CBSE 9th Chemistry | Matter-Nature and Behaviour-NCERT EXERCISE-1

September 1st, 2015 by Amit Thakur No comments »

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.

 

image

Answer.

A = Fusion (or melting)

B = Vapourisation

C = Condensation (or liquefaction)

D = Solidification (or freezing)

F = Solidification of gaseous state.

CBSE 9th Chemistry | Matter-Nature and Behaviour – NCERT solved Questions

September 1st, 2015 by Amit Thakur No comments »

Matter-Nature and Behaviour – NCERT solved Questions

Question 1. Which of the following are matter?

Chair, air, love, smell, hate, almonds, thought, cold-drink, smell of perfume.

 

Answer.

The following are matter.

Chair, air, almonds, cold-drink, smell of perfume.

[NOTE] : Love, hate, smell and thought are not matter as they neither have mass, nor occupy space.

Question 2. Give reason for the following observations. The smell of hot sizzling food reaches you several meters away, but to get smell from cold food you have to go close.

Answer.

The particles of aroma in hot sizzling food mix with the particles of air around us. Due to this spreading of particles, we are able to get the smell even at a distance (i.e., several meters away). Thus, it is the result of diffusion which increases with increase of temperature. We know that the rate of diffusion increases on heating so the smell of hot sizzling food reaches us several meters away but to get smell from cold food we have to go close.

Question 3. A diver is able to cut through water in a swimming pool. Which property of matter does this observation show?

Answer.

It shows the property of compressibility in case of liquid state (water is a liquid). Due to large interparticle space in liquids (water) diver is able to cut through water.

Question 4. What are the characteristics of particles of matter?

Answer.

The characteristics of particles of matter are :

(i) They have space between them.

(ii) They are continuously moving.

(iii) They attract each other.

Question 5. The mass per unit volume of a substance is called density.

image

Arrange the following in order of increasing density, air, exhaust from chimneys, honey, water, chalk, cotton, iron.

Answer.

In increasing order of density, they can be arranged as follows:

air < exhaust from chimneys < water < honey < cotton < iron.

Question 6.

(a) Tabulate the differences in the characteristics of states of matter,

Answer.(a)

Differences in the characteristics of matter:

Property Solids Liquids Gases
1. Shape They have a definite shape. 1. They take the shape of container and have no definite shape. 1. They take the shape of the container and have no definite shape.
2. Volume They have a fixed volume. 2. They have a fixed volume. 2. The volume is not fixed. They completely occupy the available volume. Their volume is equal to the volume of container.
3. Density The density of solids is higher than other states of matter. 3. The densities of liquids are more as compared to gases. 3. Their densities are least.
4. Intermolecular forces The intermolecular forces of attraction are maximum. 4. The intermolecular forces of attraction are stronger than those of gaseous particles. 4. The intermolecular forces of attraction are minimum.
5. Intermolecular space The intermolecular space is minimum 5. The intermolecular space is more than those of solids but less than those of liquids 5. The intermolecular space is maximum.
6. Compressibility Compressibility is least 6. Compressibility more than that of solids but less than that of liquids. 6. Compressibility is maximum.
7. K.E. Kinetic energy possessed by particles is minimum. 7. Kinetic energy possessed by particles is more than those of solids but less than those of gases. 7. Kinetic energy is maximum.
8. Diffusion Show negligible 8. In them the process of diffusion is slow. 8. In them process of diffusion is very fast.

 

(b) Comment upon the following: rigidity, compressibility, fluidity, filling a gas in a container, shape, kinetic energy and density.

Answer (b)

(i) Rigidity:

It refers to the property shown by solids. Because of, the absence of property of flow in solids they are considered as rigid. It is due to low value of K.E. in solids.

(ii) Compressibility:

It is the property which depends upon the availability of interparticle space. Because in solids the intermolecular space is quite lesi. So they are incompressible, where gases show the property of compressibility to maximum extent.

(iii) Fluidity:

It is the property to flow. It is shown by liquids and gases which are collectively known as fluids. It is maximum for gases.

(iv) Filling of gas in cylinder:

The gases have maximum compressibility and due to this a large volume of gas can be compressed under pressure to a very small volume. The LPG and CNG cylinders contain gases compressed under pressure.

(v) Shape:

Solids have a definite shape, liquids and gases do not have a definite shape and they take the shape of the container.

(vi) Kinetic energy:

It is the energy possessed by molecules by virtue of their motion. The kinetic energy is maximum in case of particles of gases and least in case of particles of solids.

(vii) Density:

It is the ratio of mass to volume. The density of solids is more than the density of liquids and gases. The density of gases is lowest due to large intermolecular space in them.

Question 7. Give reasons:

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

 

Answer. (a)

Since, the interparticle force of attraction between gaseous particles is very small. So they neither have a definite shape nor a definite volume. The gas completely fills the vessel in which it is kept and its volume is the same as that of the vessel. It is due to very high rate of diffusion in case of gases. [Note: The actual volume occupied by gaseous molecules is negligible as compared to the volume of the gas].

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

 

Answer (b)

Gaseous particles are in a state of random motion at high speed. Due to random motion particles hit each other and also the walls of the container. The pressure exerted by the gas is because of the force exerted by gas particles per unit area on the walls of the container.

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

 

Answer (c)

Since, the wooden table has a definite shape and definite volume. So, it should be classified as a solid.

The other properties associated with solids are that they have distinct boundaries, rigidity and incompressibility. These properties are also formed in case of wooden table. So, it is 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.

 

Answer (d)

Air is a gas. In gaseous state there is maximum interparticle distance and so they are highly compressible. Due to this property of compressibility we can easily move our hand in air but solids are incompressible and in them the constituent particles are quite close. So we need a large force to move our
hands through a block of solid. This can be done only by a Karate expert.

Question 8. Liquids generally have lower density as compared to solids. But you must have observed that ice floats on water. Find out why?

Answer.

Ice (a solid at room temperature) is lighter and floats on water, (a liquid at room temperature) because ice has a cage-like structure in which there are large vacant space between various water molecules which are linked to form ice. In water molecule, such spaces are negligible. Thus, we can say that ice floats on water because the structure of ice is more porous than that of water. Due to this the density of water is more than that of ice and so ice floats over water.