NCERT Solutions for Class 9 Science Chapter-1 “Matter In Our Surroundings”

TEXTUAL QUESTIONS

EXERCISE – 1.1 – 1.2 PAGE – 3

Q.1- Which of the following are matter- Chair , air, love, smell, hate, almonds, thought, cold, cold drink, smell of perfume.

Ans- Chair, Air, Almonds, Cold drink.

Q.2- Give the reasons for following observations-

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- This is because the rate of diffusion of a gas increases with the increase in temperature. In hot food, the rate of diffusion is high, but in cold food, the rate of diffusion is low.

Q.3-The diver is able to cut through water in a swimming pool, which property of matter does this observation show?

Ans- The diver is able to cut through the water in a swimming pool because of the weak forces of attraction between water molecules.

Q.4- What are the characteristics of particles of matter?

Ans- The characteristics of particles of matter are-

a) The particles of matter have intermolecular spaces between them.

b) They are always in motion.

c) They attract each other.

EXERCISE – 1.3 PAGE – 6

Q.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 chimneys, honey, water, chalk, cotton and iron.

Ans- The increasing order of density is-

Exhaust from chimneys, air, water, honey, cotton, chalk, iron.

Q.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-

b) RIGIDITY- It is the property by virtue of which a substance can retain its shape when a force is applied on it. Solids posses the property of rigidity.

COMPRESSIBILITY- It is the property by virtue of which the volume of a substance can be decreased by applying force or pressure on it. Gases have high compressibility because of the large intermolecular spaces between their molecules.

FLUIDITY- It is the property by virtue of which the molecules of a substance can flow from one point to another. Liquids and gases possess fluidity.

FILLING A GAS CONTAINER- A gas fills the container because there are negligible attractive forces between the gas molecules and the molecules of gas move in all possible directions.

SHAPE- It is the definite geometrical arrangement of constituents of a substance. Solids have definite shapes.

KINETIC ENERGY- It is the energy possessed by a particle due to its motion.

DENSITY- Density of a substance is the mass per unit volume. Density = Mass/ Volume

Q.3- Give reasons-

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

b) A gas exerts pressure on the walls of a 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) The attractive forces between the gas molecules are negligible due to which the molecules of gas move with a very high speed in all possible directions and hence fills the vessel completely.

b) The gas exerts pressure on the walls of a container because the molecules of the gas are in constant random motion due to high kinetic energy. These molecules move constantly and hit the walls of the container thereby exerting pressure on it.

c) This is because the molecules of wood are tightly packed. There are no intermolecular spaces between them and hence cannot be compressed. These are the properties of a solid. That’s why a wooden table should be called a solid.

d) We can easily move our hand in air because the molecules of air have less force of attraction between them. But in case of solids, the molecules have maximum force of attraction and the particles are tightly packed.

Q.4- Liquids generally have lower density as compared to solids. But you must have observed that ice floats on water. Find out why.

Ans- Density of ice is 0.9 g/cm cube whereas the density of water is 1 g/cm cube. Since ice has less density than water, it floats on water.

EXERCISE – 1.4 PAGE – 9

Q.1- Convert the following temperature to celsius scale- a) 300 K (b) 573 K

Ans- a) 300 K = 300 – 273 = 27° C

b) 573 K = 573 – 273 = 300° C

Q.2- What is the physical state of water at

(a) 250°C ( b) 100°C

Ans- a) 250°C- Gas (b) 100°C- Liquid and gas

Q.3- For any substance, why does the temperature remain constant during the change of state?

Ans- This is because the heat supplied is used to overcome the forces of attraction between the particles.

Q.4- Suggest a method to liquefy atmospheric gases.

Ans- The atmospheric gases can be liquefied by cooling and applying pressure on them.

EXERCISE – 1.5 PAGE – 10

Q.1- Why does a desert cooler cool better on a hot dry day?

Ans- This is because on a hot dry day, due to increase in temperature and lesser humidity the evaporation of water will be faster. Evaporation leads to cooling.

Q.2- How does the water kept in an earthen pot become cool during summer?

Ans- The earthen pot is porous. The water oozes out from these pores gets evaporated. Evaporation causes cooling effect and the water kept in the earthen pot become cool.

Q.3- Why does our palm feel cold when we put some acetone or petrol or perfume on it?

Ans- Acetone or petrol or perfume evaporate when they come in contact with air. Evaporation causes cooling.

OR

When we put some acetone or petrol or perfume on our palm, they gain heat from our palm and evaporate. This evaporation causes cooling.

Q.4- Why are we able to sip hot tea or milk faster from saucer rather than a cup?

Ans- The surface area of a saucer is more than a cup. More is the surface area, faster will be the evaporation. Hence the tea or milk in saucer cools down faster than a cup.

Q.5- What type of clothes should we wear in summer?

Ans- We should wear light coloured cotton clothes in summer. Light colour does not absorb heat. Cotton clothes have pores which absorbs sweat and allows the sweat to evaporate faster thereby giving cooling effect.

TEXTUAL EXERCISE PAGE – 12

Q.1- Convert the following temperature in Celsius scale-

(a) 300 K (b) 573 K (c) 250 K

Ans- a) 300 K = 300 – 273 = 27°C

b) 573 K = 573 – 273 = 300°C

c) 250 K = 250 – 273 = -23°C

Q.2- Convert the following temperatures to the Kelvin scale.

a) 25°C (b) 373°C

Ans- a) 25°C = 25+273 = 298 K

b) 373°C = 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 metres away.

Ans- a) Naphthalene balls disappear with time without leaving any solid because naphthalene balls undergo sublimation and directly changes from solid state into vapour state without leaving any solid.

b) Perfume contain volatile solvent which diffuses faster and can reach people sitting several metres away. That’s why we can get the smell of perfume sitting several metres away.

Q.4- Arrange the following substances in increasing order of forces of attraction between the particles- water, sugar, oxygen.

Ans- Oxygen, water, sugar.

Q.5- What is the physical state of water at

a) 25°C ( b) 0°C (c) 100°C

Ans- a) At 20°C water is liquid.

b) At 0°C water is solid (ice).

c) At 100°C water is gas ( steam).

Q.6- Give two reasons to justify that-

a) Water at room temperature is liquid.

b) An iron almirah is solid at room temperature.

Ans- a) Water is liquid at room temperature because its freezing point is 0°C and boiling point is 100°C.

Also the intermolecular forces are strong enough to keep its particles bound together and prevent them from going into vapour state.

b) An iron almirah is a solid at room temperature because melting point of iron is much more higher than the room temperature.

Q.7- Why is ice at 273 K more effective in cooling than water at the same temperature?

Ans- Ice at 273 K absorbs latent heat to overcome the fusion to become water. On the other hand water does not absorb this extra heat from the medium. Therefore the cooling effect of ice is more than the water at same temperature.

Q.8- What produces more severe burns : boiling water or steam?

Ans- This is because when water gets converted into heat at 100°C, it absorbs latent heat of vaporisation. Hence steam has more heat than water at the same temperature. Therefore steam produces more severe burns than boiling water.

Q.9- Name A, B, C, D, E and F in the following diagram showing in its state-

Ans- A – Fusion. D – Solidification

B – Vaporisation. E – Sublimation

C – Condensation. F – Sublimation

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