Class 9 Science Chapter 1 MATTER IN OUR SURROUNDINGS

Combustion and Flame

Question 1 What is “matter” according to scientists? Answer : Everything in this universe is made up of material which scientists have named “matter”.

Question 2 What are the two fundamental properties all matter possesses? Answer : All matter occupies space and has mass, meaning it has both mass and volume.

Question 3 How did early Indian philosophers classify matter? Answer : They classified matter into five basic elements — the “Panch Tatva”: air, earth, fire, sky, and water.

Question 4 What are the two bases for modern scientific classification of matter? Answer : Modern scientists classify matter based on their physical properties and chemical nature.

Question 5 What is the fundamental physical nature of matter? Answer : Matter is made up of particles.

Question 6 How small are the particles that make up matter? Answer : The particles of matter are very small, beyond our imagination.

Question 7 What evidence suggests there is space between particles of matter? Answer : The even distribution of substances like sugar or salt in water indicates that particles get into the spaces between other particles, showing enough space between them.

Question 8 What is a key characteristic of matter particles regarding their motion? Answer : Particles of matter are continuously moving.

Question 9 What type of energy do continuously moving particles possess? Answer : Continuously moving particles possess kinetic energy.

Question 10 How does increasing temperature affect the kinetic energy of particles? Answer : As temperature rises, the kinetic energy of the particles increases, making them move faster.

Question 11 What is “diffusion”? Answer : Diffusion is the intermixing of particles of two different types of matter on their own.

Question 12 How does heating affect the rate of diffusion? Answer : On heating, diffusion becomes faster.

Question 13 What force exists between particles of matter? Answer : Particles of matter have a force acting between them that keeps the particles together.

Question 14 What gives rise to the different states of matter? Answer : The variation in the characteristics of the particles of matter gives rise to its different states.

Question 15 Name the three common states of matter. Answer : The three common states of matter are solid, liquid, and gas.

Question 16 What are the defining physical properties of a solid? Answer : Solids have a definite shape, distinct boundaries, fixed volume, negligible compressibility, and are rigid.

Question 17 Why is a rubber band considered a solid even though it can be stretched? Answer : It changes shape under force and regains the same shape when the force is removed, and breaks if excessive force is applied.

Question 18 Why are individual sugar or salt crystals considered solids despite taking the shape of their container? Answer : The shape of each individual crystal remains fixed, whether placed in a hand, plate, or jar.

Question 19 How can a sponge, a solid, be compressed? Answer : A sponge has minute holes that trap air, which is expelled out when pressed, allowing it to be compressed.

Question 20 What are the defining physical properties of a liquid? Answer : Liquids have no fixed shape but a fixed volume, take the shape of their container, flow, change shape, and are fluid.

Question 21 Can gases diffuse into liquids? Answer : Yes, gases from the atmosphere diffuse and dissolve in water, which is essential for aquatic life.

Question 22 How does the rate of diffusion of liquids compare to that of solids? Answer : The rate of diffusion of liquids is higher than that of solids.

Question 23 Why do liquids diffuse faster than solids? Answer : In the liquid state, particles move freely and have greater space between them as compared to particles in the solid state.

Question 24 What is a notable characteristic of gases regarding compressibility? Answer : Gases are highly compressible as compared to solids and liquids.

Question 25 Name a common compressed gas used for cooking. Answer : Liquefied petroleum gas (LPG) is a compressed gas used for cooking.

Question 26 What causes a gas to exert pressure on the walls of its container? Answer : The force exerted by rapidly moving gas particles per unit area on the walls of the container causes the pressure exerted by the gas.

Question 27 How does the speed of diffusion of gases compare to other states of matter? Answer : Gases show the property of diffusing very fast into other gases due to the high speed of their particles and large spaces between them.

Question 28 What two physical changes can cause matter to change its state? Answer : Matter can change its state by changing temperature or pressure.

Question 29 What is the “melting point” of a solid? Answer : It is the minimum temperature at which a solid melts to become a liquid at atmospheric pressure.

Question 30 What is “fusion” in the context of state changes? Answer : Fusion is the process of melting, which is the change of a solid state into a liquid state.

Question 31 What is “latent heat of fusion”? Answer : It is the amount of heat energy required to change 1 kg of a solid into liquid at atmospheric pressure at its melting point, without showing any rise in temperature.

Question 32 Do particles in water at 0°C have more or less energy than particles in ice at 0°C? Answer : Particles in water at 0°C (273 K) have more energy as compared to particles in ice at the same temperature, due to the absorbed latent heat of fusion.

Question 33 What is the “boiling point” of a liquid? Answer : It is the temperature at which a liquid starts boiling at atmospheric pressure.

