Q 1: Which of the following are matter?
Chair, air, love, smell, hate, almonds, thought, cold, cold drink, smell of perfume.
Answer Anything that occupies space and has mass is called matter. Matter can exist in three physical states—solid, liquid, and gaseous.
Chair and almond are forms of matter in the solid state.
Cold drink is a liquid state of matter.
Air and smell of perfume are gaseous states of matter.
Q2: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.
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.
Q3 :A diver is able to cut through water in a swimming pool. Which property of matter does this observation show?
Answer :The ability of a diver to cut through water in a swimming pool shows that matter is made up of particles.
Q 4: What are the characteristics of particles of matter?
Answers
- The characteristics of particles of matter are:
- Particles of matter have spaces between them.
- Particles of matter are continuously moving.
- Particles of mater attract each other.
Page no.9
Q1 :Convert the following temperature to Celsius scale:
- a) 300 K
- b) 573 K
Answer :
a) 300 K = (300 − 273)°C = 27°C
b) 573 K = (573 − 273)°C = 300°C
Q 2:What is the physical state of water at:
- a) 250°C
- b) 100°C+
Answer:
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.
Q3:For any substance, why does the temperature remain constant during the change of state?
Answer: During a change of state, the temperature remains constant. This is because all the heat supplied to increase the temperature is utilised (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.
Q 4:Suggest a method to liquefy atmospheric gases.
Answer: By applying pressure and reducing the temperature, atmospheric gases can be liquefied.
Page no. 10
Q 1:Why does a desert cooler cool better on a hot dry day?
Answer :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 surroundings more.
That is why a desert cooler cools better on a hot dry day.
Q 2: How does water kept in an earthen pot (matka) become cool during summers?
Answer: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.
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.
Q 3:Why does our palm feel cold when we put some acetone or petrol or perfume on it?
Answer: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.
Q 4: Why are we able to sip hot tea or milk faster from a saucer than a cup?
Answer: 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 a saucer than a cup.
Q5:What type of clothes should we wear in summers?
Answer:We should wear cotton clothes in summers. During summers, 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
Q 1:Convert the following temperatures into the Celsius scale.
(a) 300 K (b) 573 K
Answer:
(a) 300 K = 300 – 273 = 27 0C
(b) 573 K = 573 – 273 = 300 0C
Q 2: Convert the following temperatures into the Kelvin scale.
(a) 25OC (b) 373OC
Answer :
(a) 25OC = 25 + 273 = 298 K
(b) 373OC = 373 + 273 = 646 K
Q 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 meters away.
Answer:
(a) Naphthalene shows the property of sublimation. Evaporation of naphthalene takes place easily and so it disappears during course of time without leaving a solid.
(b) Perfumes vaporize very fast and its vapours diffuse into air easily. That is why we can smell perfume sitting several meters away.
Q 4 Arrange the following in increasing order of forces of attraction between the particles – water, sugar, oxygen.
Answer :=Oxygen < Water < Sugar.
Q 5: What is the physical state of water at —
(a) 25°C (b) 0°C (c) 100°C?
Answer :
(a) Liquid
(b) Solid and Liquid
(c) Liquid and Vapours
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.
Answer:
(a) Water at room temperature is a liquid because it has fluidity and has definite volume but no definite shape.
(b) An iron almirah is a solid at room temperature because it is rigid and has a definite shape.
Q 7: Why is ice at 273 K more effective in cooling than water at the same temperature?
Answer: Ice at 273 K is less energetic than water. It is because of the difference in the latent heat of fusion which is present in water at the same temperature in the form of extra energy.
Q 8: What produces more severe burns, boiling water or steam?
Answer: Steam produces more severe burns than boiling water. This is because steam has more energy than boiling water, present in it in the form of latent heat of vaporization.