SEBA Class 9 Science: Matter in Our Surroundings

INTEXT QUESTION

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

Answer: The following are matter: Chair, air, almonds, cold drink, smell of perfume.

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: The smell of hot sizzling food reaches us from far because the particles of the food mix with air and spread faster. This happens due to diffusion, and it becomes faster when the temperature is high.
But cold food doesn’t spread smell quickly because the particles move slower at low temperature.

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

Answer: This shows that particles of matter have space between them. Because of this space, the diver is able to move through water easily.

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

Answer: The characteristics of particles of matter are:

1. They have space between them:
There is space between the particles of matter. That’s why substances like sugar, salt or potassium permanganate can mix in water.

2. They are continuously moving:
The particles of matter are always in motion. They move faster when the temperature increases.

3. They attract each other:
The particles of matter pull each other with a force. This force is strongest in solids, less in liquids, and very weak in gases.

INTEXT QUESTION

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.

Answer: The correct order of increasing density is:

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.

Answer:

a) Rigidity:
The property of a substance to retain its shape and not flow is called rigidity. Solids are rigid.

b) Compressibility:
The ability of a substance to be compressed or reduced in volume by applying force is called compressibility. Gases are highly compressible.

c) Fluidity:
The ability of a substance to flow is called fluidity. Liquids and gases are fluids.

d) Filling a gas container:
Gases can fill the container completely because their particles move freely in all directions.

e) Shape:
The outer form or boundary of a substance is called shape. Solids have a fixed shape; liquids and gases do not.

f) Kinetic Energy:
The energy of particles due to their motion is called kinetic energy. It increases with temperature.

g) Density:
The mass per unit volume of a substance is called density.

Density = Mass/Volume.

3. Give reasons

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

Answer: A gas fills the vessel completely because the particles of a gas move about randomly at high speed. These particles are far apart and have a very weak force of attraction, so they spread freely in all directions. As a result, gases take the shape and volume of the container in which they are kept.

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

Answer: In the gaseous state, the particles move about randomly at high speed. Due to this random movement, the particles hit each other and also the walls of the container, and due to these collisions, Gases exert pressure on the walls of the container.

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

Answer: A wooden table should be called a solid because it has a definite shape, a fixed volume, and is rigid. The particles of solids are tightly packed, and the force of attraction between them is very strong. Solids do not flow or get compressed easily, which is why the table is a solid.

(d) We can easily move our hand in the air, but to do the same through a solid block of wood, we need a karate expert.

Answer: We can move our hand in the air easily because air is a gas and its particles are far apart with a very weak force of attraction, so it offers very little resistance. But in solids like wood, the particles are tightly packed with a strong force of attraction, so more force is needed to break through them, which is why 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.

Answer: Ice floats on water because the density of ice is lower than that of water. When water freezes into ice, the particles arrange themselves in a way that creates more empty space. This makes ice less dense, even though it is a solid. That’s why ice is lighter than the same volume of water, so it floats on it.

INTEXT QUESTION

1. Convert the following temperatures to the Celsius scale: a. 300 K b. 573 K.

Answer:

a) 300k = (300-273)0C = 270C

b) 573k = (573-273)0C = 3000C

2. What is the physical state of water at: a) 2500C b) 1000C ?

a) Answer: At 2500C, water is in the gaseous state, because it is above the boiling point of water (1000C).

b) Answer: At 1000C, water is in the liquid state and starts to convert into gas, as this is the boiling point of water.

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

Answer: During the change of state, the temperature of a substance remains constant because the heat energy supplied is used to overcome the force of attraction between the particles of matter. This energy is used to change the state, not to increase the temperature. This hidden heat is called latent heat.

4. Suggest a method to liquefy atmospheric gases.

Answer: Atmospheric gases can be liquefied by applying pressure and reducing temperature. When pressure is increased, the gas particles come closer, and when temperature is reduced, their movement slows down. This allows the gases to change into a liquid state.

INTEXT QUESTION

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

Answer: A desert cooler cools better on a hot dry day because the rate of evaporation increases when the temperature is high and the humidity is low. On a dry day, the air has less water vapour, so more water from the cooler can evaporate. This evaporation causes cooling and makes the air cooler.