Question 34 What is “latent heat of vaporisation”? Answer : It is the heat energy required to change 1 kg of a liquid to gas at atmospheric pressure at its boiling point.

Question 35 Do particles in steam at 100°C have more or less energy than water at 100°C? Answer : Particles in steam at 100°C (373 K) have more energy than water at the same temperature because they have absorbed extra energy in the form of latent heat of vaporisation.

Question 36 How do you convert a temperature from the Kelvin scale to the Celsius scale? Answer : To change a temperature on the Kelvin scale to the Celsius scale, you have to subtract 273 from the given temperature.

Question 37 How do you convert a temperature from the Celsius scale to the Kelvin scale? Answer : To convert a temperature on the Celsius scale to the Kelvin scale, you have to add 273 to the given temperature.

Question 38 What is “sublimation”? Answer : Sublimation is the direct change of a solid state to a gaseous state without passing through the liquid state.

Question 39 What is “deposition”? Answer : Deposition is the direct change of a gaseous state to a solid state without passing through the liquid state.

Question 40 What is “dry ice”? Answer : Dry ice is solid carbon dioxide (CO2), which gets converted directly into the gaseous state on a decrease of pressure without coming into the liquid state.

Question 41 What is “evaporation”? Answer : Evaporation is the phenomenon of a liquid changing into vapours at any temperature below its boiling point.

Question 42 Is evaporation a surface phenomenon or a bulk phenomenon? Answer : Evaporation is a surface phenomenon.

Question 43 Name one factor that increases the rate of evaporation. Answer : An increase in surface area, an increase in temperature, a decrease in humidity, or an increase in wind speed can all increase the rate of evaporation.

Question 44 How does evaporation cause cooling? Answer : Particles of liquid absorb energy from the surroundings to regain the energy lost during evaporation, which makes the surroundings cold.

Question 45 Why should we wear cotton clothes in summer? Answer : Cotton is a good absorber of water, helping to absorb sweat and expose it to the atmosphere for easy evaporation, thereby cooling the body.

Question 46 Why do water droplets appear on the outer surface of a glass with ice-cold water? Answer : Water vapour present in the air loses energy and condenses into liquid droplets when it comes in contact with the cold glass of water.

Question 47 In which state of matter are the forces of attraction between particles maximum? Answer : The forces of attraction between the particles are maximum in solids.

Question 48 In which state of matter is the space between constituent particles maximum? Answer : The space between the constituent particles is maximum in gases.

Question 49 Why is ice at 273 K more effective in cooling than water at the same temperature? Answer : Ice at 273 K (0°C) is more effective in cooling because it absorbs latent heat of fusion from the surroundings to melt, while water at the same temperature does not.

Question 50 What produces more severe burns, boiling water or steam at 100°C? Answer : Steam at 100°C produces more severe burns because its particles have absorbed extra energy in the form of latent heat of vaporisation compared to boiling water at the same temperature.

Chapter 1: MATTER IN OUR SURROUNDINGS

1. Introduction to Matter

Definition: Everything in the universe is made up of material that scientists call “matter”.
Examples: Air, food, stones, clouds, stars, plants, animals, a drop of water, or a particle of sand are all matter.
Properties of Matter: All matter occupies space and has mass. In other words, matter has both mass and volume.
- Mass: The SI unit of mass is kilogram (kg).
- Volume: The SI unit of volume is cubic metre (m³). Common units include litre (L), where 1 L = 1 dm³, 1 L = 1000 mL, and 1 mL = 1 cm³.

2. Early Classification of Matter

Indian Philosophers: Early Indian philosophers classified matter into five basic elements, known as the “Panch Tatva”: air, earth, fire, sky, and water. They believed everything, living or non-living, was composed of these five elements.
Greek Philosophers: Ancient Greek philosophers had a similar classification of matter.
Modern Classification: Modern scientists classify matter based on its physical properties and chemical nature. This chapter focuses on physical properties.

3. Physical Nature of Matter

3.1 Matter is Made Up of Particles

Debate: For a long time, there were two prevailing schools of thought: one believed matter was continuous (like a block of wood), while the other thought it was made up of particles (like sand).
Evidence (Activity 1.1): Dissolving salt or sugar in water demonstrates that the particles of salt/sugar spread throughout the water and get into the spaces between water particles, indicating matter is made up of particles and has spaces between them.

3.2 How Small Are These Particles of Matter?

Evidence (Activity 1.2): Diluting a solution of potassium permanganate or Dettol repeatedly (5 to 8 times) shows that even a few crystals can colour a very large volume of water or that the smell can be detected even after repeated dilution.
Conclusion: This suggests that there are millions of tiny particles in one crystal of potassium permanganate that keep dividing into smaller and smaller particles. Particles of matter are exceedingly small, “beyond our imagination”.

