Intext Questions

Q.1     Which of the following are matter?
Chair, air, love, smell, hate, almonds, thought, cold, cold-drink, smell of perfume.
Sol.     Chair, air, almonds, and cold-drink are matters. Explanation: Things that occupy space and have some mass are called matter. Since chair, air, almonds and cold-drink occupy some space and have some mass, so these are matter.

---

Q.2     Give reasons for the following observation: The smell of hot sizzling food reaches you several meters away, but to get the smell from cold food you have to go close.
Sol.     Smell of anything comes because of gases emanating from the given thing. The smell reaches to us because of diffusion of gas. The rate of diffusion increases with increase in temperature. This happens because of higher kinetic energy due to higher temperature. That is why smell of hot sizzling food reaches to us from several feet. On the other hand, the kinetic energy of gases emanating from cold food is low because of lower temperature. Due to this, we need to move closer to a cold food to take its smell.

---

Q.3     A diver is able to cut through water in a swimming pool. Which property of mater does this observation show?
Sol.     Water is also a matter. We know that particles of matter have space between them. In case of fluids, the space between particles is large enough and due to this it becomes easier for a diver to cut through water in a swimming pool.

---

Q.4     What are the characteristics of the particles of matter?
Sol.     Characteristics of the particles of matter:
–  The particles of matter are very small.
–   The particles of matter have space between them.
–   The particles of matter are moving constantly.
–   The particles of matter attract one another.

---

Page 6

Q.1     The mass per unit volume of a substance is called density. (Density = mass / volume) Arrange the following in order of increasing density
air, exhaust from chimneys, honey, water, chalk, cotton and iron.
Sol.     Air < Exhaust from chimneys < cotton < Water < Honey < Chalk < Iron
Explanation : Air is the mixture of gases. Chimney exhaust is also a mixture of gases; along with some heavier particles, such as ash. This makes the density of chimney exhaust more than air. Cotton is a porous solid and which has lot of air trapped within pores. This makes its volume more than water. Therefore, it is less dense than water.

Q.2     (a) – Tabulate the differences in the characteristics of states of matter.
Sol.

Difference in the characteristics of states of matter
Solid	Liquid	Gas
Definite	Indefinite shape	Indefinite shape
Definite volume	Definite volume	Indefinite volume
Maximum force of attraction between particles	Less forces of attraction between particles compare to solid	Negligible force of attraction between particles
Particles are closely packed	Particles are loosely packed compared to solid	Particles are loosely packed
Cannot be compressed	Cannot be compressed	Can be compressed
Kinetic energy of particles is minimum	Kinetic energy of particles is more than solid	Kinetic energy of particles is maximum
Particles cannot move rather they vibrate only at  their fixed position	Particles can slide over one another	Particles can move freely
Highest density cannot flow	Density is lower than solid	Lowest density
Cannot flow	Flow	Flow

 

             (b) – Comment upon the following: rigidity, compressibility, fluidity, filling a gas container, shape, kinetic energy and density
Sol.     Rigidity: The greatest force of attraction between particles and close packing of particles make solids rigid. Rigidity is one of the unique properties of solids. Because of rigidity, a solid can resist from getting distorted. Because of rigidity a solid has definite shape and volume. Rigidity is negligible in fluid and gas.

Compressibility: Compressibility is one of the most important characteristics of gas. Because of lot of space between particles, a gas can be compressed to a great extent.

Liquid and solid cannot be compressed because of the least space between their particles.

Fluidity: The ability to flow is called fluidity. The less force of attraction and more space between particles make liquid and gas to flow. That’s why liquid and gas are called fluid.

Filling of a gas container: Liquids do not fill a gas container completely, while gases fill the gas container completely in which it is kept. This is because the particles of gas can move in all the directions.

Shape: Solids have fixed shape. Liquid and gas take the shape of the container in which they are kept. This happens because of less force of attraction and more kinetic energy between particles of liquids and negligible force of attraction and highest kinetic energy between particles of gas.

Kinetic energy: The kinetic energy of particles of solid is the minimum. They only vibrate at their fixed position. The kinetic energy of particles of liquid is more than that of solid. But they can slide above one another. The kinetic energy of particles of gas is the maximum.

Density: The mass per unit volume of a substance is called density. The density of solid is highest, of liquid is less than solid and of gas is minimum.

---

Q.3     Give reasons
           (a) A gas fills completely the vessel in which it is kept.
Sol.     The force of attraction between particles of gas is negligible. Because of this, particles of gas move in all directions. Thus, a gas fills the vessel completely in which it is kept.

           (b) A gas exerts pressure on the walls of the container.
Sol.     Because of negligible force of attraction between particles of gas, the particles of gas have the highest kinetic energy. These properties enable the particles of gas to move in all directions and hit the walls of container from all sides. Because of this a gas exerts pressure on the walls of the container in which it is kept.

           (c) A wooden table should be called a solid.
Sol.     A wooden table has fixed shape and fixed volume, which are the main characteristics of solid. Thus a wooden table should be called a solid.

