(IN TEXT QUESTIONS): (PAGE: 3)
1: Which of the following are matter : Chair, air, love, smell, hate, almonds, thought, cold drink, smell of perfume.
Ans: a) Chair and almond are forms of matter in the solid state.
b) Cold drink is a liquid state of matter.
c) Air is a gaseous state of matter.
2: Give reasons for the following observation: The smell of hot sizzling food reaches you several metres away, but to get the smell from cold food you have to go close.
Ans: 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 the air increases. This is due to an increase in the kinetic energy of the solid particles. The smell of cold food cannot reach us because the temperature of the food is cold and hence the rate of diffusion is also slow.
3: A diver is able to cut through water in a swimming pool. Which property of matter does this observation show.
Ans: The ability of a diver to cut through water in a swimming pool shows that matter is made up of particles. that attract each other, A driver cuts across these particles and enters the water.
4: What are the characteristics of particles of matter.
Ans: The characteristics of particles of matter are:
(i) Particles of matter have spaces between them.
(ii) Particles of matter are continuously moving.
(iii) Particles of matter attract each other.
(IN TEXT QUESTIONS): (PAGE 6)
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 chimney, honey, water, chalk, cotton, and iron.
Ans: The given substances in the increasing order of their densities can be represented as:
Air < Exhaust from chimney < Cotton < Water < Honey < Chalk < Iron
2: a) Tabulate the differences in the characteristics of states of matter.
b) Comment upon the following:
rigidity, compressibility, fluidity, filling a gas container, shape, kinetic energy, and density.
Ans: a) The differences in the characteristics of states of matter are given in the following table.
| S No | Solid state | Liquid state | Gaseous state |
| 1. | Definite shape and volume. | No definite shape. Liquids attain the shape of the vessel in which they are kept. | Gases have neither a definite shape nor a definite volume. |
| 2. | Incompressible. | Compressible to a small extent. | Highly compressible. |
| 3. | There is little space between the particles of a solid. | These particles have a greater space between them. | The space between gas particles is the greatest. |
| 4. | These particles attract each other very strongly. | The force of attraction between liquid particles is less than solid particles. | The force of attraction is least between gaseous particles. |
| 5. | Particles of solid cannot move freely. | These particles move freely. | Gaseous particles move in all direction. |
1. b) (i) Rigidity can be expressed as the tendency of matter to resist a change in it shape.
(ii) Compressibility is the ability to be reduced to a lower volume when force or pressure is applied.
(iii) Fluidity is the ability of a substance to flow. filling a gas container means the attainment of shape of the container by the gas.
(iv) Kinetic energy is the energy possessed by a particle due to its motion.
(v) Density is mass per unit volume.
3) Give reasons:
a) A gas fills completely the vessel in which it is kept.
b) A gas exerts pressure on the walls of the container.
c) 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.
Ans: a) There is little attraction between particles of gas. Thus, gas particles move freely in all directions. Therefore, gas completely fills the vessel in which it is kept.
b) 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 has a definite shape and volume. It is very rigid and cannot be compressed
i.e., it has the characteristics of a solid. Hence, a wooden table is being called a solid.
d) This is because air has the least force of attraction between its molecules but in solids there is a strong force of attraction between its molecules, the karate experts uses various techniques to overcome the force of attraction between the particles of solid.
4) Liquids generally have lower density as compared to solids but you must have observed that ice floats on water, find out why.
Ans: It is because ice has an open net like structure that has empty spaces. It has more volume but less mass, so it has lower density than water though it is a solid that is why it floats.
(IN TEXT QUESTIONS): (PAGE 9)
1: Convert the following temperature to Celsius scale:
- a) 300 K b) 573 K
Ans: a) 300 K = (300 − 273)°C = 27°C
- b) 573 K = (573 − 273)°C = 300°C
2: What is the physical state of water at:
- a) 250°C b) 100°C
Ans: a) Water at 250°C exists in gaseous state.
b) At 100°C, water can exist in both liquid and gaseous form. At this temperature, after getting the heat equal to the latent heat of vaporization, water starts changing from liquid state to gaseous state.
3: For any substance, why does the temperature remain constant during the change of state.
Ans: During a change of state, the temperature remains constant. This is because all the heat supplied to increase the temperature is utilized as latent heat 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.
4: Suggest a method to liquefy atmospheric gases.
Ans: By applying high pressure and reducing the temperature, atmospheric gases can be liquefied.
(IN TEXT QUESTIONS): (PAGE 10)
1: Why does a desert cooler cool better on a hot dry day.
Ans: 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 surroundings more.
2: How does water kept in an earthen pot (matka) become cool during summers.
Ans: (i) There are some pores in an earthen pot through which the liquid inside the pot evaporates. (ii) When a liquid evaporates, the particles of the liquid absorb energy from the container to compensate the loss of energy during evaporation. This makes the remaining water cool. (iii) This evaporation makes the water inside the pot cool.
3: Why does our palm feel cold when we put some acetone or petrol or perfume on it.
Ans: When we put some acetone petrol or perfume (all volatile liquids) 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.
4: Why are we able to sip hot tea or milk faster from a saucer than a cup.
Ans: 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. Greater the surface area greater will be the cooling.
5: What type of clothes should we wear in summer.
Ans: We should wear cotton clothes in summer. During summer, we sweat more. On the other hand, cotton is a 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.