4. Characteristics of Particles of Matter

4.1 Particles of Matter Have Space Between Them

Observation: Activities like dissolving salt/sugar in water (Activity 1.1) and distributing Dettol or potassium permanganate evenly in water (Activity 1.2) show that particles of one substance fit into the spaces between particles of another. Making tea, coffee, or lemonade also demonstrates this.
Conclusion: There is enough space between particles of matter.

4.2 Particles of Matter Are Continuously Moving

Kinetic Energy: Particles of matter are continuously moving, possessing kinetic energy.
Effect of Temperature: As temperature rises, particles move faster, meaning an increase in temperature increases the kinetic energy of particles.
Evidence:
- Incense Stick (Activity 1.3): The smell of a lit incense stick spreads quickly over a distance, indicating particle movement.
- Ink/Honey in Water (Activity 1.4): Ink spreading in water shows continuous movement, while honey spreads slower, indicating different rates of movement.
- Crystals in Hot/Cold Water (Activity 1.5): Crystals of copper sulphate or potassium permanganate dissolve and spread faster in hot water than in cold water, demonstrating that the rate of mixing (diffusion) increases with temperature.
Diffusion: The intermixing of particles of two different types of matter on their own (by getting into the spaces between particles) is called diffusion. Heating makes diffusion faster.
Example: The smell of hot sizzling food reaches several meters away due to faster particle movement at higher temperatures, unlike cold food.

4.3 Particles of Matter Attract Each Other

Evidence (Activities 1.6, 1.7, 1.8):
- Human Chains (Activity 1.6): Holding hands or locking arms (representing stronger forces) is harder to break than just touching fingertips.
- Breaking Substances (Activity 1.7): An iron nail is harder to break than chalk or a rubber band, implying stronger forces between iron particles.
- Cutting Water (Activity 1.8): Being unable to cut the surface of water with fingers suggests that water particles hold together.
Conclusion: Particles of matter have a force acting between them that keeps them together. The strength of this force of attraction varies from one type of matter to another.

5. States of Matter

Matter around us exists in three states: solid, liquid, and gas. These states arise from variations in the characteristics of matter’s particles.

5.1 The Solid State

Properties (Activity 1.9):
- Definite Shape: Solids have a definite shape.
- Distinct Boundaries: They have distinct boundaries.
- Fixed Volume: They have a fixed volume.
- Negligible Compressibility: It is difficult to compress them.
- Rigidity: Solids have a tendency to maintain their shape and are rigid, meaning it’s difficult to change their shape. They may break under force but resist shape change.
- Diffusion: Solids are generally not capable of diffusing into each other.
Particle Arrangement: Particles are held together with maximum force. The arrangement of particles is most ordered, and the spaces between them and kinetic energy are minimum.
Exceptions:
- Rubber band: Changes shape under force but regains it when force is removed; breaks if excessive force is applied. Still considered a solid.
- Sugar/Salt: Individual crystals retain their fixed shape regardless of the container.
- Sponge: Is a solid but compressible because it has minute holes that trap air, which is expelled when pressed.

5.2 The Liquid State

Properties (Activity 1.10):
- No Fixed Shape: Liquids have no fixed shape; they take the shape of the container.
- Fixed Volume: They have a fixed volume.
- Fluidity: Liquids flow and change shape; they are not rigid but are called fluid.
- Diffusion: Solids and gases can diffuse into liquids. The rate of diffusion of liquids is higher than that of solids.
Particle Arrangement: Particles move freely and have greater space between each other compared to solids. Forces of attraction are intermediate, as are the spaces and kinetic energy. Layers of particles can slip and slide over each other.
Importance: Dissolved gases like oxygen and carbon dioxide in water are essential for aquatic animals and plants to breathe and survive.

5.3 The Gaseous State

Properties (Activity 1.11):
- No Fixed Shape or Volume: Gases have no fixed shape and no fixed volume; they completely fill the vessel.
- High Compressibility: Gases are highly compressible compared to solids and liquids. Large volumes of gas can be compressed into small cylinders (e.g., LPG, CNG).
- Rapid Diffusion: Due to high speed of particles and large spaces between them, gases diffuse very fast into other gases.
- Pressure: Gas particles move randomly at high speed, hitting each other and the container walls. The force exerted by these particles per unit area on the walls causes the pressure exerted by the gas.
Particle Arrangement: Particles move about randomly at high speed; there is no order. Forces of attraction are minimum, and spaces between particles and kinetic energy are maximum.

6. Can Matter Change Its State?

Yes, matter can change its state. Water is a common example, existing as ice (solid), water (liquid), and water vapour (gas).

