Class 9 Science NCERT Notes – Chapter 1: Matter in Our Surroundings (PDF, MindMap, Q&A, Quizzes)

Chapter 1 (Chemistry): Matter in Our Surroundings – Class 9 NCERT Science Detailed Study Notes.

1. The Concept of Matter

• Definition: Anything in the universe that occupies space (has volume) and has mass is defined as matter. Examples range from stones, clouds, and stars to a drop of water or a particle of sand.
• Early Classifications:
  • Indian Philosophers: Classified matter into five basic elements known as the “Panch Tatva”: air, earth, fire, sky, and water. They believed all things, living and non-living, were composed of these elements.
  • Greek Philosophers: Arrived at a similar classification.
• Modern Classifications: Scientists classify matter based on its physical properties and chemical nature. This guide focuses on the classification based on physical properties.
• Standard Units:
  • The SI unit of mass is the kilogram (kg).
  • The SI unit of volume is the cubic metre (m³). A common unit is the litre (L), where 1L = 1000 mL and 1 mL = 1 cm³.

2. The Physical Nature of Matter

• Particulate Nature: Matter is composed of extremely small particles. This is contrary to the older belief that matter was continuous like a block of wood.
• Evidence for Particles: When salt or sugar is dissolved in water, the particles of the solute spread out and occupy the spaces between the particles of water, without changing the water level.
• Size of Particles: The particles of matter are incredibly small, beyond our imagination.
  • Evidence: A few crystals of potassium permanganate can color a large volume of water (approx. 1000 L) through repeated dilution. Similarly, the smell of 2 mL of Dettol can be detected in water even after repeated dilutions. This shows that a single crystal or a small amount of substance contains millions of tiny particles.

3. Characteristics of Particles of Matter

1. Particles Have Space Between Them: When substances like sugar, salt, Dettol, or potassium permanganate dissolve in water, their particles get evenly distributed by occupying the spaces between the water particles. This is also observed when making tea, coffee, or lemonade.
2. Particles are Continuously Moving:
   • Particles of matter possess kinetic energy, which causes them to be in constant, random motion.
   • Effect of Temperature: As temperature increases, the kinetic energy of particles also increases, causing them to move faster.
   • Diffusion: The intermixing of particles of two different types of matter on their own is called diffusion. It occurs as particles move into the spaces between other particles. Heating increases the rate of diffusion.
   • Examples: The smell of an unlit incense stick requires one to be close, but the smell of a lit incense stick spreads across a room quickly. The smell of hot sizzling food travels several metres, while the smell of cold food does not.
3. Particles Attract Each Other:
   • There is a force of attraction acting between the particles of matter, which holds them together.
   • The strength of this force of attraction varies from one type of matter to another.
   • Evidence: It is harder to break an iron nail than a piece of chalk, indicating stronger forces of attraction between particles in the iron nail. A diver can cut through water because the forces of attraction between water particles are weak enough to be overcome.

4. The Three States of Matter

Matter exists in three primary states: solid, liquid, and gas. These states arise due to variations in the characteristics of their constituent particles.

Characteristic	Solid State	Liquid State	Gaseous State
Shape	Definite shape, distinct boundaries.	No fixed shape; takes the shape of the container.	No definite shape.
Volume	Fixed volume.	Fixed volume.	No definite volume; fills the container completely.
Compressibility	Negligible.	Low (almost negligible).	Highly compressible.
Rigidity/Fluidity	Rigid; difficult to change shape.	Not rigid; can flow (fluid).	Not rigid; can flow (fluid).
Particle Arrangement	Tightly packed in an ordered manner.	Particles are close but can move and slide past each other.	Particles are far apart and move randomly.
Inter-particle Space	Minimum.	Intermediate.	Maximum.
Force of Attraction	Maximum.	Intermediate.	Minimum.
Kinetic Energy	Minimum.	Intermediate.	Maximum.
Diffusion Rate	Very slow.	Higher than solids.	Very fast.
Density	Generally high.	Generally lower than solids.	Very low.

