punjab text book notes 9th chemistry

1. Chapter 5 physical states of matter
Creative Science Academy ADNAN SOHAIL
M.PHIL CHEMISTRY
1. Why the rate of diffusion of gases is rapid than that of liquids?
Ans. Rate of diffusion is inversely proportional to the density and intermolecular forces of a
substance. Gases have low density and weak intermolecular forces than liquids. So they
have high rate of diffusion than liquids.
2. Why are the gases compressible?
Ans. Gases are highly compressible due to empty spaces between their molecules.
3. What do you mean by Pascal. How many Pascals are equal to 1 atm?
Ans. Pressure has SI unit of Nm-2
. It is also called Pascal (Pa)
One Pascal (Pa) = 1 Nm-2
101325 Pa = 1 atm
4. Why the density of a gas increases on cooling?
Ans. As temperature decreases, the attractive forces among gas molecules increase. This
results decrease in volume and increase in density of a gas.
5. Why is the density of gas measured in g dm-3
while that of a liquid in g cm-3
?
Ans. The density of gas measured in g dm-3
while that of a liquid in g cm-3
because liquids
and solids are 1000 times denser than gases.
6. Convert the following
• 70 cm Hg to atm
• 3.5 atm to torr
• 1.5 atm to Pa
Ans. Conversion of 70 cm Hg to atm
We know that
760 mm 0f Hg = 1 atm
76 cm of Hg = 1 atm
1 cm of Hg = 1/76 atm
70 cm of Hg = (1/76) x70 atm
70 cm of Hg = 0.92 atm
Conversion of 3.5 atm to torr
We know that ,
1 atm = 760 torr
3.5 atm = 3.5x760 torr
3.5 atm = 2660 torr
Conversion of 1.5 atm to Pa
We know that
1 atm = 101325Pa
1.5 atm = 101325 x 1.5 Pa
1.5 atm = 151987.5 Pa
1

2. Chapter 5 physical states of matter
Creative Science Academy ADNAN SOHAIL
M.PHIL CHEMISTRY
1. Is the Boyle’s law applicable to liquids?
Ans. No, Boyle’s law is not applicable to liquids because these are not compressible.
2. Is the Boyle’s law valid at very high temperature?
Ans. Yes, Boyle’s law is valid at high temperature because temperature is kept constant in
this law.
3. What will happen if the pressure on a sample of gas is raised three times and its
temperature is kept constant
Ans. According to Boyle’s law, at constant temperature when pressure on a gas is raised
three times its volume will reduce to three times
1. Which variables are kept constant in Charles’s law?
Ans. In Charles’s law the mass of a gas and pressure are kept constant.
2. Why volume of a gas decreases with increase of pressure?
Ans. There are large empty spaces among gas molecules. When pressure is applied
attractive forces are developed. The empty spaces are reduced and gas occupies less
volume.
3. What is absolute zero?
Ans. It is the temperature at which an ideal gas would have zero volume.
0 K equal to --273 °C and 273 K is equal to 0 °C
4. Does Kelvin scale show a negative temperature?
Ans. No, Kelvin scale does not show a negative temperature.
5. When a gas is allowed to expand, what will be its effect on its temperature?
Ans. When a compressed gas is allowed to expand into the region of low pressure, it gets
cool.
6. Can you cool a gas by increasing its volume?
Ans. No, it is not possible to cool a gas by increasing of its volume. A gas can be cooled by
decreasing its temperature.
2

3. Chapter 5 physical states of matter
Creative Science Academy ADNAN SOHAIL
M.PHIL CHEMISTRY
1. Why does evaporation increase with the increase of temperature?
Ans. At high temperature kinetic energy of the molecules increases so high that they
overcome the intermolecular forces and escape from liquid surface i.e.evaporate rapidly
2. What do you mean by condensation?
Ans. Condensation is a process in which a gas changes into liquid state.
3. Why is vapour pressure higher at high temperature?
Ans. At high temperature, the kinetic energy of the molecules increases enough to enable
them to vaporize and exert pressure on the surface of liquid.
4. Why is the boiling point of water higher than that of alcohol?
Ans. Boiling point of water (100 °C) is higher than that of alcohol (78 °C) because of more
and stronger hydrogen bonding in water.
5. What do you mean by dynamic equilibrium ?
Ans. An equilibrium state at which the number of molecules evaporating is equal to the
number of molecules coming back (condensing) to liquid. This state is called dynamic
equilibrium.
6. Why are the rates of diffusion in liquids slower than that of gases?
Ans. Stronger the intermolecular forces slower will be the rate of diffusion. The rate of
diffusion in liquids is slower than that of gases because they have very strong intermolecular
forces in them as compared to gases.
7. Why does rate of diffusion increase with increase of temperature?
Ans. The rate of diffusion increases with increase of temperature because at high
temperature molecules of a substance get high kinetic energy. The intermolecular forces
also become weak. They can move and diffuse rapidly easily from one place to another.
8. Why are the liquids mobile?
Ans. Liquids do not have definite volume.The molecules of liquids have weak intermolecular
forces as compared to solids. They are in constant state of motion. Due to this they are
mobile.
3

