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Question 1.
A rubber band can change its shape on stretching. Will you classify it as solid or not? Justify?
Answer
Rubber band changes shape under force and regains the shape when the force is removed. So, it is classified as a solid.

Question 2.
Sponge, though compressible, is a solid?
Answer
A sponge has minute holes in which air is trapped. When we press it, the air is expelled out and we are able to compress it. On releasing pressure, it again regains its shape. So, it is classified as a solid.

Question 3.
Gases completely fill the vessel in which they are kept. Give reasons.
Answer
In the gaseous state, particles move freely and have greater space between them. So they occupy the entire space available. Hence, gases completely fill the vessel in which they are kept.

Question 4.
Under what conditions gases can be liquefied? In which form LPG is filled in gas cylinder?
Answer
By applying pressure and reducing temperature, gases can be liquefied. The liquefied petroleum gas (LPG) is filled in gas cylinders in the compressed gas form.

Question 5.
Liquid generally have lower density as compared to solids, but ice floats on water. Find out, why.
Answer
Ice and water both are the same substance but ice has greater volume than that of the same amount of water.
As density is inversely proportional to the volume, ice is less denser than water.
Objects having density less than water float on the surface of water.

Question 6.
What is dry ice?
Answer
Solid CO2 is called dry ice, because solid CO2 gets converted directly into gaseous state without coming into liquid state on decreasing pressure to one atmosphere.

Question 7.
Explain why temperature remains constant during interconversion of states of matter?
Answer
Heat supplied to a substance is getting used during changing its state to overcome the force of attraction between the particles. The excess heat is absorbed by the particles in the form of latent heat.

Question 8.
Give reason to explain why it takes longer time to dry wet clothes in humid weather?
Answser
Rate of evaporation depends on humidity present in air. Humid air already has large amount of water vapours, so rate of evaporation is slow.

Question 9.
Why should we wear cotton clothes during summer?
Answer
We perspire more during summer. Cotton is a good absorber of water. It absorbs sweat and exposes it for easy evaporation. As a result body feels cool and comfortable. So, we should wear cotton clothes during summer.

Question 11.
Why do people sprinkle water on the roof after a hot sunny day?
Answer
After a hot sunny day, people sprinkle water on the roof or open ground because the large latent heat of vaporisation of water helps to cool the surface. Water takes away the heat from the ground and gives cooling effect.

Question 12.
Write any three differences between evaporation and boiling?
Answer

Question 13.
Why does ice at 0°C appear colder than water at same temperature?
Answer
Particles in water at 0° C (273K) have more energy as compared to the particles in ice at the same temperature. It is due to the latent heat o fusion.

Question 14.
Why mixture does not have a fixed melting point or a fixed boiling point? Give two reasons?
Answer
It does not have fixed composition and it does not have uniform ordered arrangement of particles.

Question 17.
A gas exerts pressure on the walls of container, why?
Answer
Gas molecules collide with each other as well as with the walls of container. Therefore, gas exerts pressure.

Question 18.
The smell of hot sizzling food reaches you several meters away, but to get the smell from cold food you have to go close.
Answer
Solids diffuse at a very slow rate. But, if the temperature of the solid is increased, then the rate of diffusion of the solid particles into air increases. This is due to an increase in the kinetic energy of solid particles. Hence, the smell of hot sizzling food reaches us even at a distance, but to get the smell from cold food we have to go close.

Question 19.
Comment upon the following: rigidity, compressibility, fluidity, filling a gas container, shape, kinetic energy, and density.
Answer

Question 21.
Why does our palm feel cold when we put some acetone or petrol or perfume on it?
Answer
Acetone, petrol, and perfume evaporate at low temperatures. When some acetone, petrol, or perfume is dropped on the palm, it takes heat from the palm and evaporates, thereby making the palm cooler.

Question 22.
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.
Answer
Naphthalene balls disappear with time without leaving any solid because they undergo sublimation easily i.e., the change of state of naphthalene from solid to gas takes place easily.
Perfumes has high degree of vaporization and its vapour diffuse into air easily. Therefore, we can get the smell of perfume sitting several metre away.

Question 23.
Give two reasons to justify-
(a)Water at room temperature is a liquid.
(b)An iron almirah is a solid at room temperature.
Answer
Water at room temperature is a liquid because it has fluidity also it has no shape but has a fixed volume that is, it occupies the shape of the container in which it is kept.
Iron almirah is a solid at room temperature it has rigid and fixed shape.

Question 24.
A glass tumbler containing hot water is kept in the freezer compartment of a refrigerator (temperature < 0⁰C). If you could measure the temperature of the content of the tumbler, which of the following graphs would correctly represent the change in its temperature as a function of time.

Answer
Since ice and water are in equilibrium, the temperature would be zero. When we heat the mixture, energy supplied is utilized in melting the ice and the temperature does not change till all the ice melts because of latent heat of fusion. On further heating, the temperature of the water would increase. Therefore the correct option is (d).
Question 25.
i.Convert the following Kelvin temperature to degrees Celsius.
a. 173 K
b. 273 K
c. 400 K
ii.Convert the following Celsius temperature to Kelvin temperature.
a. -73 ° C
b. -23 ° C
c. 100 ° C
Answer

i. We know that ° Celsius = K – 273
a. ° C= K−273=173−273K−273=173−273 =-100 ° C
b. ° C= K−273=273−273K−273=273−273= 0 ° C
c. ° C= K−273=400−273K−273=400−273= 127° C

ii. We know that K = ° C + 273
a. K=° C + 273 = -73 + 273 = 200 K
b. K=° C + 273 = -23 + 273 = 250 K
c. K=° C + 273 = 100 + 273 = 373 K