Physical change:
A temporary change, in which no new substance is formed and the chemical composition of the original substance remains the same, even though some of its physical properties like color, state, shape, size, etc. may change. For example, the melting of ice.
Melting of iron is a physical change because when the iron is heated it changes from one state to other but the chemical properties remain the same.
Chemical change:
A permanent change in which new substances are formed whose chemical and physical properties are completely different from that those of the original substance. For example, the burning of fuel.
During rusting of iron the new substance i.e rust (red-brown solid) is formed when iron comes in contact with air and water.
Thus rusting of iron is a chemical change.

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.

Electric power, p = \( \frac{V^{2}}{R}\), where R
V = Potential difference across the circuit
For 40 W bulb, 40 =\(\frac{V^{2}}{R_{1}0}\), where
R₄₀ = Resistance offered by the 40 W bulb 
For 60 W bulb, 60 =\(\frac{V^{2}}{R_{6}0}\) , where
R₄₀ = Resistance offered by the 60 W bulb
Therefore,  \(R_{10}=\frac{V^{2}}{40}\) and \(R_{60}=\frac{V^{2}}{60}\)  (as the voltage difference remains the same)
Therefore, R₄₀ > R₆₀
So, the 40 W bulb has a higher electrical resistance when in use. 

A measuring instrument used to measure potential difference between two points in an electric circuit.

The flow of electric charge is known as electric current. Electric current is carried by moving electrons through a conductor. Electric current flows in opposite direction to the movement of electrons.

A catalyst changes the rate of reaction without taking part in the reaction itself.

i. Neutralization reaction
ii. Oxidation reaction

3Fe(s) + 4H₂O(g) →→ Fe₃O₄(s)  + 4H₂(g)

(a) When magnesium is heated in oxygen of air, it leaves white powder of magnesium oxide which dissolves in water. The magnesium oxide being alkaline in nature turns red litmus to blue. Also, the metal in ribbon form is Magnesium [Mg]. Thus 'X' is identified ad magnesium and 'Y' is identified as magnesium oxide.
(b)The chemical reaction is:
2Mg(s)+O₂(g)\(\rightarrow\)2MgO

Magnesium ribbon reacts with oxygen present in air to form a protective and inert layer of magnesium oxide on its surface. This layer is unreactive and prevents the ribbon from burning. Hence, it needs to be cleaned with sand paper before burning in air.

N₂ + H₂ \(\rightarrow \)  NH₃ (Unbalanced equation).
(i) Examine the number of atoms of different elements present in the unbalanced equation.
(ii) In the above reaction, left hand N₂ and H₂ of both the sides are unbalanced.
(iii) To balance hydrogen, H₂ is multiplied by 3 on side. It makes 6H-atoms on the left hand side.
(iv) Now to balance hydrogen atoms on the right hand side, NH₃ should be multiplied by 2. It makes 6H-atoms on this side.
(v) Now to balance nitrogen atoms, they are counted separately for both the sides and we will find that, nitrogen atoms are 2 on both the sides.
(vi) Balanced chemical equation will be :
N₂ + H₂ \(\rightarrow \)  NH₃.

(a) Short circuiting is the term used for defining the situation in which the neutral and live wires of an electric circuit come in direct contact. When too many electrical appliances are connected to a single socket, the appliances drawn much more current or power that the permissible limit from the main supply. This situation is termed as overloading.
(b) Resistivity of an alloy is much higher than its constituent metals Moreover, alloys do not oxidise as easily as the constituent metals at high temperature. Because of these reasons, coils of electric testers are made of an alloy rather than a pure meta.

(a)
  
(b) Electric current flowing through a bulb = \(\frac { Power\quad of\quad the\quad bulb }{ Voltage\quad across\quad the\quad bulb } \)
\(i.e.,\quad I=\frac { P }{ V } \)
Therefore, \({ I }_{ 60 }=\frac { 60W }{ 220V } =\frac { 3 }{ 11 } A\quad and\quad { I }_{ 40 }=\frac { 40W }{ 220V } =\frac { 2 }{ 11 } A\)
Therefore, total current flowing through the circuit  \(={ I }_{ 60 }+{ I }_{ 40 }=\frac { 3 }{ 11 } +\frac { 2 }{ 11 } =\frac { 5 }{ 11 } A=0.45A\)
(c) Total energy consumed = Power \(\times \) Time
      That is, E = P \(\times \) T
                      = (40W+60W)\(\times \)1h
                    = 100 Wh
                    = 0.1 kWh

