CBSE : CLASS 9 : Science : Chapter - " Matter in Our Surroundings " study notes

Chapter Name : "Matter in Our Surroundings"

NCERT CHAPTER PDF

1. Introduction to Matter

Matter: Anything that occupies space and has mass. Matter is made up of particles.

Characteristics of particles of matter:

1. Particles of matter are very small.
2. They have spaces between them.
3. Particles of matter are in continuous motion.
4. They attract each other.

2. States of Matter

There are three states of matter:

1. Solid:

1. Definite shape and volume.
2. Particles are closely packed.
3. Least kinetic energy.
4. Strong intermolecular forces.

2. Liquid:

1. No fixed shape but definite volume.
2. Particles are less closely packed compared to solids.
3. Moderate kinetic energy.
4. Weaker intermolecular forces than solids.

3. Gas:

1. No fixed shape or volume.
2. Particles are far apart.
3. Maximum kinetic energy
4. Negligible intermolecular forces.

3. Change of State of Matter

• Effect of Temperature:

When a substance is heated, its particles gain kinetic energy and move more vigorously, leading to changes in the state of matter.

Melting: Solid to liquid (at melting point).

Boiling: Liquid to gas (at boiling point).

Sublimation: Direct conversion of solid to gas (e.g., camphor, dry ice).

• Effect of Pressure:

By applying pressure and reducing temperature, gases can be liquefied.

Example: Liquefaction of gas for use in LPG cylinders.

4. Evaporation

Evaporation: The process of conversion of a liquid into vapor at any temperature below its boiling point.

Factors affecting evaporation:

1. Surface area (larger surface area, faster evaporation).
2. Temperature (higher temperature, faster evaporation).
3. Humidity (lower humidity, faster evaporation).
4. Wind speed (higher wind speed, faster evaporation).

Cooling effect of evaporation: As particles of a liquid evaporate, they take away heat from the remaining liquid, causing a cooling effect.

5. Latent Heat

Latent Heat of Fusion: The amount of heat energy required to change 1 kg of solid into liquid at its melting point.

Latent Heat of Vaporization: The amount of heat energy required to change 1 kg of liquid into vapor at its boiling point.

6. Condensation and Freezing

Condensation: Gas to liquid, occurs when a gas loses heat.

Freezing: Liquid to solid, occurs when a liquid loses heat.

7. Applications of Change of State of Matter

• Sublimation in dry ice (solid CO₂) used for refrigeration.
• Liquefaction of gases in industrial applications like LPG and compressed oxygen.

8. Plasma and Bose-Einstein Condensate (BEC)

Plasma: Ionized state of matter, consisting of super-energetic particles, found in stars and neon signs.

Bose-Einstein Condensate: State of matter achieved at temperatures close to absolute zero, where particles condense into a single quantum state.

9. Diffusion

Diffusion: The intermixing of particles of two different types of matter on their own.

It is faster in gases, slower in liquids, and slowest in solids due to particle arrangement and energy levels.

Key Definitions:

• Melting Point : Temperature at which a solid turns into a liquid.
• Boiling Point : Temperature at which a liquid turns into gas.
• Latent Heat : Heat required to change the state of a substance without changing its temperature.
• Matter is made up of small particles.
• The matter around us exists in three states—solid, liquid and gas.
• The forces of attraction between the particles are maximum in solids, intermediate in liquids and minimum in gases.
• The spaces in between the constituent particles and kinetic energy of the particles are minimum in the case of solids, intermediate in liquids and maximum in gases.
• The arrangement of particles is most ordered in the case of solids, in the case of liquids layers of particles can slip and slide over each other while for gases, there is no order, particles just move about randomly.
•  The states of matter are inter-convertible. The state of matter can be changed by changing temperature or pressure.
• Sublimation is the change of solid state directly to gaseous state without going through liquid state.
• Deposition is the change of gaseous state directly to solid state without going through liquid state.
• Boiling is a bulk phenomenon. Particles from the bulk (whole) of the liquid change into vapour state.
• Evaporation is a surface phenomenon. Particles from the surface gain enough energy to overcome the forces of attraction present in the liquid and change into the vapour state.
• The rate of evaporation depends upon the surface area exposed to the atmosphere, the temperature, the humidity and the wind speed.
•  Evaporation causes cooling.
• Latent heat of vaporisation is the heat energy required to change
• 1 kg of a liquid to gas at atmospheric pressure at its boiling point.
• Latent heat of fusion is the amount of heat energy required to change 1 kg of solid into liquid at its melting point.

