chapter5

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

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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.

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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.

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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.

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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

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(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

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