• Note on Solids:
  • Rubber Band: Changes shape under force but regains it when the force is removed. It breaks if excessive force is applied. It is considered a solid.
  • Salt/Sugar: Individual crystals have a fixed shape, even when they take the shape of the container they are in. They are solids.
  • Sponge: It is a solid with minute holes trapping air. When pressed, the air is expelled, allowing it to be compressed.

5. Changes in the State of Matter

The state of a substance is determined by temperature and pressure. States are inter-convertible.

5.1 Effect of Temperature

• Melting (Fusion): The process of a solid changing into a liquid.
  • Mechanism: Increasing the temperature of a solid increases the kinetic energy of its particles. They vibrate faster, overcoming the forces of attraction, and the solid converts to a liquid.
  • Melting Point: The minimum temperature at which a solid melts to become a liquid at atmospheric pressure. The melting point of ice is 273.15 K (0°C).
  • Latent Heat of Fusion: The amount of heat energy required to change 1 kg of a solid into a liquid at its melting point and atmospheric pressure. This “hidden” heat is used to overcome particle attraction and does not cause a rise in temperature until all the solid has melted.
• Boiling (Vaporisation): The process of a liquid changing into a gas.
  • Mechanism: When heat is supplied to a liquid, particles move faster. At the boiling point, particles throughout the bulk of the liquid gain enough energy to break free from forces of attraction and change into the vapour state.
  • Boiling Point: The temperature at which a liquid starts boiling at atmospheric pressure. Boiling is a bulk phenomenon. The boiling point of water is 373 K (100°C).
  • Latent Heat of Vaporisation: The amount of heat energy required to change 1 kg of a liquid into a gas at its boiling point and atmospheric pressure. Particles in steam at 373 K have more energy than water at the same temperature because of this extra absorbed energy.
• Sublimation & Deposition:
  • Sublimation: The change of state directly from solid to gas without passing through the liquid state. Example: camphor, solid carbon dioxide (dry ice).
  • Deposition: The change of state directly from gas to solid without passing through the liquid state.

5.2 Effect of Pressure

• Liquefaction of Gases: Applying pressure and reducing temperature can liquefy gases. High pressure brings the gas particles closer together, and low temperature reduces their kinetic energy, allowing forces of attraction to take over.
• Solid Carbon Dioxide (Dry Ice): Stored under high pressure. When the pressure is decreased to 1 atmosphere, it converts directly into a gaseous state without becoming a liquid.

6. Evaporation

• Definition: The phenomenon of a liquid changing into vapours at any temperature below its boiling point. It is a surface phenomenon.
• Mechanism: Particles at the surface of a liquid with higher kinetic energy can break away from the forces of attraction of other particles and escape as vapour.
• Factors Affecting Evaporation: The rate of evaporation increases with:
  1. An increase in surface area: Spreading clothes out helps them dry faster.
  2. An increase in temperature: More particles gain sufficient kinetic energy to vaporise.
  3. An increase in wind speed: Particles of water vapour are carried away by the wind, reducing the amount of vapour in the surroundings.
  4. A decrease in humidity: Humidity is the amount of water vapour in the air. If the air is already saturated (high humidity), the rate of evaporation decreases.
• Evaporation Causes Cooling:
  • During evaporation, the liquid particles absorb energy from their surroundings to regain the energy lost in changing to a vapour.
  • This absorption of energy from the surroundings makes the surroundings cool.
  • Examples: Pouring acetone on your palm makes it feel cool. Sprinkling water on a roof on a hot day helps cool the surface. We wear cotton clothes in summer because cotton absorbs sweat, and when the sweat evaporates, it takes heat from our body, leaving us cool.

