# NCERT Solutions Class 9 Science Chapter 1 Matter In Our Surroundings

#### In-Text Questions Page 03

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

Ans:-  Chair, air, almonds, and cold-drink.
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:- At higher temperature, particles have high kinetic energy and move faster. So, the particles of hot vapors from hot food move and diffuses faster in air. Hence, the smell of hot sizzling food reaches us several metres away, but to get the smell from cold food we have to go close.
3. A diver is able to cut through water in a swimming pool. Which property of matter does this observation show?
Ans:- A diver is able to cut through water in a swimming pool. This shows that the particles of water have more intermolecular space and has less force of attraction.
4. What are the characteristics of the particles of matter?
Ans:- The characteristics of the particles of matter are:-
(i) Particles of matter have space between them.
(ii) Particles of matter are continuously moving.
(iii) Particles of matter attract each other.

#### In-Text Questions Page 06

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:-  Increasing density:
air < exhaust from chimneys < 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 the three states of matter are:-

(b)
(i) Rigidity: It is the property of a substance to continue to remain in its shape when treated with an external force.
(ii) Compressibility: It is the property of the matter in which its volume is decreased by applying force.
(iii) Fluidity: It is the ability of a substance to flow or move about freely.(iv) Filling the gas container: The particles in a container take its shape as they randomly vibrate in all possible directions.
(v) Shape: It is the definite structure of an object within an external boundary.
(vi) Kinetic energy: The energy possesed by the particles due to their motion is known as kinetic energy. The increasing order of kinetic energy possessed by various states of matter is: Solids < Liquids < Gases
(vii) Density: The mass per unit volume of an object is known as its density.

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) Kinetic energy possessed by gas particles is very high which allows them to randomly move across all directions. Hence, the particles of gas fills the vessel entirely.
(b) Gas molecules possess high kinetic energy, due to which they are under constant motion inside the container in random directions which causes them to hit the walls of the container. These collisions with the walls of the container generate pressure.
(c) A wooden table should be called a solid as it possesses all the properties of a solid such as:

• Definite size and shape
• High intermolecular force of attraction between closely packed particles.
• It is rigid and cannot be compressed.

(d) Particles of the air have large spaces and very less force of attraction between them. On the other hand, wood has little space and high force of attraction between its particles. So, we can easily move our hands in the air but to do the same through a solid block of wood we need a karate expert.

4. Liquids generally have lower density as compared to solids. But you must have observed that ice floats on water. Find out why ?
Ans:-Though ice is a solid, it has large number of empty spaces between its particles. So, density of ice is less than the density of water and hence ice floats on water.

#### In-Text Questions Page 09

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. 250ºC b. 100ºC
Ans:- (a) Gaseous state as it is beyond its boiling point.
(b) It would be present in both liquid and gaseous state.
3. For any substance, why does the temperature remain constant during the change of state?
Ans:- It is due to the latent heat as the heat supplied is used up to overcome the force of attraction between the particles and to transform the state of matter of the substance. So, the temperature remains constant during the change of state.
4. Suggest a method to liquefy atmospheric gases.
Ans:- It can be achieved by either increasing the pressure or decreasing the temperature which ultimately leads to the reduction of spaces between molecules.

#### In-Text Questions Page 10

1. Why does a desert cooler cool better on a hot dry day?
Ans:- The temperature is high and humidity is less on a hot dry day which increases the rate of evaporation. During evaporation energy is absorbed from the surrounding. So, desert cooler cools better on a hot dry day.
2. How does the water kept in an earthen pot (matka) become cool during summer?
Ans:- An earthen pot is porous in nature. These tiny pores facilitate penetration of water and hence evaporation occurs from the pot surface. The process of evaporation requires energy which is contributed by water in the pot as a result of which water turns cooler.
3. Why does our palm feel cold when we put some acetone or petrol or perfume on it?
Ans:- Acetone, petrol, and perfume are volatile substances that get evaporated when they come in contact with air. Evaporation is facilitated as it uses energy from palm hence leaving a cooling effect on our palms.
4. Why are we able to sip hot tea or milk faster from a saucer rather than a cup?
Ans:- A saucer has a larger surface area than a cup which promotes quicker evaporation hence the tea or milk in a saucer cools down faster.
5. What type of clothes should we wear in summer?
Ans:- In summer, it is preferred to wear light-colored cotton clothes because light colour reflects heat and cotton materials have pores that absorb sweat, facilitating their evaporation hence causing a cooling effect in the skin.

#### Chapter End Exercises

1. Convert the following temperatures to the 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. Convert the following temperatures to the Kelvin scale:
(a) 25°C (b) 373°C
Ans:- (a) 25°C = (25+273) K = 298 K
(b) 373°C = (373+273) K = 646 K
3. Give reason 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.
Ans:- (a) At room temperature, naphthalene balls undergo sublimation wherein they directly get converted from a solid to a gaseous state without having to undergo the intermediate state, i.e., the liquid state.
b) Molecules of air move at a higher speed and have large intermolecular spaces. Perfumes comprise of flavored substances that are volatile which scatter quickly in air. Hence we are able to smell perfume sitting several meters away.
4. Arrange the following substances in increasing order of forces of attraction between the particles— water, sugar, oxygen.
Ans:- Oxygen (gas) < water (liquid) < sugar (solid)
5. What is the physical state of water at—
(a) 25°C (b) 0°C (c) 100°C ?
Ans:-
(a) At 25°C, the water will be in liquid form (normal room temperature)
(b) At 0°C, the water is at its freezing point, hence both solid and liquid phases are observed.
(c) At 100°C, the water is at its boiling point, hence both liquid and gaseous state of water (water vapor) are observed.
6. Give two reasons to justify—
(a) water at room temperature is a liquid.
(b) an iron almirah is a solid at room temperature.
Ans:- (a) Transition in the states of matter of water occurs at 0°C and 100°C. At room temperature, water is in the liquid state, thereby exhibiting all the properties of a liquid such as:-
➨ Water flows at this temperature
➨ It has a fixed volume and it takes the shape of its container
(b) The melting and boiling points of iron are as high as 1538°C and 2862°C respectively. The room temperature is about 20-25 °C. Hence iron almirah at room temperature shows all the properties of solid such as:-
➨ It has a definite and rigid shape.
➨ It has a fixed shape.
7. Why is ice at 273 K more effective in cooling than water at the same temperature?
Ans:- Ice at the 273 K has less energy than water ( although both are at same temperature). Water possesses the additional latent heat of fusion . Hence, at 273 K, ice is more effective in cooling than water at the same temperature.
8. What produces more severe burns, boiling water or steam?
Ans:- Steam has more energy than boiling water. It possesses the additional latent heat of vaporisation. Therefore, stream produces more severe burns than boiling water.
9. Name A,B,C,D,E and F in the following diagram showing change in its state

Ans:- A: Melting (or) fusion
B: Evaporation (or) vaporization
C: Condensation
D: Solidification
E: Sublimation
F: Sublimation