2. How does the water kept in an earthen pot (matka) become cool during summer?

Answer: In summer, water kept in an earthen pot (matka) becomes cool because the pot has tiny pores in its walls. Water slowly evaporates through these pores. For evaporation, water takes heat energy from the remaining water in the pot. This process of evaporation causes cooling, and that is why the water becomes cool.

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

Answer: When we put acetone, petrol or perfume on our palm, it evaporates quickly. During evaporation, the liquid takes heat energy from our palm to change into vapour. This loss of heat from the skin causes a cooling effect, and that is why our palm feels cold.

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

Answer: We are able to sip hot tea or milk faster from a saucer than from a cup because the surface area is more in a saucer. Due to the larger surface area, evaporation happens faster, and the tea cools quickly. This is why it becomes easier to sip from a saucer.

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

Answer: In summer, we should wear cotton clothes because cotton is a good absorber of water. It absorbs sweat from our body and allows it to come in contact with air. The sweat evaporates, and during evaporation, it takes away heat from our body, which helps in cooling us down and makes us feel comfortable.

EXERCISE QUESTION

1. Convert the following temperatures to the Celsius scale. (a) 293 K (b) 470 K

Answer: (a) 293K= (293 – 273)°C = 20°C

(b) 470 K = (470 – 273)°C = 197°C

2. Convert the following temperatures to the Kelvin scale. (a) 25°C (b) 373°C

Answer: (a) 25°C = (25+273)K = 298K

(b) 373°C = (373+273)K = 646K

3. Give a reason for the following observations.

(a) Naphthalene balls disappear with time without leaving any solid.

Answer: Naphthalene balls disappear over time without leaving any solid because they undergo the process of sublimation. In sublimation, a solid changes directly into its gaseous state without passing through the liquid state. Therefore, the solid naphthalene turns into gas and slowly disappears from sight.

(b) We can get the smell of perfume sitting several metres away.

Answer: We are able to smell perfume from several metres away because the particles of the perfume diffuse into the air. Since the particles of gases are always moving rapidly and have large spaces between them, they spread out quickly and reach us, even at a distance. This process is called diffusion.

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

Answer: The correct increasing order of forces of attraction between the particles is:
Oxygen < Water < Sugar

5. What is the physical state of water at— (a) 25°C (b) 0°C (c) 100°C ?

Answer: (a) At 25°C, water is in the liquid state, as it is above the melting point and below the boiling point.
(b) At 0°C, water exists in the solid and liquid state — it is the melting point, so both ice and water can be present.
(c) At 100°C, water is in the liquid and gaseous state, as it is the boiling point, where water starts changing into steam.

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 at room temperature is a liquid because it has a definite volume but no fixed shape. It takes the shape of the container and can flow easily. These are the main characteristics of liquids, which prove that water is in the liquid state.

(b) An iron almirah is a solid because it has a fixed shape, fixed volume, and is rigid. Its particles are tightly packed with a strong force of attraction, and it does not flow. These are the main properties of solids, so the iron almirah is in the solid state.

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

Answer: Ice at 273 K is more effective in cooling than water at the same temperature because ice absorbs extra heat from the surroundings to change into water. This extra heat is called the latent heat of fusion. Even though both ice and water are at 273 K, ice uses this hidden heat energy during melting, which helps in cooling the surroundings more than water does.

8. What produces more severe burns, boiling water or steam?

Answer: Steam produces more severe burns than boiling water because steam has more heat energy than boiling water. At 373 K, both steam and water are at the same temperature, but steam contains extra hidden heat called the latent heat of vaporisation. When steam condenses on the skin, it releases this extra heat, causing more severe burns than boiling water.

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

Answer: (A) Solid to Liquid → Melting or fusion.

(B) Liquid to Gas → Evaporation or vaporisation.

(C) Gas to liquid → Condensation or liquefaction.

(D) Liquid to Solid → Solidification or freezing.

(E) Solid to Gas → Sublimation.

(F) Gas to Solid → Deposition.