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Short-Answer Quiz (25 Questions)

1. What is the scientific definition of matter?
2. According to early Indian philosophers, what are the five basic elements (“Panch Tatva”)?
3. What are the SI units for mass and volume?
4. Describe the experiment with salt and water that demonstrates that matter is made of particles.
5. How does the experiment with potassium permanganate show that particles of matter are very small?
6. What are the three primary characteristics of the particles of matter?
7. Define the process of diffusion and explain how temperature affects it.
8. Why can a diver cut through water but not through a block of wood?
9. What is meant by the compressibility of a substance? Compare the compressibility of solids, liquids, and gases.
10. Explain why a gas exerts pressure on the walls of its container.
11. Why is a sponge, which can be compressed, still considered a solid?
12. Define density. In general, how does the density of liquids compare to that of solids?
13. What happens to the kinetic energy of particles when a substance is heated?
14. Define melting point. What is the melting point of ice in Kelvin?
15. What is latent heat of fusion? Explain why the temperature of melting ice does not rise despite continuous heating.
16. Define boiling point. Why is boiling considered a “bulk phenomenon”?
17. Which has more energy: water at 100°C or steam at 100°C? Justify your answer.
18. Explain the process of sublimation and give an example of a substance that sublimes.
19. What is dry ice and why is it called so?
20. How can atmospheric gases be liquefied?
21. What is the key difference between boiling and evaporation?
22. List the four factors that increase the rate of evaporation.
23. Explain how evaporation causes a cooling effect.
24. Why is it advisable to wear cotton clothes during the summer?
25. Why do water droplets form on the outer surface of a glass containing ice-cold water?

Answer Key for Short-Answer Quiz

1. Matter is anything that occupies space (has volume) and has mass.
2. The five basic elements or “Panch Tatva” are air, earth, fire, sky, and water.
3. The SI unit for mass is the kilogram (kg), and the SI unit for volume is the cubic metre (m³).
4. When salt is dissolved in water, the salt particles spread out and occupy the spaces between the water particles. This even distribution happens without a noticeable change in the water level, proving matter is particulate with spaces between particles.
5. A few crystals of potassium permanganate can colour a large volume of water (1000 L) through repeated dilutions. This shows one crystal must contain millions of tiny particles that keep dividing and spreading.
6. The three characteristics are: (1) particles have space between them, (2) they are continuously moving, and (3) they attract each other.
7. Diffusion is the spontaneous intermixing of particles of two different types of matter. As temperature increases, the kinetic energy of particles increases, making them move faster and thus increasing the rate of diffusion.
8. A diver can cut through water because the force of attraction between water particles is relatively weak and can be overcome. In a block of wood, the particles are held together by much stronger forces, making it rigid and impenetrable by hand.
9. Compressibility is the ability of a substance to be reduced in volume by applying force. Gases are highly compressible, while liquids and solids have negligible compressibility.
10. Gas particles move randomly at high speeds, causing them to collide with each other and the walls of the container. The force exerted by these particles per unit area on the container’s walls creates pressure.
11. A sponge is a solid because it has a definite shape and volume. It can be compressed because it has tiny holes filled with air; pressing it expels this air, not compresses the solid material itself.
12. Density is the mass per unit volume of a substance (density = mass/volume). Generally, liquids have a lower density than solids.
13. When a substance is heated, the heat energy is absorbed by the particles, which increases their kinetic energy. This causes them to move or vibrate more rapidly.
14. The melting point is the minimum temperature at which a solid becomes a liquid at atmospheric pressure. The melting point of ice is 273.15 K.
15. Latent heat of fusion is the heat energy required to change 1 kg of a solid to a liquid at its melting point. The temperature remains constant because the supplied heat energy is used up to overcome the forces of attraction between particles, facilitating the change of state, rather than increasing their kinetic energy.
16. The boiling point is the temperature at which a liquid starts boiling at atmospheric pressure. It is a bulk phenomenon because particles from the entire volume (bulk) of the liquid gain enough energy to change into the vapour state, not just particles at the surface.
17. Steam at 100°C has more energy than water at 100°C. This is because the particles in steam have absorbed extra energy in the form of the latent heat of vaporisation during the change of state from liquid to gas.
18. Sublimation is the direct change of a substance from a solid state to a gaseous state without passing through a liquid state. An example is solid carbon dioxide (dry ice) or camphor.
19. Dry ice is the common name for solid carbon dioxide (CO₂). It is called so because it turns directly into a gas when its pressure is lowered, without melting into a liquid.
20. Atmospheric gases can be liquefied by applying high pressure and reducing the temperature. This brings the particles closer together and slows them down, allowing intermolecular forces to convert the gas to a liquid.
21. Boiling is a bulk phenomenon that occurs at a specific temperature (the boiling point), converting a liquid to gas. Evaporation is a surface phenomenon that can occur at any temperature below the boiling point.
22. The rate of evaporation increases with: an increase in surface area, an increase in temperature, a decrease in humidity, and an increase in wind speed.
23. During evaporation, particles of the liquid absorb energy from the surroundings to overcome forces of attraction and escape as vapour. This absorption of heat from the surroundings leads to a cooling effect.
24. Cotton is a good absorber of water. In summer, we perspire, and cotton clothes absorb the sweat, exposing it to the atmosphere for easy evaporation, which cools the body.
25. Water vapour present in the air comes into contact with the cold outer surface of the glass. The air particles lose energy, cool down, and get converted into the liquid state, appearing as water droplets.

