NCERT Class 9 Science So lutions: Chapter 1 – Matter in Our Surroundings Part 2

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Image of States of Matter Solids and Gases for Matter in our …

Image of States of Matter

Question 3:

Give reasons:

(a) A gas fills completely the vessel in which it is kept.

A. There is little attraction between particles of gas. Thus, gas particles movefreely in all direction there gas completely fills the vessel in which it is kept.

(b) A gas exerts pressure on the walls of the container.

A. particles of gas move randomly in all directions at high speed. As a result, the particles hit each other and also hit the walls of the container with a force. Therefore, gas exerts pressure on the walls of the container.

(c) A wooden table should be called a solid.

A. A wooden table has a definite shape and volume. It is very rigid and cannot be compressed i.e., it has the characteristics if a solid. Hence, 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.

A. Particles of air have large spaces between them. On the other hand, wood has little space between its particles. Also, it is rigid. For this reason, we can easily move our hands in air, but to do the same through a solid block of wood, we need a karate expert.

Question 4:

Liquids generally have lower density as compared to solids. But you must have observed that ice floats on water. Find out why.

A. The mass per unit volume of a substance is called density (density = mass/volume). As the volume of a substance increases, its density decreases.

Though ice is solid, it has large number of empty spaces between its particles. These spaces are large as compared to the spaces present between the particles of water. Thus, the volume of ice is greater than that of water. Hence, the density of ice is less than that of water. A substance with lower density than water can float on water. Therefore, ice floats on water.

Question 1:

Q. Convert the following temperature to Celsius scale:

(a) 300 K

A.

(b) 573 K

A.

Question 2:

Q. What is the physical state of water at:

(a) 250

A. Water at 250 oC exists in gaseous state.

(b) 100

A. at 100, water can exist in both liquid and gaseous form. At this temperature, after getting the heat of vaporization, water starts changing from liquid state to gaseous state.

Question 3:

Q. For any substance, why does the temperature remain constant during the change of state?

A. During a change of state, the temperature remains constant. This is because all the heat supplied to increase the temperature is utilised in changing the state by overcoming the forces of attraction between the particles. Therefore, this heat does not contribute in increasing the temperature of the substance.

Question 4:

Q. Suggest a method to liquefy atmospheric gases.

A. By applying pressure and reducing the temperature, atmospheric gases can be liquefied.

Question 1:

Q. Why does a desert cooler cool better on a hot dry day?

A. When a liquid evaporates, the particles of the liquid absorb energy from the surroundings to compensate the loss of energy during evaporation. This makes the surroundings cool.

In a desert cooler, the water inside it is made to evaporate. This leads to absorption of energy from the surroundings, thereby cooling the surroundings. Again, we know that evaporation depends on the amount of water vapour present in air (humidity). If the amount of water vapour present in air is less, then evaporation is more. On a hot dry day, the amount of water vapour present in air is less. Thus, water present inside the desert cooler evaporates more, thereby cooling the surrounding more. That is why a desert cooler cools better on a hot dry day.

Question 2:

Q. how does water kept in an earthen pot (matka) become cool during summers?

A. there are some pores in an earthen pot through which the liquid inside the pot evaporates. This evaporation makes the water inside the pot cool. In this way, water kept in an earthen pot becomes cool during summers.

Question 3:

Q. why does our palm feel cold when we put some acetone or petrol or perfume on it?

A. when we put some acetone or petrol or perfume on our palm, it evaporates. During evaporation, particles of the liquid absorb energy from the surrounding or the surface of the palm to compensate for the loss of energy, making the surroundings cool. Hence, our palm feels cold when we put some acetone or petrol or perfume on it.

Question 4:

Q. why are we able to sip hot tea or milk faster from a saucer than a cup?

A. a liquid has a larger surface area in a saucer than in a cup. Thus, it evaporates faster and cools faster in a saucer than in a cup. For this reason, we are able to sip hot tea or milk faster from saucer than cup.

Question 5:

Q. What type of clothes should we wear in summers?

A. We should wear cotton clothes in summers. During summers, we sweat more. On the other hand, cotton is good absorber of water. Thus, it absorbs sweat from our body and exposes the liquid to the atmosphere, making evaporation faster. During this evaporation, particles on the surface of the liquid gain energy from our body surface, making the body cool.

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