[3 Mark Questions]

Question 13 Marks

A household uses the following electric appliances:
$i$. The refrigerator of rating $400 W$ for $10 h$ each day and Two electric fans of rating $80 W$ each for $6 h$ daily
$ii$. Six electric tubes of rating $18 W$ each for $6 h$ daily.
Calculate the electricity bill for the household for the month of June, if the cost of electrical energy is $Rs. 3$ per unit.

Answer

$i$. Energy consumed per day by refrigerator
$=0.4 kW \times 10 h\ [\therefore$ power of refrigerator $=400 W \frac{400}{1000} kW=0.4 kW]$
$=4 kWh $
Energy consumed per day by fans
$=2 \times 0.08 kW \times 6 h \ [\therefore$ power of each fan $=\frac{80}{1000}=0.08 kW]$
$=0.96 kWh $
$ii$. Energy consumed per day by electric tubes
$=6 \times 0.018 kWh \times 6 h \ [\therefore $ power of each electric tube $=\frac{18}{1000}=0.018 kW]$
$=0.648 kWh $
Total energy consumed per day $= 4 + 0.96 + 0.648 = 5.608 kWh$
Energy consumed in $30$ days $=30 \times 5.608=168.24 kWh$
Cost of $168.24 kWh$
Cost of $168.24$ units $ ₹ 3.00=168.24 \times 3= ₹ 504.72$

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Question 23 Marks

$i$. A wire of resistance $2$ has been connected to a source of $50 V$ as its two ends. What is the current flowing through the wire?
$ii.$ An electric refrigerator rated $400 W$ operates $8$ hour/day. What is the cost of the energy to operate it for $30$
days at $₹ 3.00 \ \text{per kWh}$ ?

Answer

$i$. Zero, current flows due to potential difference and not due to potentials.
$ii.$ The total energy consumed by the refrigerator in $30$ days would be
$400 w \times 8.0 \text { hour } / \text { day } \times 30 \text { days }=96000 Wh$
$=96 kWh $
Thus the cost of energy to operate the refrigerator for $30$ days is
$96 kW h \times ₹ 3.00\ \text{per kWh} = ₹ 288.00$

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Question 33 Marks

The refractive indices 1.0003, 1.31 1.5 respectively of Air, Ice and Benzine in which of these does the light travels fastest?

Answer

Refractive indices of Air, Ice and Benzene are 1.003, 1.31 and 1.5 respectively. Velocity of light in a medium is inversely proportional to refractive index of the medium. Light will travel fastest in air (having least refractive index i.e. 1.0003) and slowest in Benzene (having maximum refractive index i.e. 1.5)

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Question 43 Marks

Explain Mendel's observation when he crossed a homozygous tall (TT) plant with homozygous dwarf (tt) plant followed by self-cross.

Answer

When Mendel crossed a homozygous tall (TT) plant with homozygous dwarf (tt) plant, all plants in F1 generation were tall (Tt).
Self crossing of F1 gives F2. F2
Since presence of dwarf allele was masked by tall allele in F1,tall allele (T) was dominant over dwarf allele (t). generation had 3 tall : 1 recessive plants.

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Question 53 Marks

i. Why is nutrition a necessity for an organism? State three reasons.
ii. What is likely to happen if green plants disappear from Earth?

Answer

i. Nutrition is necessary because
a. it helps in the growth of new cells, survival and maintenance of cells
b. it is needed to develop resistance against diseases.
c. it provides energy by the oxidation of food for metabolic processes.
ii. The disappearance of green plants from Earth would mean a total disaster for the ecosystem.It will cause a decrease in
concentration of oxygen to such a low level that it would not be sufficient for all the living organism for breathing. Green
plants are the source of energy for all organisms. All other organisms directly or indirectly depend on them for food. So, if
they disappear from the Earth, all the herbivores will die due to starvation and so will the carnivore. It would result in the
extinction of life from the Earth.

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Question 63 Marks

$i$. Write the electron $-$ dot structures for sodium, oxygen and magnesium.
$ii$. Show the formation of $\ce{Na _2 O}$ and $\ce{MgO}$ by the transfer of electrons.
$iii$. What are the ions present in these compounds?

Answer

$i$.

