
because, it goes against the law of conservation of mass.
15. A chemical equation in which the number of atoms of each
ellement are same on the side of reactants and products, is
called a balanced chemical equation.
ellement are same on the side of reactants and products, is
called a balanced chemical equation.
16. A balanced chemical equation tells us :
(i) about the substances taking part in a chemical reaction
and the products formed.
(ii) about the symbols and formulae of the reactants and products.
iii) about the number of atoms or molecules of the reactants and
products involved in the chemical reaction.
(iv) about the weights of the reactants and products involved
in the chemical reaction.
(i) about the substances taking part in a chemical reaction
and the products formed.
(ii) about the symbols and formulae of the reactants and products.
iii) about the number of atoms or molecules of the reactants and
products involved in the chemical reaction.
(iv) about the weights of the reactants and products involved
in the chemical reaction.
17. A balanced chemical equation does not tell us :
(i) the physical state of the reactants and products.
(ii) whether the reaction will come to completion or not.
(iii) about the speed of the chemical reaction.
(iv) about the physical conditions which bring about the
chemical reaction.
(v) about changes, such as precipitation, change in colour,
evolution of heat, light, etc., during the chemical reaction.
(i) the physical state of the reactants and products.
(ii) whether the reaction will come to completion or not.
(iii) about the speed of the chemical reaction.
(iv) about the physical conditions which bring about the
chemical reaction.
(v) about changes, such as precipitation, change in colour,
evolution of heat, light, etc., during the chemical reaction.
18. When one or more substances (elements or compounds)
undergo a chemical change, with the absorption or release
of energy (generally, heat energy) so as to form one or
more products, then the change which takes place, is called
a chemical reaction.
undergo a chemical change, with the absorption or release
of energy (generally, heat energy) so as to form one or
more products, then the change which takes place, is called
a chemical reaction.
19. Chemical composition reaction : When two elements or
compounds react chemically to form a single new compound,
the chemical reaction which takes place is called a chemical
composition reaction or a chemical combination reaction.
A + B AB.
compounds react chemically to form a single new compound,
the chemical reaction which takes place is called a chemical
composition reaction or a chemical combination reaction.
A + B AB.
20. Chemical decomposition reaction : When a chemical
compound decomposes on heating or absorbing some other
kind of energy, so as to form two or more new substances
(elements or compounds), then the chemical reaction which
takes place is called a chemical decomposition reaction
or a chemical decombination reaction. AB A + B Chemical
decomposition reaction can be further classified into three types.
(a) Thermal decomposition reaction : When a chemical compound
decomposes on heating so as to form one or more substances
(elements or compounds), then the chemical reaction is called a
thermal decomposition reaction.
(b) Photo-decomposition reaction : When a chemical compound
decomposes on absorbing light energy, so as to form two or more
different substances, then the reaction which takes place is called
a photo-decomposition reaction.
(c) Electrochemical reaction : When a chemical compound in an
aqueous or fused state decomposes into two different substances
on the passage of electric current then the reaction is called an
electrochemical reaction.
compound decomposes on heating or absorbing some other
kind of energy, so as to form two or more new substances
(elements or compounds), then the chemical reaction which
takes place is called a chemical decomposition reaction
or a chemical decombination reaction. AB A + B Chemical
decomposition reaction can be further classified into three types.
(a) Thermal decomposition reaction : When a chemical compound
decomposes on heating so as to form one or more substances
(elements or compounds), then the chemical reaction is called a
thermal decomposition reaction.
(b) Photo-decomposition reaction : When a chemical compound
decomposes on absorbing light energy, so as to form two or more
different substances, then the reaction which takes place is called
a photo-decomposition reaction.
(c) Electrochemical reaction : When a chemical compound in an
aqueous or fused state decomposes into two different substances
on the passage of electric current then the reaction is called an
electrochemical reaction.
