NCERT BOOK QUESTIONS AND EXERCISES

Chapter 1 : Matter in Our Surroundings

Part – 3

Question 1- Convert the following temperature to the Celsius scale :

(a)                      293 K              (b) 470 K

Answer – (a) Temp. On Kelvin scale = Temp. on Celsius scale + 273

293 = temperature on Celsius scale + 273

So, temperature on Celsius scale = 293 – 27

= 20oC

(b) Temp. On Kelvin scale = Temp. on Celsius scale + 273

470 = temperature on Celsius scale + 273

So, temperature on Celsius scale = 470 – 273

= 197oC

Question 2 – Convert the following temperature to the Kelvin scale -      (a)                      25oC        (b) 373oC

Answer – (a) Temperature  On Kelvin scale = Temperature on Celsius scale + 273

= 25 + 273

= 298 K

(b) Temperature  On Kelvin scale = Temperature on Celsius scale + 273

= 373 + 273

= 646 K

Question 3 – Give reasons for the following observations –

(a) Naphthalene balls disappear with time without leaving any solid.

(b)   We can get the smell of perfume sitting several metres away.

(a) Naphthalene is a volatile solid organic compound which can undergo sublimation (change from solid states directly in to vapour state). The solid naphthalene balls keep subliming slowly. After a certain time, the naphthalene balls sublime completely forming naphthalene vapours and hence they disappear without leaving any solid residue.

(b) We can get the smell of perfume sitting several metres away due to the diffusion of perfume vapours into air. This can be explained as follows : Perfume is a volatile liquid. When liquid perfume is applied by a person, it quickly changes in to vapours. The perfume vapours move very rapidly in all directions in air, mix up with air particles and spread in the air by diffusion. When this air containing perfume vapours reaches several metres away, we can smell the perfume.

Question 4 – Arrange the following substances in increasing order of forces of attraction between the particles – water, sugar, oxygen.

Answer – The forces of attraction between the particles in a solid are the strongest, in liquids are less strong whereas in gases are the weakest. Now out of water sugar and oxygen :

(i) Oxygen is a gas, so it has the weakest forces of attraction between its particles.

(ii) Water is a liquid, so it has a stronger forces of attraction between its particles.

(iii) Sugar is a solid, so it has the strongest forces of attraction between its particles.

Thus, the increasing order of forces of attraction between the particles of water, sugar and oxygen will be :

oxygen < water < sugar

Question 5 – What is the physical state of water at :

(a)  25oC         (b) 0oC        (c) 100oC

Answer – (a) The physical state of water at 25oC is liquid.

(b) 0oC is the melting point of ice as well as the freezing point of water. So, the physical state of water at 0oC can be either a solid or liquid.

(c) 100oC is the boiling point of water as well as the condensation temperature of steam. So, the physical state of water at 100oC can be either a liquid or a gas.

Question 6 – Give two reasons to justify :

(a) Water at room temperature is a liquid.

(b) An iron almirah is a solid at room temperature.

Answer – (a) The two general properties of liquids are that liquids have a fixed volume but no fixed shape. Now water is a liquid at room temperature because :

(i)              Water has a fixed volume.

(ii)           Water has no fixed shape.

(b)The two general properties of solids are that solids have a fixed shape and a fixed volume. An almirah is a solid at room temperature because :

(i) an almirah has a fixed shape.

(ii) an almirah has a fixed volume.

Question 7 – Why is ice at 273K more effective in cooling than water at the same temperature?

Answer – First of all please note that the temperature of 273K is equal to 0oC and it is the melting point of ice at the same temperature. Another point to be noted is that the latent heat of melting of ice is 3.34 x 105 joules per kilogram. Now ice at 273K is more effective in cooling a substance than water at the same temperature of 273 K because for melting each kilogram of ice takes 3.34 x 105 joules of its latent heat from that substance and hence cools the substance more effectively. On the other hand, water at the same temperature of 273K cannot take away any such latent heat from the substance and hence does not cool it more effectively.

Question 8 – What produces more severe burns – boiling water or steam ?

Answer – When water changes in to steam at its boiling point, it absorbs latent heat of vaporisation. This means that steam at 100oC contains much more heat than boiling water at same temperature of 100oC. So, when steam falls on our skin and condenses to form water, it gives out 22.5 x 105 joules per kilogram of more heat than boiling water at the same temperature. Since steam gives out more heat than boiling water, it cause more severe burns.

Question 9 – Name A, B, C, D, E, and F in the following diagram showing changes in state :

(i) In process A , a solid is changing into a liquid, so A is melting.

(ii) In process B, a liquid is changing in to a gas, so B is vaporisation.

(iii) In process C, a gas is changing into a liquid, so C is condensation.

(iv) In process D, a liquid is changing in to a solid, so D is freezing.

(v) In process E, a solid is directly changing into a gas, So E is sublimation.

(vi) In process F, a gas is changing directly into a solid, so F is also sublimation.