Matter-Nature and Behaviour-NCERT EXERCISE-1
Question 1. Convert the following temperature to celcius scale.
(a) 300 K
(b) 573 K
Answer.
To change temperature on Kelvin scale (K) to celcius scale (°C) you have to subtract 273.16 from the given temperature on Kelvin scale.
(a) 300 K
300K = (300-273.16)°C
= 26.84°C(or 27°C)
(b) 573 K
573K = (573-273.16)°C
= 299.84°C(or 300°C)
[Note: In ordinary calculations we use 273.0 instead of 273.16].
Question 2. What is the physical state of water at
(a) 250°C
(b) 100°C
Answer.(a)
The boiling point of water is 100°C and above this temperature it is in gaseous state. At 250°C, water is present in gaseous state only.
Answer.(b)
At 100°C, there is equilibrium between gaseous state and liquid state i.e., both
states liquid and gaseous are present.
Question 3. For any substance why does the temperature remain constant during the change of state?
Answer.
The temperature remains constant during the change of state because the heat supplied is used up in changing the state by overcoming the intermolecular forces of attraction. This heat energy is known as latent heat.
Question 4. Suggest a method to liquefy atmospheric gases
Answer.
The atmospheric gases can be liquified by applying pressure and reducing temperature
Question 5. Why does a cooler cool better on a hot dry day?
Answer.
On a hot dry day the amount of water vapour present in air is less, (i.e., humidity is low) and so, the rate of evaporation increases. Due to this a cooler cool better on a hot dry day.
Question 6. How does the water kept in an earthen pot (matka) become cool during summer?
Answer.
The earthen pot has a large number of tiny pores which provide a large surface area for evaporation of water kept in it. Evaporation cause cooling. The rate of evaporation increases with rise of temperature. Since, the temperature is high during summer so rate of evaporation increases and the water kept in an earthen pot become cool.
Question 7. When we pour some acetone or perfume on our palm, we get a cooling sensation. Assign reason.
Answer.
Acetone or perfume particles gain energy from our palm for evaporation and this gives a feeling of coolness and we feel a cooling sensation.
Question 8. We can sip hot tea from a saucer faster than from a cup?
Answer.
Surface area of saucer is more as compared to that of a cup. Due to increase in surface area, the rate of evaporation increases and thus the temperature of the hot tea in saucer gets lowered and we can sip it faster than from a cup.
Questioon 9. Convert the following temperature to Celcius scale.
(a) 293 K
(b) 470 K
Answer.
(a)
293K = (293-273.16)°C
= 19.84°C [or20°C]
(b)
470K = (470-273.16)°C
= 196.84°C [orl97°C]
Question 10. Convert the following temperature to Kelvin scale.
(a) 25°C
(b) 373°C
Answer.
(a)
25°C = (25 + 273.16)K
= 298.16 K [or 298.0 K]
(b)
373°C=(373+273.16)K
= 646.16 K [or 646.0 K]
Question 11. Give reasons for the following observations :
(a) Naphthalene balls disappear with time without leaving any solid.
Answer. (a)
Naphthalene is a sublime substance. It changes from solid state to gaseous state without passing through the liquid state. When kept in open the naphthalene balls get sublimed and the vapours go to atmosphere and no solid is left as the balls disappear.
(b) We can get the smell of perfume sitting several meters away.
Answer. (b)
The particles of perfume which is a mixture of number of pleasant smelling vapour mix with the particles of air around us and spread out. Due to this spreading we are able to get the smell even sitting several meters away.
Question 12. Arrange the following substances in the increasing order of forces of attraction between the particles – water, sugar, oxygen.
Answer.
We know that the forces of attraction between particles are maximum in case of solids and minimum in case of gases. The correct order is
oxygen < water < sugar
(gas)< (liquid)< (solid)
Question 13. What is the physical state of water at
(a) 25°C
(b) 0°C
(c) 100°C
Answer.(a)
Freezing point of water is 0°C and its boiling point is 100°C between these two temperatures it is in liquid state i.e. at 25°C water is in liquid state.
Answer (b)
It represent freezing point of water and both solid and liquid states are in equilibrium.
Answer (c)
It is boiling point of water and at this temperature both liquid and vapour (gaseous) state are in equilibrium.
Question 14. Give two reasons to justify:
(a) water at room temperature is a liquid.
Answer. (a)
At room temperature water is liquid because it has no definite shape. It takes the shape of the container. It has a definite volume. Water shows the property of fluidity.
(b) an iron almirah is solid at room temperature.
Answer (b)
An iron almirah has a definite shape and also a definite volume. So, it is a solid at room temperature.
Question 15. Why is ice at 273 K more effective in cooling than water at the same temperature?
Answer.
When ice (a solid) melts at 273 K (m.p. of ice) it absorbs energy from its surroundings in order to overcome the forces of attraction present in solid particles. Due to this the temperature of surroundings gets lowered and cooling is caused. In case of liquid, water at 0°C there is no such tendency of changing to vapour state and so it has almost negligible tendency to absorb any energy from surroundings. Therefore, it will cause comparatively less cooling.
Question 16. Why does steam produce more severe burns on the skin as compared to boiling water?
Answer.
When water is boiled at 100°C (373 K) it absorbs certain amount of heat energy {latent energy) to get converted into steam. It results is more energy in steam than in water at 100°C. Due to this (i.e., more energy) steam produces more severe burns on the skin as compared to boiling water.
Question 17. Name A, B, C, D, E and F in the following diagram showing change of state.
Answer.
A = Fusion (or melting)
B = Vapourisation
C = Condensation (or liquefaction)
D = Solidification (or freezing)
F = Solidification of gaseous state.