21. Chemical displacement reaction : When a more active element
displaces a less active element from its aqueous ionic solution,
the reaction which takes place is called a chemical displacement
reaction.
displaces a less active element from its aqueous ionic solution,
the reaction which takes place is called a chemical displacement
reaction.
22. Metals arranged in tabular form in the order of their
decreasing chemical activity is called metal activity series.
K > Na > Ca > Mg > Al > Zn > Fe > Sn > Pb > [H] > Cu > Hg > Ag > Au.
The above list constitutes the metal activity series in which
potassium is the most active metal and gold is the least
active metal.
decreasing chemical activity is called metal activity series.
K > Na > Ca > Mg > Al > Zn > Fe > Sn > Pb > [H] > Cu > Hg > Ag > Au.
The above list constitutes the metal activity series in which
potassium is the most active metal and gold is the least
active metal.
23. Chemical double displacement reaction : A chemical reaction
in which two ionic compounds in their aqueous solutions, react
by exchanging their ions/radicals, to form two new compounds
is called a chemical double displacement reaction.
A+B– + C+D– A+D– + C+B–.
Chemical double displacement reaction can be further classified
into two kinds.
(a) Precipitation reaction : When aqueous solutions of two ionic
compounds react by exchanging their ions/radicals, to form
two or more compounds, such that one of the products formed
is an insoluble salt, and hence, forms a precipitate, the double
displacement reaction is said to be a precipitation reaction
. (b) Neutralisation reaction : When an aqueous solution of an
acid reacts with a base (alkali) by exchanging their ions/ radicals
to form salt and water as the only products, the reaction which
takes place is called a neutralisation reaction.
in which two ionic compounds in their aqueous solutions, react
by exchanging their ions/radicals, to form two new compounds
is called a chemical double displacement reaction.
A+B– + C+D– A+D– + C+B–.
Chemical double displacement reaction can be further classified
into two kinds.
(a) Precipitation reaction : When aqueous solutions of two ionic
compounds react by exchanging their ions/radicals, to form
two or more compounds, such that one of the products formed
is an insoluble salt, and hence, forms a precipitate, the double
displacement reaction is said to be a precipitation reaction
. (b) Neutralisation reaction : When an aqueous solution of an
acid reacts with a base (alkali) by exchanging their ions/ radicals
to form salt and water as the only products, the reaction which
takes place is called a neutralisation reaction.
24. Exothermic reactions : A chemical reaction which proceeds
with the release (evolution) of heat energy, is called an
exothermic reaction.
with the release (evolution) of heat energy, is called an
exothermic reaction.
25. Endothermic reaction : A chemical reaction which proceeds
with the absorption of heat energy, is called an endothermic
reaction.
with the absorption of heat energy, is called an endothermic
reaction.
26. Oxidation reaction : When a substance gains oxygen or loses
hydrogen, the reaction taking place is called an oxidation reaction
.
hydrogen, the reaction taking place is called an oxidation reaction
.
27. Reduction reaction : When a substance gains hydrogen or
loses oxygen, the reaction taking place is called as reduction
reaction.
loses oxygen, the reaction taking place is called as reduction
reaction.
28. Corrosion of metals : Formation of layers of undesired
compounds, such as metallic oxides or hydroxides on the
surface of metals is called a corrosion of metals.
compounds, such as metallic oxides or hydroxides on the
surface of metals is called a corrosion of metals.
29. Rusting : Slow conversion of iron into hydrated ferric oxide,
in the presence of moisture and air is called rusting.
in the presence of moisture and air is called rusting.
30. Rust : The flaky, non sticky brown powder formed on the
surface of iron, when iron is exposed to moist air, is called rust.
surface of iron, when iron is exposed to moist air, is called rust.
31. Conditions for rusting : Iron should be exposed to :
(i) water, (ii) air, at the same time.
(i) water, (ii) air, at the same time.
32. Rancidity : The oxidation of food materials, so that they
become stale and start smelling is called rancidity.
become stale and start smelling is called rancidity.
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