### Science, Grade 9 - Chapter 1: Matter in our surroundings

Questions, Page 3

1. Which of the following are matter? Chair, air, love, smell, hate, almonds, thought, cold, cold drink, smell of perfume.
Solution: 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.
Solution: Particles of matter are continuously moving. They possess the kinetic energy. As the temperature rises, particles move faster. Thus, particles that carry smell of hot sizzling food move faster than the smell of the cold food. Therefore, the smell of hot sizzling food can reach us several metres away, but to get smell from a cold food you 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?
Solution: This shows that the particles of matter have space between them. This space is the maximum in the gases and the minimum in liquid. Thus, one cannot cut through a solid easily but a diver is able to cut easily through water in a swimming pool.

4. What are the characteristics of the particles of matter?
Solution: the characteristics of the particles of matter are:
1. Particles of matter have space between them.
2. Particles of matter are continuously moving.
3. Particles of matter attract each other.

Questions, Page 6

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.
Solution: Exhaust from chimneys, air, cotton, water, honey, chalk, and iron
2. (a) Tabulate the differences in the characteristics of states of matter.
Solution:
(Click the table for a larger view)

(b) Comment upon the following: rigidity, compressibility, fluidity, filling a gas container, shape, kinetic energy and density.
Solution: Rigidity:
Rigidity is the physical property of being stiff and resistance to bending or flowing. Solids are highly rigid. Liquids and gases are not rigid. Thus, they can easily flow and bend. Thus, liquids and gases are called fluids.
Compressibility:
Compressibility is the property of being able to occupy less space. Solids cannot be compressed easily. Liquids can be compressed but only to a limit. Gases are highly compressible.
Fluidity:
Fluidity is the ability to flow. Liquids and gases can flow. Thus, they are known as fluids.
Filling a gas container:
Gases are highly compressible. Thus, large volumes of gases are compressed and stored in containers so that they are easier to store and transport.
Shape:
Solids have definite shape and retain their shape till an outside force changes its shape. Liquids and gases do not have a definite shape. They take the shape of the container in which they are kept.
Kinetic Energy:
Kinetic energy is the energy possessed by objects by virtue of their motion. The greater the speed of motion, the larger is the kinetic energy. The particles of matter are continuously moving. They possess the kinetic energy. The motion of the particles is the maximum in solid and is the minimum in gases. Thus, the particles of solid have the maximum kinetic energy and the particles of gases have the minimum kinetic energy. The particles of liquid have the kinetic energy that is intermediate that of the solids and gases.
Density:
Density is defined as the mass per unit volume. Density refers to the amount of matter that is present per unit volume. Solids are known to have highest density and gases lowest density. Liquids have a density that is intermediate that of the solids and liquids.

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.
Solution: (a) The particles of gases are free to move. Gas particles have least amount of attraction between them. Thus, the particles of gases fill completely the vessel in which they are kept.
(b) The particles of gas are in continuous motion. They collide with each other and with the walls of container. These collisions of the gas particles with the walls of the container exert pressure on the walls of the container.
(c) A wooden table has a definite shape and size. Also, it is a non-compressible rigid body. Thus, a wooden table has all the characteristics of a solid.
(d) The particles of air have least amount of attraction between them. Thus, one can easily move the hand air and push the particles of air apart. However, the particles of solid have maximum amount of attraction between them. A greater amount of force is required to move the particles of solid apart. Thus, a much greater force is required to move hand through a solid block of wood.

4. Liquids generally have lower density as compared to solids. But you must have observed that ice floats on water. Find out why.
Solution: Density of water is the maximum at 4°C. The density of ice is less than that of the water. Thus, ice floats on water.

Questions, Page 9

1. Convert the following temperature to Celsius scale: a. 300 K b. 573 K.
Solution:
Celsius scale = Kelvin scale ¬– 273
(a) Celsius scale = 300 – 273 = 27°C
(b) Celsius scale = 573 ¬– 273 = 300°C

2. What is the physical state of water at: a. 250ºC b. 100ºC?
Solution: The physical state of water at
(a) 250°C is gaseous
(b) 100°C might be gaseous or liquid. Steam and water coexist at 100°C.

3. For any substance, why does the temperature remain constant during the change of state?
Solution: The temperature remains constant during the change of state. The temperature of the substance does not increase (or decrease) as the heat is used (or released) to overcome the force of attraction (or as the particles come closer). This heat energy absorbed (or released) is known as the latent heat. The temperature of the substance remains the same as latent heat is supplied to it or is released by it.