           (d) We can easily move our hand in air but to do the same through a solid block of wood we need a karate expert.
Sol.     Since, air is gas, so its particles are loosely packed and there is negligible force of attraction between its particles. Because of that we can easily move our hand in air.
But wood is a solid, so the force of attraction between its particles is greatest. The particles of wooden block are closely packed. That’s why we cannot move our hand through a solid block of wood. However a karate expert can exert required pressure to break the great force of attraction of the particles of a solid wooden block.

---

Q.4     Liquids generally have lower density as compared to solids. But you must have observed that ice floats on water. Find out why.
Sol.     During freezing of water, some space between the particles of water is left vacant with some air trapped between them. These empty spaces having air in them makes the density of ice; lower than that of water. That’s why ice floats on water.

---

Page 9

Q.1     Convert the following temperature to Celsius scale.
           (a)300 K
           (b) 573 K
Sol.     (a) Temperature on Kelvin scale = Temperature on Celsius scale + 273 Therefore,
Temperature on Celsius scale = Temperature of Kelvin scale – 273
Hence, Temperature on Celsius scale = 300K – 273 = 27⁰C

Sol.     (b) Temperature on Kelvin scale = Temperature on Celsius scale + 273
Therefore, Temperature on Celsius scale = Temperature of Kelvin scale – 273
Hence, Temperature on Celsius scale = 573K – 273 = 300⁰C

---

Q.2     What is the physical state of water at
            (a) 250⁰C
            (b) 100⁰C
Sol.      (a) Since, water boils at 100⁰C, thus it is in gaseous state at 250⁰C.
Sol.      (b) Since, water boils at 100⁰C, thus at 100⁰C water is in transition phase, i.e. in both liquid and gaseous states.

---

Q.3     For any substance why does the temperature remain constant during the change of state.
Sol.     During the change of state of any substance, the heat supplied or released is utilised in phase change. Such heat is called latent heat. So, the temperature of any substance remains constant during the change of state.

---

Q.4     Suggest a method to liquefy atmospheric gas.
Sol.     Atmospheric gas is liquefied by increasing pressure and decreasing temperature.

---

Page 10

Q.1     Why does a desert cooler cool better on a hot dry day?
Sol.      Desert cooler works on the basis of evaporation. In hot and dry days the moisture level is very low in atmosphere which increases the rate of evaporation. Because of faster evaporation, cooler works well. That’s why desert cooler cool better on a hot dry day.

---

Q.2     How does the water kept in an earthen pot (matka) become cool during summer?
Sol.     Water from porous wall of earthen pot evaporates continuously, which lowers the temperature of water kept in the earthen pot. In summer moisture level is very low in the atmosphere, which increases the rate of evaporation as evaporation is inversely proportional to the moisture level in atmosphere. That is why in summer water kept in earthen pot becomes cool.

---

Q.3     Why does our palm feel cold when we put some acetone or petrol or perfume on it?
Sol.     When acetone or petrol or perfume is poured over palm, it evaporates quickly as these are volatile liquids. The evaporation lowers the temperature of palm and our palm feels cold.

---

Q.4     Why are we able to sip hot tea or milk faster from a saucer rather than a cup?
Sol.     When hot tea or milk is kept in a saucer, the liquid is exposed over a larger surface area as compared to in case of the liquid being kept in a cup. The larger surface area enables the faster cooling. That’s why we are able to sip hot tea or milk faster from a saucer rather than from a cup.

 

Exercise

 

Q.1     Convert the following temperature to the Celsius scale.
           (a) 293 K
           (b) 470 K
Sol.     (a) Temperature in Celsius scale = Temperature in Kelvin scale – 273
⇒ 293 K = 293 K – 273 = 20⁰C

           (b) Temperature in Celsius scale = Temperature in Kelvin scale – 273
⇒ 470 K = 470 K – 273 = 197⁰C

---

Q.2     Convert the following temperature to the Kelvin scale.
           (a) 25⁰C
           (b) 373⁰C
Sol.     (a) Temperature in Kelvin scale = Temperature in Celsius scale + 273
= 25⁰C + 273 = 298 K

           (b) Temperature in Kelvin scale = Temperature in Celsius scale + 273
= 373⁰C + 273 = 646 K

---

Q.3     Give reason for the following observations.
           (a) Naphthalene balls disappear with time without leaving any solid.
           (b) We can get the smell of perfume sitting several meters away.
Sol.     (a) Naphthalene ball is a sublimate and a sublimate turns into vapour without changing into liquid. Thus, naphthalene balls disappear with time without leaving any solid.
          (b) Perfume turns into gas at room temperature. The vapour of perfume travels up to several meters because of diffusion. That’s why we can get the smell of perfume sitting several meters away.

---

Q.4     Arrange the following substances in increasing order of forces of attraction between the particles  Water, Sugar, Oxygen.
Sol.     Oxygen < Water < Sugar
Explanation: Oxygen is a gas, thus force of attraction is negligible between particles. Water is a liquid, thus force of attraction between particles is more than liquid and less than solid. Sugar is a solid, thus force of attraction between particles is greatest.