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Multiple-Choice Quiz (MCQ)

1. Which of the following is considered matter?
   • a) Love b) Thought c) Smell of perfume d) Hate
2. The intermixing of particles of two different types of matter on their own is called:
   • a) Fusion b) Diffusion c) Evaporation d) Sublimation
3. Which state of matter has the maximum force of attraction between particles?
   • a) Solid b) Liquid c) Gas d) All have the same force
4. The change of state from solid directly to gas is known as:
   • a) Vaporisation b) Deposition c) Fusion d) Sublimation
5. What is the SI unit of temperature?
   • a) Celsius b) Litre c) Kelvin d) Pascal
6. Liquids are called fluids because:
   • a) They have a fixed shape b) They are highly compressible c) They can flow d) They have high density
7. A temperature of 300 K on the Celsius scale is:
   • a) 30°C b) 27°C c) 573°C d) -27°C
8. Which of the following will increase the rate of evaporation of water?
   • a) Increasing the humidity b) Decreasing the temperature of the water c) Decreasing the surface area of the water d) Increasing the wind speed over the water
9. The “hidden” heat absorbed by a substance during a change of state without a change in temperature is called:
   • a) Specific heat b) Kinetic energy c) Latent heat d) Potential energy
10. Which of the following is an example of a compressed gas used in homes?
    • a) CNG b) Oxygen c) Air d) LPG
11. The physical state of water at 100°C is:
    • a) Solid only b) Liquid only c) Gas only d) Both liquid and gas
12. The rigidity of a substance is its tendency to:
    • a) Flow easily b) Fill its container c) Be compressed d) Maintain its shape
13. The smell of hot sizzling food reaches you from several metres away due to:
    • a) High kinetic energy and rapid diffusion of food particles b) The low density of hot food c) The process of sublimation d) The high pressure of the food vapour
14. What happens to the particles of matter when pressure is applied to a gas?
    • a) They move farther apart b) They come closer together c) Their kinetic energy increases d) They stop moving
15. Boiling is a:
    • a) Surface phenomenon b) Particle phenomenon c) Bulk phenomenon d) Chemical phenomenon
16. Which substance is also known as dry ice?
    • a) Solid water b) Solid nitrogen c) Solid carbon dioxide d) Solid camphor
17. Aquatic animals can breathe underwater because:
    • a) They don’t need oxygen b) Oxygen is dissolved in water c) They can breathe carbon dioxide d) Water is made of hydrogen and oxygen
18. Evaporation from a saucer is faster than from a cup because:
    • a) The saucer has a larger surface area b) The cup has a higher temperature c) The saucer has a lower volume d) The cup has higher humidity
19. The process of a gas changing directly into a solid is called:
    • a) Condensation b) Solidification c) Deposition d) Sublimation
20. At its melting point, the particles in water at 0°C have more energy than particles in ice at 0°C because of:
    • a) Higher kinetic energy b) Latent heat of vaporisation c) Higher pressure d) Latent heat of fusion

Answer Key for MCQ

1. c) Smell of perfume (The particles of perfume are matter)
2. b) Diffusion
3. a) Solid
4. d) Sublimation
5. c) Kelvin
6. c) They can flow
7. b) 27°C (300 – 273 = 27)
8. d) Increasing the wind speed over the water
9. c) Latent heat
10. d) LPG (Liquefied Petroleum Gas)
11. d) Both liquid and gas (It is the temperature at which boiling occurs, so both states can coexist)
12. d) Maintain its shape
13. a) High kinetic energy and rapid diffusion of food particles
14. b) They come closer together
15. c) Bulk phenomenon
16. c) Solid carbon dioxide
17. b) Oxygen is dissolved in water
18. a) The saucer has a larger surface area
19. c) Deposition
20. d) Latent heat of fusion

