[2 Mark Question Answer]

Question 12 Marks

A given quantity of a liquid is heated. Which of the following quantity will vary and how ?
$(a)$ mass, $(b)$ volume and $(c)$ density

Answer

When a given quantity of liquid is heated
$1.$ Mass : does not change.
$2.$ Volume : changes and increases with rise in temperature.
$3.$ Density : Changes and decreases.
Density $=$ Mass $/$ volume

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Question 22 Marks

A balloon filled with hydrogen rises in air. Explain the reason.

Answer

Hydrogen's density and weight is less than that of air. A balloon filled with hydrogen rises to a certain height as it displaces more weight of air than the weight of balloon but as it rises higher density of air decreases and upthrust becomes less and ultimately upthrust becomes equal to the weight of balloon and balloon stops rising further.

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Question 32 Marks

Explain why it is easier to lift a stone under water than in air.

Answer

A stone is easier to lift in water because the buoyant force of the water acts on the stone which counter$-$balances the weight of the stone acting downward and this makes the stone lighter.
In the air, the gravity tries to pull the stone downwards and hence we require more force to lift up the stone

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Question 42 Marks

Icebergs floating on sea water are dangerous for ships. Explain the reason.

Answer

Icebergs are huge masses of ice floating in the sea $[$density of ice being $0.917 g Cm^{-3}]$ with about $9/10$ portion of ice below seawater and only $1/10$ portion of it above. Since such a large portion of the iceberg is submerged underwater it becomes difficult for the sailor to estimate the size of the iceberg increasing the danger of the ship colliding in them.

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Question 52 Marks

Explain why an iron needle sinks in water, but a ship made of iron floats on water.

Answer

Density of iron is more than density of water,
$\therefore$ weight of iron nail is more than the weight of water displaced by it and nail sinks. While the shape of the iron ship is made in such a way that it displaces more weight of water than its own weight. Secondly, the ship is $\text{HOLLOW}$ and $\text{THE EMPTY SPACE}$ contains air which makes the average density of ship less than that of water and hence ship floats on water.

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Question 62 Marks

Define the term Density of a substance.

Answer

Density of a substance is defined as "Mass per Unit volume". Density $[d]=\frac{\text { mass of the substance }}{\text { volume of the substance }}$
$d =\frac{m}{v}$

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Question 72 Marks

The density of water is $1.0 g\ \ cm ^{-3}$. The density of iron is $7.8 \times 10^{\prime \prime}\ 3 g\ \ cm ^{-3}$. The density of mercury is $13.6 g\ \ cm ^{-3}$
Ans the following :
$(b)$ Will a piece of iron float or sink in mercury?

Answer

Density of water $1.0\ g\ Cm ^{-3}$
Density of piece of iron $=7.8 \times 10^{-3}$
Density of mercury is $13.6 \times 10-3 g\ Cm ^{-3}$
Since $7.8 \times 10-3<13.6 \times 10-3$
$\therefore$ Density of piece of iron is $\text{LESS}$ than density of mercury
$\therefore$ Piece of iron will $\text{FLOAT}$ in mercury

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Question 82 Marks

The density of water is $1.0\ g\ cm ^{-3}$. The density of iron is $7.8 \times 10^{\prime \prime} 3\ g /Cm ^{-3}$. The density of mercury is $13.6\ g/ Cm ^{-3}$.
Ans the following : Will a piece of iron float or sink in water?

Answer

Density of water $1.0g/ Cm ^{-3}$
Density of piece of iron $=7.8 \times 10^{-3} g/ Cm ^{-3}$
$7.8 \times 10^{-3}=\frac{7.8}{1000}=0.0078\ g/cm^3<1.0\ g/cm^3$
$\therefore$ Density of piece of iron is $\text{LESS}$ than density of water.
Hence, a piece of iron will $\text{FLOAT}$ in water.

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Question 92 Marks

What is the law of floatation?

Answer

When a body floats in a liquid, the weight of the liquid displaced by its immersed part is equal to the total weight of the body. This is the law of floatation, i.e. while floating. Weight of the floating body $=$ Weight of the liquid displaced by its immersed part.

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Question 102 Marks

Which of the following will sink or float on water? $($Density of water $= 1 g\ Cm^{-3})$
body $D$ having density $0.85\ g m^{-3}$

Answer

Density of water $=1 g\ Cm _{-3}$
Density of body $D =0.85\ g m ^{-3}<1.0 g\ Cm ^{-3}$
Density of body $D$ is less than the density of water hence body $D$ will $\text{FLOAT}$ on water

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Question 112 Marks

Which of the following will sink or float on water? (Density of water $=1 g \ cm ^{-3}$ )
body $C$ having density $1100 \ kg m ^{-3}$

Answer

Density of water $=1 g\ Cm ^{-3}$
Density of body $C =1100 \ kg m ^{-3}=1100 \times 1 / 1000=1.1 g\ Cm ^{-3}$ is greater than water.
Hence, body $C$ will sink in water.