Q. Why should we wear cotton clothes in summer?
During summer, we perspire more because of the mechanism of our body which keeps us cool. We know that during evaporation, the particles at the surface of the liquid gain energy from the surroundings or body surface and change into vapour. The heat energy equal to the latent heat of
vaporisation is absorbed from the body leaving the body cool. Cotton, being a good absorber of water helps in absorbing the sweat and exposing it to the atmosphere for easy evaporation.

Q. HOW DOES EVAPORATION CAUSE COOLING?

In an open vessel, the liquid keeps on evaporating. The particles of liquid absorb energy from the  surrounding to regain the energy lost during evaporation. This absorption of energy from the surroundings make the surroundings cold.

Q. Why do we see water droplets on the outer surface of a glass containing ice-cold water?

Let us take some ice-cold water in a tumbler. Soon we will see water droplets on the outer surface of the tumbler. The water vapour present in air, on coming in contact with the cold glass of water, loses energy and gets converted to liquid state, which we see as water droplets.

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

A desert cooler works on the principle of evaporative cooling. On a hot, dry day:

• The temperature is high, which increases the rate of evaporation.

• The humidity is low (the air is dry), which allows the air to hold more water vapor, further speeding up evaporation.

As the water in the cooler evaporates, it absorbs heat from the surrounding air, making the air blown out by the fan much cooler.

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

An earthen pot is porous, meaning it has thousands of extremely tiny pores in its walls.

• Water constantly seeps through these pores to the outside surface of the pot.

• This surface water evaporates by absorbing the necessary latent heat of vaporization from the water remaining inside the pot.

• Loss of heat from the internal water causes its temperature to drop, keeping it cool.

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

Acetone, petrol, and perfume are volatile liquids, meaning they have very low boiling points and evaporate quickly at room temperature.

• When you put them on your palm, the particles gain energy (latent heat) from your skin to change into a vapor state.

• This absorption of heat from your palm leaves your skin feeling significantly colder.

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

Evaporation is a surface phenomenon. The rate of evaporation increases as the surface area increases.

• A saucer has a much larger surface area compared to a narrow cup.

• Because of this larger area, the hot liquid evaporates much faster in a saucer.

• Since evaporation causes cooling, the tea or milk reaches a drinkable temperature much more quickly.

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

We should wear light-colored cotton clothes during the summer for two main reasons:

• Absorption and Evaporation: Cotton is a good absorber of water. it absorbs sweat from our body and exposes it to the atmosphere for easy evaporation, which keeps our body cool.

• Heat Reflection: Light colors (like white) are poor absorbers and good reflectors of heat, which helps in keeping the body temperature down compared to dark-colored fabrics.

EXERCISE

Here are the exercises for Class 9 CBSE Science, Chapter 1: "Matter in Our Surroundings":

A. Multiple Choice Questions (MCQs)

1. Which of the following is an example of matter?

(a) Air

(b) Light

(c) Sound

(d) None of these

Answer: (a) Air

2. Which state of matter has a definite shape and volume?

(a) Solid

(b) Liquid

(c) Gas

(d) Plasma

Answer: (a) Solid

3. The process of a liquid changing into vapor at any temperature below its boiling point is called:

(a) Boiling

(b) Evaporation

(c) Condensation

(d) Freezing

Answer: (b) Evaporation

4. The temperature at which a solid melts to become a liquid at atmospheric pressure is known as:

(a) Melting Point

(b) Boiling Point

(c) Sublimation Point

(d) Freezing Point

Answer: (a) Melting Point

5. Which of the following is not an example of sublimation?

(a) Camphor

(b) Dry ice

(c) Iodine

(d) Water

Answer: (d) Water

B. Fill in the Blanks

1. Matter is made up of ________.

Answer: particles

2. A gas can be liquefied by applying ________ and lowering ________.

Answer: pressure, temperature

3. The intermixing of particles of two different types of matter is known as ________.

Answer: diffusion

4. The latent heat of ________ is the amount of heat required to change 1 kg of solid into liquid without any temperature change.

