Q.
What is chemical equation?
Ans.
A chemical equation is symbolic representation of chemical reaction. It uses
chemical symbols and formulae to present the reactants and products.
For
example
2H2 + O2 > 2H2O
Above
mentioned example is a symbolic expression of a chemical equation. It has two
sides – LHS and RHS. Both the sides are separated by arrow ( à) symbol. Substances on LHS are
Reactants, while substances on RHS are Products.
In
the above chemical reaction, 2H2O and O2 ( on LHS ) are Reactants, whereas 2H2O ( on RHS ) are Products.
The
equation tells us when 2 molecules of hydrogen react with 1 molecule of oxygen;
it produces 2 molecules of water.
 |
| Law of conservation of mass |
Q. What is law of conservation of mass?
Ans.
Law of conservation of mass states that
in any system, mass can neither be created nor be destroyed. It comes out from
Einstein’s theory of Relativity. The law is much useful in creating a
balanced chemical equation.
 |
| skeletal equation |
Q. What is a Skeletal equation?
Ans.
A Skeletal equation states which substances are reactants and which are
products only. It is not a balanced chemical equation. It needs to be further
balanced in terms of number of elemental atoms on both the sides. It is only a
rough expression of reactants and products.
For
example
Fe
+ H2O > Fe2O4 + 4H2
In
the above mentioned reaction we can clearly see that number of atoms of Fe, H,
O are not equal in both the sides.
As
it only tells us the reactants and products without keeping the atoms of
elements equal. It is skeletal equation.
 |
| balanced chemical equation |
Q. What is a balanced chemical equation?
Ans.
A balanced chemical equation is that equation in which number of elemental
atoms is equal on both the sides of an equation.
For
example
2H2 + O2 à 2H2O
Let’s
understand the above example more closely,
·
H2 means one molecule of hydrogen has
two hydrogen atoms.
·
O2 means one molecules of oxygen has
two oxygen atoms.
·
2H2 means two molecules of hydrogen,
and two hydrogen moles means four hydrogen atoms
So,
LHS has 4 atoms of hydrogen and two atoms of oxygen. Now on RHS,
·
There are two molecules of water
·
One molecule of water has 2 hydrogen atoms and 1 oxygen
atom,
·
Two molecule of water has 4 atoms of hydrogen and 2
atoms of oxygen.
So,
LHS = 4 hydrogen atoms, 2 oxygen atoms
RHS=
4 hydrogen atoms, 2 oxygen atoms.
Thus
we can say above mentioned equation is a balanced chemical equation.
NCERT QUESTION (page 13)
 |
| Why does the color of copper sulphate change when an iron nail is dipped in it? |
Q1.
Why does the color of copper sulphate change when an iron nail is dipped in it?
Ans.
The
color of copper sulphate changes because iron replaces copper in this process
the quantity of copper sulphate decreases. It results change in colour in the
solution. Iron also gets brownish.
Fe
+ CuSO4 à
FeSO4 + Cu
 |
| Give an example of double displacement reaction other than the one given in Activity 1.10. |
Q2.
Give an example of double displacement reaction other than the one given in
Activity 1.10.
Ans.
When
Sodium Thiosulfate ( Na2S) reacts with hydrochloric acid (HCl) it
forms Sodium Chloride (NaCl) and Hydrogen Sulfide (H2S).
Na2S
+ HCl à
NaCl + H2S
In
the above example,
Sodium
(S) replaces Chlorine (Cl) resuting
Sodium
Thiosulfate becomes Hydrogen Sulfate.
Na2S
- H2S
Chlorine
(Cl) replaces Sodium resulting
Hydrochloric
acid ( HCl) becomes Sodium Chloride (NaCl)
HCl
- NaCl
As
two elements are displacing each other simultaneously, this is an example of
double displacement reaction.
 |
| Identify the substances that are oxydised and the substances that are reduced in the following reaction: |
Q3.
Identify the substances that are oxydised and the substances that are reduced
in the following reaction:
(1)
4Na + O2 > 2Na2O
(2)
CuO + H2 > Cu + H2O
Ans.
(1)
Substances that are oxidised are, Na ; H2
(2)
Substances that are reduced are, O2 ; CuO
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