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Essay Questions & Answers

1. Compare and contrast the three states of matter—solid, liquid, and gas—in terms of particle arrangement, inter-particle forces, and kinetic energy.
2. Answer: The three states of matter—solid, liquid, and gas—are distinguished by fundamental differences in their particle characteristics. In the solid state, particles are arranged in a highly ordered, tightly packed structure with minimal space between them. This arrangement is maintained by very strong forces of attraction, which restrict particles to vibrating in fixed positions, resulting in the lowest kinetic energy among the three states.
3. In the liquid state, particles are still close together, but they are not held in fixed positions. The forces of attraction are intermediate, strong enough to keep the particles together but weak enough to allow them to slide past one another. This gives liquids a fixed volume but an indefinite shape. Their particles have more kinetic energy than solids, allowing for movement and flow.
4. In the gaseous state, particles are separated by large distances and have no ordered arrangement. The forces of attraction between them are negligible, allowing them to move randomly and at high speeds. Consequently, gas particles possess the highest kinetic energy. This high energy and random motion cause gases to have no fixed shape or volume, and they will expand to fill any container they occupy.
5. Explain in detail the concept of “latent heat” by describing both the latent heat of fusion and the latent heat of vaporisation.
6. Answer: Latent heat is the energy absorbed or released by a substance during a change in its physical state that occurs without changing its temperature. The term “latent” means hidden, as this energy does not contribute to a temperature increase but is instead used to overcome the forces of attraction between particles.
7. Latent heat of fusion is the amount of heat energy required to change 1 kilogram of a substance from a solid state to a liquid state at its melting point and at atmospheric pressure. For example, when ice at 0°C is heated, its temperature does not rise until all the ice has melted into water. The continuous supply of heat energy is absorbed by the ice to break the bonds holding the water molecules in a fixed crystal structure, thereby converting it into liquid water at 0°C.
8. Latent heat of vaporisation is the amount of heat energy required to change 1 kilogram of a substance from a liquid state to a gaseous state at its boiling point and at atmospheric pressure. When water reaches its boiling point of 100°C, further heating does not raise its temperature. Instead, the energy is absorbed to overcome the forces of attraction between the liquid particles, allowing them to escape into the gaseous phase as steam. This is why steam at 100°C contains significantly more energy than boiling water at the same temperature and can cause more severe burns.
9. Describe the process of evaporation. List the four main factors that affect its rate and explain why each has an effect.
10. Answer: Evaporation is the process where a liquid changes into its vapour state at any temperature below its boiling point. It is a surface phenomenon, meaning it primarily occurs at the surface of the liquid. Particles at the surface that possess higher kinetic energy are able to break free from the intermolecular forces of attraction holding them in the liquid and escape into the air as vapour.
11. The rate of evaporation is influenced by four main factors:
    1. Surface Area: An increase in surface area increases the rate of evaporation. Since evaporation occurs at the surface, a larger exposed area means more particles are available to escape into the vapour phase at any given time. This is why wet clothes are spread out to dry faster.
    2. Temperature: An increase in temperature accelerates evaporation. Higher temperatures mean that a greater number of particles in the liquid have sufficient kinetic energy to overcome the forces of attraction and convert into vapour.
    3. Wind Speed: An increase in wind speed enhances the rate of evaporation. The wind carries away the water vapour particles from above the liquid’s surface, decreasing the concentration of vapour in the immediate surroundings and making it easier for more liquid particles to escape.
    4. Humidity: A decrease in humidity increases the rate of evaporation. Humidity is the amount of water vapour present in the air. If the air is already saturated with water vapour (high humidity), it cannot hold much more, thus slowing down the rate at which the liquid can evaporate.
12. Explain how both temperature and pressure can be used to change the state of matter. Use water and carbon dioxide as examples.
13. Answer: Temperature and pressure are the two primary factors that determine the physical state of a substance. For water, changing the temperature at standard atmospheric pressure demonstrates this clearly. Below 0°C (273 K), water exists as a solid (ice). By supplying heat and increasing the temperature, ice melts into liquid water at 0°C and remains a liquid until it reaches 100°C (373 K), at which point it boils and turns into a gas (steam).
14. Pressure can also change the state of matter. For most substances, increasing pressure favors the denser state. A more dramatic example is the liquefaction of gases. Gases like oxygen or nitrogen can be converted into liquids by applying very high pressure, which forces the particles close together, and simultaneously reducing the temperature to decrease their kinetic energy.
15. Carbon dioxide (CO₂) is an excellent example of how pressure and temperature determine the state. At standard atmospheric pressure, solid CO₂ (dry ice) does not melt; instead, it sublimes directly into a gas. However, if solid CO₂ is kept under high pressure, it can exist as a liquid. It is stored as a solid under high pressure, and when this pressure is reduced to one atmosphere, it converts directly to its gaseous state. This illustrates that the state of a substance is a result of the interplay between both temperature and pressure.
16. A diver is able to cut through water in a swimming pool. Which property of matter does this observation show? Explain this property in the context of the particles of matter.