A chemical change is represented by a chemical equation. When the number of atoms of different elements on reactant and product side are equal, then the chemical equation is called a balanced chemical equation.
It is important to balance a chemical equation because
(i) to validate the law of conservation of mass which states that the mass of reactants should be equal to the mass of the products. The total mass of a system is thus conserved.
This law holds true only if number of atoms of reactants reacting together is equal to number of product atoms formed.
(ii) a balanced chemical equation tells us about the physical state of the reactants and products whether they are solid (s), liquid (l) or gas (g) or aqueous(aq).
(iii) it tells us about heat changes that can take place in a chemical reaction. \(\Delta\) is the symbol of heat. Hence, it is endothermic or exothermic can be deduced from a balanced chemical equations.

(i) H₂ + Cl₂ \(\rightarrow\) 2HCI
(ii) 3BaCl₂ + Al₂(SO₄)₃ \(\rightarrow\) 3BaSO₄ \(\downarrow\)+ 2AlCl₃
(iii) 2Na + 2H₂O \(\rightarrow\) 2NaOH + H₂

(a) Since two \(8\Omega \) resistors are in parallel, their effective resistance (\({ R }_{ P }\)) is given by,
\(\frac { 1 }{ { R }_{ P } } =\frac { 1 }{ 8 } +\frac { 1 }{ 8 } +\frac { 1 }{ 4 } \) or \({ R }_{ P }=4\Omega \)
(b) Total resistance in the circuit
\(R=4\Omega +{ R }_{ P }=4\Omega +4\Omega =84\Omega \)
Current through the electric circuit
\(I=\frac { V }{ R } =\frac { 8V }{ 8\Omega } =1A\)
(c) The potential difference \(4\Omega \) resistor V
 I \(=IR=1\times 4=4V\) 
(d) Power dissipated in \(​​​​4\Omega \) resistor P=\({ (I) }^{ 2 }\)
\(R={ (1) }^{ 2 }(4)=4W\)
(e) There is no difference in the readings of ammeters \({ A }_{ 1 }\) and  \({ A }_{ 2 }\) as same current flows through all elements in a series circuit.

Hydrogen gas and zinc sulphate are formed when zinc granules are treated with sulphuric acid
\(\underset{Zinc }{Zn(s)}+\underset{Sulphuric\ acid}{H2SO4(l)} \rightarrow\underset{Zinc\ sulphate}{ZnSO_4(aq)}+\underset{Hydrogen}{H_2(g)}\)
Zinc chloride and hydrogen gas are formed, when zinc granules are treated with hydrochloric acid.
\(\underset{Zinc}{Zn(s)} + \underset{Hydrochloric\ acid}{2HCI(l)}\rightarrow \underset{Zinc}{ZnCI2(s)} +\underset{Water}{ H_2(g)}\)
Similarly, hydrogen gas and zinc nitrate are formed when zinc granules are treated with nitric acid.
\(\underset{Zinc}{Zn(s)} + \underset{Nitricic\ acid}{2HNO_3(l)}\rightarrow \underset{Zinc\ nitrate}{Zn(NO_3)_2(s)} +\underset{Hydrogen}{ H_2}\)
Since, zinc is less reactive than sodium, thus no reaction takes place when zinc granules are treated with sodium chloride.
Sodium zinc ate and hydrogen gas are formed when zinc granules are treated with sodium hydroxide and water.
\(Zn + 2H_2O + 2NaOH\rightarrow Na_2Zn(OH) _4 + H_2\)

(a) Carbon dioxide turns lime water milky as it forms calcium carbonate with lime water. This is the characteristics test for carbon dioxide.
Ca(OH)₂ + CO₂ --> CaCO₃ + H₂O
(b) The smell of sulphur dioxide gas is like burning of sulphur, which is the characteristics test for sulphur dioxide. Sulphur dioxide turns moist litmus paper from blue to red.
(c) When a burnt candle or match stick is brought near the oxygen gas, it starts burning brightly. This is the characteristic test for oxygen gas.
(d) When a burnt candle or match stick is brought near the hydrogen gas, it burns with pop sound, which is the characteristic test for hydrogen gas.