NCERT Class 9 Science Chapter 1 Matter in Our Surroundings

Chapter 1 Matter in Our Surroundings

Intext Questions
Question 1: Which of the following are matter?
Chair, air, love, smell, hate, almonds, though, cold, lemon water, smell of perfume
Answer:
The following are matter:
Chair, air, almonds, lemon water, smell of perfume

Question 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.
Answer:
Solids diffuse at a very slow rate. If temperature of the solids is increased, the rate of diffusion of solid particles in air increases. This is due to an increase in kinetic energy of solid particles. Therefore, the smell of hot food reaches us several metres away, but to get the 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:
A diver is able to cut through water in a swimming pool. This shows that the particles of water have intermolecular space and has less force of attraction.

Question 4: What are the characteristics of particles of matter?
Answer:
The characteristics of particles of matter are:
• Particles of matter have space between them.
• Particles of matter are continuously moving.
• Particles of matter attract each other.

Question 5: 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.
Answer:
Air, exhaust from chimneys, cotton, water, honey, chalk, iron

Question 6:
a) Tabulate the differences in characteristics of states of matter.
b) Comment upon the following:
rigidity, compressibility, fluidity, filling a gas container, shape, kinetic energy and density.
Answer:

a)

Chapter 1 Matter in Our Surroundings Question 6 Answer


b)
Rigidity: It is a property of matter to resist the change of its shape.
Compressibility: It is a property of matter to decrease the volume of matter by applying force.
Fluidity: It is the ability of matter to flow.
Filling a gas container: The particles in a gas take the shape of the container as they vibrate in all the directions.
Shape: Definite structure of an object within an external boundary.
Kinetic energy: It is the energy possessed by the particles of matter due to its motion.
Density: The mass per unit volume of a substance is called density.

Question 7: 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.
Answer:

a) The force of attraction between the particles of gas is negligible. Due to this, particles of gas move in all the direction. And hence, a gas fills the vessel completely in which it is kept.

b) Particles of gas move randomly in all the directions at a very high speed. Due to this, the particles hit each other and the walls of the container. Therefore, a gas exerts pressure on the walls of the container.

c) The wooden table shows the main characteristics of solids, i.e. it has a definite shape and fixed volume. Hence, the wooden table is a solid.

d) Particles of the air have a lot of space between them while the particles of solid block of wood has the least space between them. This is the reason why it is easy to move hand in air.

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:
Generally, the volume of a liquid is more than the volume of a solid because liquid particles are more free to move. Ice has a maximum density of water at 4°C. Ice is lighter than water and has a lower density. As a result, ice floats on water.

Question 9: Convert the following temperature to celsius scale:
a. 300 K
b. 573 K
Answer:

a. 300 K = 300 - 273 = 27°C
b. 573 K = 573 - 273 = 300°C

Question 10: What is the physical state of water at:
a. 250ºC
b. 100ºC
Answer:

a. The physical state of water at 250ºC is gaseous state because it is above its boiling point.
b. The physical state of water at 100ºC is mix of liquid and gaseous state because it is at the boiling point.

Question 11: For any substance, why does the temperature remain constant during the change of state?
Answer:
During the change of any substance, the average kinetic energy of it particles remains constant and thus the temperature also remains constant since it is directly proportional to the kinetic energy of those particles.

Question 12: Suggest a method to liquefy atmospheric gases.
Answer:
It can be done either by increasing the pressure or decreasing the temperature which leads to reduction of space between the particles.

Question 13: Why does a desert cooler cool better on a hot dry day?
Answer:
This is because the temperature is higher and it is less humid on a hot dry day which provides better evaporation. This helps in better cooling.

Question 14: How does the water kept in an earthen pot (matka) become cool during summer?
Answer:
All the earthen pot are porous in nature. These tiny pores help in penetration of water This water evaporates continuously and takes latent heat required for vapourisation from the remaining water. In this way, the remaining water loses heat and gets cold.

Question 15: Why does our palm feel cold when we put some acetone or petrol or perfume on it?
Answer:
Acetone, petrol, perfume are volatile substances that get evaporation when they come in contact with air. Evaporation gets facilitated as it uses energy from palm hence leaving a cooling effect on our palm.

Question 16: Why are we able to sip hot tea or milk faster from a saucer rather than a cup?
Answer:
A saucer has a larger surface area than a cup which promotes quicker evaporation. Therefore, the tea or milk in a saucer cools down faster.

Question 17: What type of clothes should we wear in summer?
Answer:
We should wear cotton clothes and light-coloured clothes in summer because cotton is a good absorber of sweat and light-coloured clothes reflect the heat.

Exercise Questions
Question 1: Convert the following temperatures to the celsius scale.
a) 293 K
b) 470 K
Answer:

a) 293 K = 293 - 273 = 20°C
b) 470 K = 470 - 273 = 197°C

Question 2: Convert the following temperatures to the kelvin scale.
a) 25°C
b) 373°C
Answer:

a) 25°C = 25 + 273 = 298 K
b) 373°C = 373 + 273 = 646 K

Question 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 metres away.
Answer:

a) At room temperature, naphthalene balls undergo the process of sublimation where they change solid state to gaseous state directly.

b) When perfume is released, it changes to vapour state. The kinetic energy of gas molecules is high and rate of diffusion is also high. Hence, we are able to smell the perfume.

Question 4: Arrange the following substances in increasing order of forces of attraction between the particles -  water, sugar, oxygen.
Answer:
Oxygen < Water < Sugar

Question 5: What is the physical state of water at -
a) 25°C
b) 0°C
c) 100°C
Answer:

a) It will be in liquid form.
b) It will be freezing, both liquid and solid form can be observed.
c) It will be boiling, both liquid and gas form can be observed.

Question 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 is at liquid state because its melting point is lower than room temperature and its boiling point is higher.
→ Water has fixed volume and not fixed shape.
→ Water flows freely.

b) Due to its melting point and boiling point are above the room temperature, almirah is at solid state.
→ Almirah has fixed shape and fixed volume.
→ Particles of almirah don’t move freely.

Question 7: Why is ice at 273 K more effective in cooling than water at the same temperature?
Answer:
Ice at 273 K will absorb heat energy (latent energy) from the medium to overcome the fusion to become water. Hence, the cooling effect of ice is more than the water at same temperature because water doesn’t absorb extra heat from the medium.

Question 8: What produces more severe burns, boiling water or steam?
Answer:
Steam produces severe burns because it is an exothermic reaction than releases high amount of heat which it had consumed during vapourisation.

Question 9: Name A, B, C, D, E and F in the following diagram showing change in its state.

Chapter 1 Matter in Our Surroundings Question 9

Answer:
A → Fusion
B → Vapourisation
C → Condensation
D → Solidification
E → Sublimation
F → Deposition

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