17. Answer: This observation shows two key properties of matter in the liquid state. First, it demonstrates that the particles of matter have spaces between them. Second, it shows that the force of attraction between the particles of a liquid is not as strong as in solids.
18. In the context of particles, water is a liquid, and its constituent particles are held together by forces of attraction. However, these forces are weak enough that they can be overcome by an external force, such as the movement of a diver’s hand. When the diver moves through the water, their hand pushes the water particles aside, moving into the spaces between them. Because the attractive forces are not rigid, the particles can easily move and then come back together after the diver has passed. This is in contrast to a solid, where the forces are so strong that particles are locked in place, making it impossible to pass a hand through it.
19. Why does a desert cooler function more effectively on a hot, dry day compared to a hot, humid day?
20. Answer: A desert cooler works on the principle of cooling by evaporation. It pulls in hot, dry air from the outside and passes it over water-soaked pads. The water on the pads evaporates, and during this process, it absorbs a large amount of heat (latent heat of vaporization) from the air. This causes the air to cool down significantly before it is blown into the room.
21. On a hot, dry day, the rate of evaporation is very high. The “dry” condition means the humidity (amount of water vapour in the air) is low. Low humidity allows for rapid evaporation, leading to a greater absorption of heat and a more significant cooling effect.
22. Conversely, on a hot, humid day, the air already contains a high amount of water vapour. This high humidity slows down the rate of evaporation from the cooler’s pads because the surrounding air is nearly saturated and cannot easily absorb more water vapour. As a result, less heat is absorbed from the air, and the cooling effect of the desert cooler is greatly diminished.
23. Give reasons for the following: a) A gas completely fills the vessel in which it is kept. b) A wooden table should be called a solid.
24. Answer: a) A gas completely fills the vessel in which it is kept because of two main characteristics of its particles. Firstly, the forces of attraction between gas particles are negligible, so they are not held together. Secondly, gas particles possess high kinetic energy, causing them to move randomly and rapidly in all directions. Due to this constant, high-speed random motion and lack of attraction, they spread out and occupy all the available space within the vessel.
25. b) A wooden table should be called a solid because it exhibits the key properties of the solid state. It has a definite shape and a fixed volume, which do not change regardless of its container. Furthermore, its particles are tightly packed with very strong intermolecular forces of attraction, making it rigid and non-compressible under normal force. While a strong enough force can break it, it resists any change in its shape.
26. Explain why steam at 373 K (100°C) produces more severe burns than boiling water at the same temperature.
27. Answer: Both boiling water and steam exist at a temperature of 373 K (100°C), so the burns they cause are not due to a difference in temperature. The difference in severity comes from the extra energy stored in steam in the form of latent heat of vaporisation.
28. When water turns into steam at its boiling point, it must absorb a significant amount of heat energy to overcome the forces of attraction between its molecules and change into the gaseous state. This energy does not increase the temperature but is stored within the steam particles. When steam comes into contact with skin, it first condenses back into liquid water, releasing this large amount of latent heat onto the skin. After that, the resulting hot water at 100°C releases further heat as it cools down. Boiling water, on the other hand, can only release the heat it contains as it cools. The additional release of the latent heat of vaporization by steam results in a much greater transfer of energy to the skin, causing more severe burns.
29. Describe the experiment that demonstrates that particles of matter are continuously moving, using an incense stick as an example.
30. Answer: The continuous motion of particles of matter can be demonstrated with a simple experiment involving an incense stick. First, an unlit incense stick is placed in a corner of a room. To get its smell, one must go very close to it, as the particles are diffusing into the air very slowly at room temperature.
31. Next, the incense stick is lit. The heat from burning provides energy to the particles of the incense smoke. This energy increases the kinetic energy of these particles, causing them to move much faster and more randomly. These fast-moving particles mix with the particles of the air (diffusion) and spread rapidly throughout the entire room. As a result, the smell of the lit incense stick can be detected from a considerable distance, even in the far corners of the room, in a very short time. This observation confirms that particles are in constant motion and that this motion becomes faster as temperature increases.
32. What is sublimation? Name a substance that undergoes sublimation and describe a simple activity to observe this process.
33. Answer: Sublimation is the process of transition of a substance directly from the solid to the gaseous state, without passing through the intermediate liquid state. A common substance that undergoes sublimation is camphor (or naphthalene balls).
34. A simple activity to observe the sublimation of camphor is as follows:
    1. Take some crushed camphor in a china dish.
    2. Place an inverted glass funnel over the china dish to cover it.
    3. Plug the stem of the funnel with a piece of cotton to prevent the camphor vapour from escaping.
    4. Heat the china dish slowly on a low flame.
35. Upon heating, the solid camphor will directly turn into gaseous vapour. This vapour will rise and come into contact with the cooler, inner walls of the funnel. There, it will cool down and undergo deposition (the reverse of sublimation), changing directly from a gas back into a solid, which can be observed as solidified camphor collected on the funnel’s inner surface. This activity clearly demonstrates the direct solid-to-gas and gas-to-solid transitions.