4. Suggest a method to liquefy atmospheric gases
Solution: Atmospheric gases liquefy at very low temperatures. It is not possible to attain such low temperatures. However, we can liquefy atmospheric gases without attaining such low temperatures. Atmospheric gases can be liquefied by compressing them. When the atmospheric gases are compressed, the particles of gases come closer to each other and as we keep on compressing the gases, the particles keep coming closer and closer to each other. When the pressure on the atmospheric gases becomes large enough, then the gases liquefy. Thus, atmospheric gases can be liquefied by compressing them.

Questions, Page 10

1. Why does a desert cooler cool better on a hot dry day?
Solution: A desert cooler increases the humidity of the surrounding air. The water particles in the air take the heat from the surrounding objects and evaporate, thereby making them cooler.

2. How does the water kept in an earthen pot (matka) become cool during summer?
Solution: An earthen pot (matka) has small pores in it. Water seeps through these pores and reaches the outer surface of the earthen pot. This water then evaporates by taking heat from earthen pot, thereby making it cooler.

3. Why does our palm feel cold when we put some acetone or petrol or perfume on it?
Solution: Acetone, petrol, and perfume evaporate at low temperatures. When some acetone, petrol, or perfume is dropped on the palm, it takes heat from the palm and evaporates, thereby making the palm cooler.
4. Why are we able to sip hot tea or milk faster from a saucer rather than a cup?
Solution: Evaporation is a surface phenomenon. If the surface area is increased, then the rate of evaporation increases. Evaporation has a cooling effect. Thus, when hot tea or milk is poured in a saucer, it evaporates faster. Thus, it is cooled faster.

5. What type of clothes should we wear in summer?
Solution: One should wear cotton clothes in summer as cotton is a good sweat absorber. Sweat is absorbed by the cotton and is exposed to the atmosphere. There the sweat evaporates, thereby making one feel cooler.

End of chapter Exercises, Page 12

1. Convert the following temperatures to the Celsius scale. (a) 300 K (b) 573 K
Solution: Celsius scale = Kelvin scale ¬– 273
(a) Celsius scale = 300 – 273 = 27°C
(b) Celsius scale = 573 ¬– 273 = 300°C

2. Convert the following temperatures to the Kelvin scale. (a) 25°C (b) 373°C.
Solution: Kelvin scale = Celsius scale ¬+ 273
(a) Kelvin scale = 25 + 273 = 298 K
(b) Kelvin scale = 373 ¬+ 273 = 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 metres away.
Solution:
(a) Naphthalene or moth balls sublimate (change from a solid to a gas). So, they disappear with time without leaving any solid.
(b) Smell of perfume is carried by air particles. Air particles are free to move (they possess kinetic energy). Thus, the air particles carrying the smell of perfume can move to someone sitting several metres away.

4. Arrange the following substances in increasing order of forces of attraction between the particles— water, sugar, oxygen.
Solution: Oxygen, water, sugar

5. What is the physical state of water at— (a) 25°C (b) 0°C (c) 100°C?
Solution:
(a) Liquid
(b) Solid or liquid
(c) Liquid or gas

6. Give two reasons to justify— (a) Water at room temperature is a liquid. (b) An iron almirah is a solid at room temperature.
Solution:
(a) Water at room temperature has a definite volume, but it does not have a definite shape. Thus, water at room temperature is a liquid.
(b) An iron almirah at room temperature has a definite volume as well as a definite shape. Thus, an iron almirah at room temperature is a solid.
7. Why is ice at 273 K more effective in cooling than water at the same temperature?
Solution: Ice at 273 K would absorb latent heat as well as the heat energy in order to attain the room temperature. Water at 273 K would just absorb heat energy in order to attain the room temperature. Thus, the water at 273 K would absorb less amount of heat energy from the surroundings as compared to ice at 273 K. Therefore, ice at 273 K is more effective in cooling than water at the same temperature.

8. What produces more severe burns, boiling water or steam?
Solution: Steam would release heat energy to first liquefy and then would release heat energy to attain the room temperature. Boiling water would just release heat energy to attain the room temperature. Thus, steam would release more heat energy than boiling water. Therefore, steam 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.
Solution:
A. Fusion
B. Vaporisation
C. Condensation
D. Solidification
E. Sublimation
F. Sublimation

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