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Question 122 Marks

Which of the following will sink or float on water? $($Density of water $= 1 g\ Cm^{-3})$ body $B$ having density $2520\ kg m^{-3}$

Answer

Density of water $=1 g\ Cm ^{-3}$
Density of body $B=2520\ kg m ^{-3}=2520 \times 1 / 1000=2.52g\ Cm ^{-3}$
Density of body $B$ is more than the density of water and hence $B$ will Sink in water

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Question 132 Marks

Which of the following will sink or float on water? $($Density of water$)=1 g\ Cm ^{-3}$
$(a)$ body $A$ having density of $500\ kg m ^{-3}$

Answer

The density of water $=1 g\ Cm ^{-3}$
$(a)$ Density of body $A =500\ kg\ m ^{-3}=500 \times=0.5=0.5 g\ Cm ^{-3}$
Density of body $A_i s$ less than density of water hence $A$ will float on water

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Question 142 Marks

A cork piece floats on water surface while an iron nail sinks in it. Explain the reason.

Answer

Cork floats on water as the density of cork is $\text{LESS}$ than the density of water.
Iron nail sinks in water as the density of iron nail is $\text{MORE}$ than the density of water.

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Question 152 Marks

How does the density of a body and that of a liquid determine whether the body will float or sink into that liquid?

Answer

The density of an object determines whether it will float or sink in another substance. An object will float If the density of a body is $\text{LESS}$ than the density of $\text{LIQUID}$. An object will sink If the density of a body is MORE than the density of liquid it is placed in.

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Question 162 Marks

Explain the meaning of the statement ‘Relative density of aluminium is $2.7’.$

Answer

The statement ‘Relative density of aluminium is $2.7’$ means a piece of aluminium of any volume has mass $2.7$ times that of an equal volume of water. i.e. Aluminium is $2.7$ times heavier than water.

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Question 172 Marks

Define the term relative density of a substance.

Answer

RELATIVE DENSITY: “is the ratio of the density of a substance to the density of water at $4^\circ C.”$
Or
RELATIVE DENSITY “is the ratio of the mass of the substance to the mass of an equal volume of water at $4^\circ C.”$

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Question 182 Marks

Calculate the density of solid from the following data :
$(a)$ Mass of solid $= 72\ g$
$(b)$ Initial volume of water in measuring cylinder $= 24\ ml$
$(c)$ Final volume of water when solid is completely immersed in water $= 42\ ml$

Answer

Mass of solid $(M)=72 g$
Initial volume of water $V _1=24\ ml$
Final volume of water $V _2=42\ ml$
Volume of solid $( V )= V _2- V _1=42-24=18 \ cm^3$
Density of solid $(D) =$ ?
$D =\frac{M}{V}=\frac{72}{18}=4.0 g\ cm ^{-3}$

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Question 192 Marks

Calculate the volume of wood of mass $6000\ kg$ if the density of wood is $0.8 g\ Cm^{-3}$

Answer

Volume of wood $( V )=$ ?
Mass of wood $(M) =6000 \ kg$
Density of wood $D=0.8 g / Cm ^3$
$D =0.8 g / Cm ^3=0=0.8 \times 1000=800 \ kg / m ^3$
$ D =\frac{M}{V}$
$\therefore V =\frac{M}{D}=\frac{6000}{800}=7.5\ M^3$

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Question 202 Marks

A piece of wood of mass $150\ g$ has a volume of $200 \ cm^3$. Find the density of wood in $\text{C.G.S}.$ unit and $\text{S.I.}$ unit.

Answer

Mass of wood $(M) = 150 g$
Volume of wood $(V) = 200\ Cm^3$
Density $(D) =?$
$ D =\frac{ M }{ V }=\frac{150}{200}=0.75 \frac{ g }{ cm ^3}$
In $\text{S.I.}$ system $=0.75 \times 1000$
$=750\ kg / m ^3$

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Question 212 Marks

A piece of zinc of mass $438.6\ g$ has a volume of $86\ Cm^3$. Calculate the density of zinc.

Answer

Mass of Zinc $(M) = 438.6\ g$
Volume$ V = 86\ Cm^3$
Density $(D) = ?$
$D =\frac{M}{V}=\frac{438.6}{86}=5.1 g / \ cm ^3$

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Question 222 Marks

The density of alcohol is $600\ kg/ m ^{-3}$. Express it in $g/ cm ^{-3}$.

Answer

Density of alcohol is $=600 \ kg / m ^3$ In $g / \ cm ^3=\frac{600}{1000}=0.60 g / \ cm ^3$

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Question 232 Marks

The density of aluminium is $2.7\ g /cm^3$. Express it in kg $m^{-3}$

Answer

Density of aluminium $= 2.7\ g/Cm^3$
In $kg / m ^3=\frac{27 \times 1000}{10}=2700\ kg / m ^3$

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Question 242 Marks

The dimensions of a hall are $10 m \times 7 m \times 5 m$. If the density of air is $1.11 \ kg/ m ^{-3}$, find the mass of air in the hall.

Answer

The dimensions of hall $10 m \times 7 m \times 5 m$
i.e. $V\ 350 \ m ^3$
Density of air $(D)=1.11\ kg/ m ^{-3}$
$M=V \times D =350 \times 1.11=388.5 \ kg$

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Question 252 Marks

The density of air is $1.28 g / litre$. Express it in $kg/ m ^{-3}$

Answer

$1.28 g / litre =\frac{1.28}{1000} \times 1000=1.28\ kg/ m ^{-3}$

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Question 262 Marks

The density of air is $1.28\ g/Iitre$ . Express it in $g/ cm^{-3}$

Answer

The density of air is $I .\ 28\ g/litre$
It is in $g/ cm ^{-3}=\frac{1.28}{1000}=0.00128\ g/ cm ^{-3}$

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