Metal	Symbol	Atomic number	Electronic configuration $K, L, M, N$	No. of outermost electrons	Electron dot structures
Sodium	$Na$	$11$	$2, 8, 1$	$1$	$Na.$
Oxygen	$O$	$8$	$2, 6$	$6$	$:O::$
Magnesium	$Mg$	$12$	$2, 8, 6$	$2$	$Mg:$

$ii.$ Formation of $\ce{Na _2 O}$
The atomic number of sodium is $11$ and it has only one valence electron.
Hence, electronic configuration of ${ }_{11} N a$ is $2, 8, 1.$
The atomic number of oxygen is $8$ and it has $6$ electrons in its valence shell.
Hence, electronic configuration of ${ }_8 O$ is $2, 6$.
Sodium has a tendency to lose the valence electron and oxygen has a tendency to gain the electron lost by sodium. Since,
sodium can lose only one electron of the valence shell, and oxygen atom needs two electrons to complete its octet in the
valence electron, two atoms of sodium combine with one atom of oxygen.
By losing valence electron, sodium is changed into
$N a^{+}$ and by gaining two electrons lost by two sodium atoms, oxygen atom is changed into an oxide anion, $O _2$. In this process, both the atoms, sodium and oxygen, obtain the stable electronic configuration of the noble gas neon.
$\underset{2,8,1}{ Na } \rightarrow \underset{2,8}{ Na ^{+}}+e^{-} O ^{2-}$
$O _{2,6}+2 e^{-} \rightarrow O _{2,8}^{2-}$
$2 Na ^{+}+ O ^2 \xrightarrow{\rightarrow} 2 Na ^{+} O ^{2-} \text { or } \ce{Na a_2 O} $
The oppositely charged sodium ion $Na ^{+} O ^{2-}$ , and oxide ion $O^{2-}$ , are now held together by electrostatic force of attraction or by ionic or electrovalent bond. $\ce{Na _2 O}$ is, therefore, an ionic or electrovalent compound.

It has two electronic in its outermost shell.
So, the magnesium atom donates its $2$ valence electrons and forms a stable
magnesium ion $Mg ^{2+}$ , to attain the electronic arrangement of neon atom.
$\underset{2,8,2}{\operatorname{Mg}} \rightarrow \underset{2,8}{\stackrel{2+}{M g}}+2^{e-}$
The atomic number of oxygen $= 8$
Electronic configuration $ = \underset{2}{K}, \underset{6}{L}$
It has $6$ electrons in its valence shell.
Therefore, it requires $2$ more electrons to attain the stable electronic arrangement of neon gas.
Thus, oxygen accepts $2$ electrons donated by magnesium atom and forms a stable oxide ion $O^{2-}$.

The oppositely charged magnesium ions $Mg ^{2+}$ , and oxide ions, are held together by a strong force of electrostatic attraction to form magnesium oxide compound.
$\ce{MgO}$ is ionic compound.
$iii$. The ions present in $\ce{Na _2 O}$ are sodium ions $\left(\ce{2 N a}^{+}\right)$ and oxide ion $O ^{2-}$.
The ions present in $\ce{M g O} $ are magnesium ion $\ce{(Mg2+)}$ and oxide ion $O ^{2-}$.

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Question 73 Marks

i. Define non-metals. Give five examples of non-metals.
ii. Name a non-metal which conducts electricity.
iii. Name a non-metal having lustre (shining surface).
iv. Name a non-metal which is extremely hard.
v. How do non-metals react with oxygen? Explain with an example. Give equation of the reaction involved.
What is the nature of the product formed? How will you demonstrate it?

Answer

i. Non-metals are the elements that do not conduct heat and electricity and are neither malleable nor ductile. Example: Carbon,
sulphur, phosphorus, silicon, and oxygen.
ii. Carbon is a non-metal which conducts electricity.
iii. Iodine is a non-metal having lustre.
iv. Carbon (Diamond) is a hard non-metal.
v. Non-metals react with oxygen to form acidic oxides or neutral oxides. Carbon burns in air to form carbon dioxide. The nature of the product formed is acidic. When carbon dioxide dissolves in water, it forms carbonic acid. It turns blue litmus to red which shows it is acidic in nature.

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Question 83 Marks

State three reasons for the following facts:
$i.$ Sulphur is a non$-$metal.
$ii.$ Magnesium is a metal.
One of the reasons must be supported with a chemical equation.

Answer

$i.$ Sulphur is a non$-$metal because of the following reason:
$a.$ It is a poor conductor of electricity.
$b.$ Sulphur is neither malleable nor ductile.
$c.$ Sulphur forms acidic oxide.
$ S + O_2 \longrightarrow SO_2$
$SO_2+ H _2 O \longrightarrow H_2SO _4$
$ii.$ Magnesium is metal because of the folowing reason
$a.$ It is a good conductor of electricity.
$b.$ Magnesium is malleable nor ductile.
$c.$ It forms basic oxides
$2 Mg + O _2 \longrightarrow 2 MgO$

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