---

Q.5     What is the physical state of water at
           (a) 25⁰C
           (b) 0⁰C
           (c) 100⁰C
Sol.     (a) At 25⁰C – water is in liquid state.
(b) At 0⁰C – water is in solid state.
(c) At 100⁰C – water is in transition state, i.e. in liquid and gas both.

---

Q.6     Give two reasons to justify
           (a) Water at room temperature is a liquid.
(b) An iron almirah is a solid at room temperature.
Sol.     (a) At room temperature:
(i) Water has definite volume, but not definite shape as it takes the shape of the container in which it is kept.
(ii) Water flows at room temperature.

           (b) An iron almirah is a solid at room temperature because:
(i) It has definite shape.
(ii) It has definite volume.

---

Q.7     Why is ice at 273K more effective in cooling than water at the same temperature.
Sol.     At 273K ice requires more latent heat to melt into water, while water at 273K requires less latent heat; to come to the room temperature. So, ice at 273 K is more effective in cooling than water at the same temperature.

---

Q.8     What produces more severe burns, boiling water or steam?
Sol.     Steam produces more severe burns than boiling water, because steam has more latent heat than boiling water.

---

Q.9     Name A,B,C,D,E and F in the following diagram showing change in its state.

 

Sol.     A – Heating – Melting

B – Heating – Vapourisation

C – Cooling – Condensation – Liquefaction

D – Cooling – Freezing

E – Sublimation

F – Solidification

 

Exemplar

Multiple Choice Questions :

Q.1 Which one of the following sets of phenomena would increase on raising the temperature?

(a) Diffusion, evaporation, compression of gases

(b) Evaporation, compression of gases, solubility

(c) Evaporation, diffusion, expansion of gas

(d) Evaporation, solubility, diffusion, compression of gasses

Sol. (c) Evaporation, diffusion, expansion of gas

---

Q.2 Seema visited a Natural Gas Compressing Unit and found that the gas can be liquefied under specific conditions of temperature and pressure. While sharing her experience with friends she got confused. Help her to identify the correct set of conditions

(a) Low temperature, low pressure

(b) High temperature, low pressure

(c) Low temperature, high pressure

(d) High temperature, high pressure

Sol. (c) Low temperature, high pressure

---

Q.3 The property to flow is unique to fluids. Which one of the following statements is correct?

(a) Only gases behave like fluids

(b) Gases and solids behave like fluids

(c) Gases and liquids behave like fluids

(d) Only liquids are fluids

Sol. (c) Gases and liquids behave like fluids

---

Q.4 During summer, water kept in an earthen pot becomes cool because of the phenomenon of

(a) diffusion

(b) transpiration

(c) osmosis

(d) evaporation

Sol. (d) Evaporation

---

Q.5 A few substances are arranged in the increasing order of ‘forces of attraction’ between their particles. Which one of the following represents a correct arrangement?

(a) Water, air, wind

(b) Air, sugar, oil

(c) Oxygen, water, sugar

(d) Salt, juice, air

Sol. (c) Oxygen, water, sugar

---

Q.6 On converting 25°C, 38°C and 66°C to kelvin scale, the correct sequence of temperature will be

(a) 298 K, 311 K and 339 K

(b) 298 K, 300 K and 338 K

(c) 273 K, 278 K and 543 K

(d) 298 K, 310 K and 338 K

Sol.  (a) 298 K, 311 K and 339 K

---

Q.7 Choose the correct statement of the following

(a) conversion of solid into vapours without passing through the liquid state is called vapourisation.

(b) conversion of vapours into solid without passing through the liquid state is called sublimation.

(c) conversion of vapours into solid without passing through the liquid state is called freezing.

(d) conversion of solid into liquid is called sublimation.

Sol. (b) conversion of vapours into solid without passing through the liquid state is called sublimation.

---

Q.8 The boiling points of diethyl ether, acetone and n-butyl alcohol are 35°C, 56°C and 118°C respectively. Which one of the following correctly represents their boiling points in kelvin scale?

(a) 306 K, 329 K, 391 K

(b) 308 K, 329 K, 392 K

(c) 308 K, 329 K, 391 K

(d) 329 K, 392 K, 308 K

Sol. (c) 308 K, 329 K, 391 K

---

Q.9 Which condition out of the following will increase the evaporation of water?

(a) Increase in temperature of water

(b) Decrease in temperature of water

(c) Less exposed surface area of water

(d) Adding common salt to water

Sol. (a) Increase in temperature of water

---

Q.10 In which of the following conditions, the distance between the molecules of hydrogen gas would increase?

(i) Increasing pressure on hydrogen contained in a closed container

(ii) Some hydrogen gas leaking out of the container

(iii) Increasing the volume of the container of hydrogen gas

(iv) Adding more hydrogen gas to the container without increasing the volume of the container

(a) (i) and (iii)

(b) (i) and (iv)

(c) (ii) and (iii)

(d) (ii) and (iv)

Sol. (c) (ii) and (iii)