Answer: fusion

5. The change of state directly from solid to gas is called ________.

Answer: sublimation

C. True or False

1. Solids have a definite shape and volume.

Answer: True

2. Liquids have definite shape but no definite volume.

Answer: False

3. Particles of a gas move at a lower speed compared to solids.

Answer: False

4. Evaporation causes cooling.

Answer: True

5. Dry ice undergoes sublimation to form water.

Answer: False

D. One Word Answers

1. The process by which a liquid changes into gas at any temperature below its boiling point.

Answer: Evaporation

2. The change of a gas to liquid on cooling.

Answer: Condensation

3. The amount of heat energy required to change 1 kg of solid into liquid at its melting point.

Answer: Latent Heat of Fusion

4. The state of matter in which particles are closely packed in a fixed position.

Answer: Solid

5. The temperature at which a liquid turns into gas at atmospheric pressure.

Answer: Boiling Point

E. Short Answer Questions

1. Define matter.

Answer: Matter is anything that occupies space and has mass. It is made up of tiny particles which are in continuous motion and have spaces between them.

2. What is sublimation? Give two examples.

Answer: Sublimation is the process by which a solid directly changes into gas without passing through the liquid state. Examples: Camphor, Dry ice.

3. List the factors affecting the rate of evaporation.

Answer: The factors affecting the rate of evaporation are:

1. Surface area
2. Temperature
3. Humidity
4. Wind speed

4. What is diffusion? Give an example.

Answer: Diffusion is the intermixing of particles of two different substances on their own. Example: Mixing of perfume in air.

5. Why do solids have a definite shape?

Answer: Solids have a definite shape because the particles in solids are tightly packed and have strong intermolecular forces, restricting their movement to only vibrations in their fixed positions.

E. Long Answer Questions

1. Explain the process of evaporation and describe the factors that affect the rate of evaporation.

Answer: Evaporation is the process by which a liquid changes into vapor at any temperature below its boiling point. The particles on the surface of the liquid gain enough energy to overcome intermolecular forces and escape into the air as vapor.

The factors affecting the rate of evaporation are -

Surface Area: Larger surface area increases the rate of evaporation because more particles can escape from the surface.

Temperature: Higher temperature increases the rate of evaporation as the particles gain more energy.

Humidity: Lower humidity increases the rate of evaporation because dry air can absorb more vapor.

Wind Speed: Higher wind speed increases the rate of evaporation as it removes the water vapor from the surface.

2. What is latent heat? Differentiate between latent heat of fusion and latent heat of vaporization.

Answer: Latent heat is the heat energy required to change the state of a substance without changing its temperature.

Latent Heat of Fusion: It is the amount of heat energy required to change 1 kg of solid into liquid at its melting point.

Latent Heat of Vaporization: It is the amount of heat energy required to change 1 kg of liquid into vapor at its boiling point.

3. Discuss the differences between solids, liquids, and gases in terms of their properties.

Answer:

Solids: Have definite shape and volume, particles are tightly packed, least kinetic energy, and strong intermolecular forces.

Liquids: Have definite volume but no definite shape, particles are less tightly packed than solids, moderate kinetic energy, and weaker intermolecular forces.

Gases: Have no definite shape or volume, particles are far apart, highest kinetic energy, and negligible intermolecular forces.

4. Describe the change of state from solid to liquid and from liquid to gas. What are the processes called?

Answer:

The change of state from solid to liquid is called melting. When a solid is heated, its particles gain energy, overcome the forces holding them in fixed positions, and start moving freely, turning into a liquid.

The change of state from liquid to gas is called boiling or vaporization. When a liquid is heated, its particles gain energy, move rapidly, and break free from the liquid surface into gas.