4. Chapter 5 physical states of matter
Creative Science Academy ADNAN SOHAIL
M.PHIL CHEMISTRY
1. Which form of sulphur exists at room temperature?
Ans. Rhombic sulphur exists at room temperature.
2. Why is white tin available at room temperature?
Ans. white tin is available at room temperature because above 13.2C (at room temperature)
white tin is stable.
3. Why the melting point of a solid is considered its ‘identification’ characteristic?
Ans. The melting point of a solid is considered its identification characteristic because each
pure solid has definite and unique value of its melting point.
4. Why amorphous solids do not have sharp melting points while crystalline solids do
have?
Ans. Amorphous means shapeless. Solids in which the particles are not regularly arranged
or their regular shapes are destroyed are called amorphous solids. Due to this, they do not
have sharp melting points.
Solids in which particles are arranged in a definite three-dimensional pattern are
called crystalline solids. They have definite surfaces or faces. Each face has definite angle
with the other. This is the reason that they have sharp melting points.
5. Which is lighter one aluminium or gold?
Ans. Aluminium is lighter than gold because of its low density.
6. Write the molecular formula of a sulphur molecule?
Ans. The molecular formula of sulphur is S8 .
7. Which allotropic form of carbon is stable at room temperature (25 °C)?
Ans. The most stable allotropic form of carbon is graphite.
8. State whether allotropy is shown by elements or compounds or both?
Ans. Allotropy is shown by elements only.
SHORT ANSWER QUESTIONS
1. What is diffusion, explain with an example?
Ans. Diffusion is defined as spontaneous mixing up of molecules by random motion and
collisions to form a homogeneous mixture.
For example, when a few drops of ink are added in a beaker of water, ink molecules move
around and after a while spread in whole of the beaker. Thus diffusion has taken place.
Liquids diffuse like gases but the rate of diffusion of liquid is very slow.
2. Define standard atmospheric pressure. What are its units? How it is related to Pascal?
Ans. It is defined as the pressure exerted by a mercury column of 760 mm height at sea
level.
4

5. Chapter 5 physical states of matter
Creative Science Academy ADNAN SOHAIL
M.PHIL CHEMISTRY
3. Why are the densities of gases lower than that of liquids?
Ans. Gases have low density than liquids and solids. It is due to light mass and more
volume occupied by the gas molecules.
4. What do you mean by evaporation how it is affected by surface area?
Ans. The process of changing of a liquid into a gas phase is called evaporation.
Surface area: Greater is surface area, greater is the evaporation and vice verse. For
example, sometimes a saucer is used if tea is to be cooled quickly. This is because
evaporation from the larger surface area of saucer is more than that from the smaller surface
area of a tea cup.
5. Define the term allotropy with examples?
Ans. The existence of an element in more than one form in same physical state is
called allotropy.
Allotropes of oxygen are oxygen (O2) and ozone (O3), sulphur has two allotropic forms
rhombic and monoclinic.
6. In which form sulphur exists at 100 °C?
Ans. Transition temperature of sulphur is 96 °C. Below this temperature rhombic form is
stable. If rhombic form is heated above 96 °C, its molecules rearrange themselves to give
monoclinic form. So at 100 °C monoclinic form exists.
7. What is the relationship between evaporation and boiling point of a liquid?
Ans. The boiling point of a liquid is constant at a given pressure. If more heat is supplied to
liquid, it increases rate of evaporation.
NUMERICALS
1. Convert the following units:
(a) 850 mm Hg to atm (b) 205000 Pa to atm
(c) 560 torr to cm Hg (d) 1.25 atm to Pa
2. Convert the following units:
(a) 750 °C to K (b) 150 °C to K
(c) 100K to°C (d) 172K to°C.
Ans. 1. Convert the following units:
(a) 850 mm Hg to atm
760 mm Hg = 1 atm
1 mm Hg = 1atm
760
850 mm Hg = 1 x 850
760
850 Hg = 1.12 atm
(b) 205000 Pa to atm
101325 Pa = 1 atm
1 Pa = 1_ atm
101325
205000 Pa =
1x205000
101325)
205000 Pa = 2.02 atm
5

6. Chapter 5 physical states of matter
Creative Science Academy ADNAN SOHAIL
M.PHIL CHEMISTRY
(c) 560 torr to cm Hg
1mm Hg = 1 torr
1 mm = 1 cm
10
560 torr = 560x1cm Hg
10
560 torr = 56 cm Hg
(d) 1.25 atm to Pa
1 atm = 101325 Pa
1.25 atm =
1.25x 101325
1.25 atm = 126656 Pa
Ans.2. Convert the following units:
(a) 750 °C to K
K = °C + 273
750 °C = 750 + 273
750 °C = 1023 K
(b) 150 °C to K
K = °C + 273
150 °C = 150 + 273
150 °C = 423 K
(c) 100K to°C
°C = K – 273
100 K = 100 – 273
100 K = – 173 °C
(d) 172K to°C
°C = K – 273
172 K = (172 – 273) °C
172 K = – 101 °C
6