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Glossary of Key Terms

Term	Definition
Atmospheric Pressure	The pressure exerted by the air in the atmosphere. The normal atmospheric pressure at sea level is 1 atmosphere.
Boiling	A bulk phenomenon in which particles from the whole of the liquid change into the vapour state at a fixed temperature.
Boiling Point	The temperature at which a liquid starts boiling at the atmospheric pressure.
Compressibility	The property of matter that allows its volume to be decreased by applying pressure. Gases are highly compressible.
Density	The mass per unit volume of a substance (density = mass/volume).
Deposition	The direct change of a substance from a gaseous state to a solid state without changing into a liquid.
Diffusion	The intermixing of particles of two different types of matter on their own.
Evaporation	A surface phenomenon in which a liquid changes into vapours at any temperature below its boiling point.
Fluidity	The property of a substance to flow. Liquids and gases are fluids.
Fusion	The process of melting, where a solid state changes into a liquid state.
Humidity	The amount of water vapour present in the air.
Kinetic Energy	The energy that particles of matter possess due to their continuous motion.
Latent Heat of Fusion	The amount of heat energy required to change 1 kg of a solid into a liquid at its melting point and at atmospheric pressure.
Latent Heat of Vaporization	The amount of heat energy required to change 1 kg of a liquid into a gas at its boiling point and at atmospheric pressure.
Mass	The amount of matter in an object. Its SI unit is the kilogram (kg).
Matter	Anything that occupies space and has mass. It is made up of small particles.
Melting Point	The minimum temperature at which a solid melts to become a liquid at atmospheric pressure.
Panch Tatva	The five basic elements (air, earth, fire, sky, and water) that early Indian philosophers believed everything was made of.
Rigidity	The property of a substance to resist a change in its shape. Solids are rigid.
Sublimation	The change of a solid state directly to a gaseous state without going through the liquid state.
Volume	The amount of space occupied by an object. Its SI unit is the cubic metre (m³).