6.1 Effect of Change of Temperature

Melting (Solid to Liquid) – Fusion:
- On increasing the temperature of a solid, the kinetic energy of its particles increases, making them vibrate faster.
- The heat energy supplied overcomes the forces of attraction, causing particles to leave fixed positions and move freely, leading to melting.
- Melting Point: The minimum temperature at which a solid melts to become a liquid at atmospheric pressure. It indicates the strength of attraction forces between particles.
- Latent Heat of Fusion: During melting, the temperature remains constant even with continuous heating. This hidden heat energy, absorbed to overcome attractive forces and change state, is called latent heat. The amount of heat energy required to change 1 kg of a solid into liquid at atmospheric pressure at its melting point.
- Example: Particles in water at 0°C (273 K) have more energy than particles in ice at the same temperature, due to absorbed latent heat of fusion.
Boiling (Liquid to Gas) – Vaporisation:
- When heat energy is supplied to water, particles move faster until they gain enough energy to break free from attractive forces and change into gas.
- Boiling Point: The temperature at which a liquid starts boiling at atmospheric pressure.
- Bulk Phenomenon: Boiling affects particles from the entire bulk of the liquid.
- Latent Heat of Vaporisation: Similar to latent heat of fusion, particles in steam (water vapour) at 373 K (100°C) have more energy than water at the same temperature because they’ve absorbed extra energy in the form of latent heat of vaporisation.
Temperature Units:
- SI Unit: Kelvin (K).
- Conversion: 0°C = 273.15 K (conveniently taken as 273 K).
  - Celsius to Kelvin: Add 273.
  - Kelvin to Celsius: Subtract 273.

6.2 Sublimation and Deposition

Sublimation: The direct change of a solid to a gas without passing through the liquid state.
- Example: Camphor undergoes sublimation (Activity 1.13). Naphthalene balls disappear over time due to sublimation.
Deposition: The direct change of a gas to a solid without passing through the liquid state.
Dry Ice: Solid carbon dioxide (CO₂) is stored under high pressure. It gets converted directly into a gaseous state when pressure decreases to 1 atmosphere without becoming liquid, which is why it’s called dry ice.

6.3 Effect of Change of Pressure

Compressing Gases: Increasing pressure and reducing temperature can liquefy gases.
Particle Distance: The difference in states of matter is due to the distances between constituent particles. Applying pressure can bring gas particles closer together.
Conclusion: Pressure and temperature together determine the state of a substance (solid, liquid, or gas).

7. Evaporation

Definition: The phenomenon of change of liquid into vapours at any temperature below its boiling point. It is a surface phenomenon.
Mechanism: Particles at the surface of the liquid, having higher kinetic energy, break away from the forces of attraction and convert into vapour.
Factors Affecting Evaporation (Activity 1.14): The rate of evaporation increases with:
- Increase in surface area: Spreading out clothes to dry increases surface area, thus increasing evaporation.
- Increase in temperature: More particles gain enough kinetic energy to go into the vapour state.
- Decrease in humidity: Lower humidity (less water vapour in the air) allows for faster evaporation.
- Increase in wind speed: Wind carries away water vapour particles, decreasing the amount of vapour in the surroundings, which speeds up evaporation.

7.1 How Does Evaporation Cause Cooling?

Process: During evaporation, liquid particles absorb energy from the surroundings (or the body surface) to regain the energy lost during the process.
Result: This absorption of energy makes the surroundings (or the surface from which evaporation occurs) feel cold.
Examples:
- Acetone/Petrol on Palm: Particles gain energy from the palm and evaporate, making the palm feel cool.
- Sprinkling water on roofs: Water’s large latent heat of vaporisation helps cool hot surfaces.
- Wearing cotton clothes in summer: Cotton absorbs sweat, which then evaporates from the body, absorbing latent heat and leaving the body cool.
- Water in earthen pots (matkas): Water seeps through pores and evaporates from the surface, cooling the remaining water.
- Sipping tea from a saucer: Increased surface area in a saucer leads to faster evaporation and quicker cooling.

Key Learnings (Summary)

Matter is composed of small particles.
Matter exists in three states: solid, liquid, and gas.
Forces of attraction: Maximum in solids, intermediate in liquids, minimum in gases.
Space between particles and kinetic energy: Minimum in solids, intermediate in liquids, maximum in gases.
Particle arrangement: Most ordered in solids, layers can slip in liquids, no order and random movement in gases.
States are inter-convertible by changing temperature or pressure.
Sublimation is solid to gas directly; deposition is gas to solid directly.
Boiling is a bulk phenomenon; evaporation is a surface phenomenon.
Evaporation depends on surface area, temperature, humidity, and wind speed.
Evaporation causes cooling.
Latent heat of fusion (solid to liquid) and latent heat of vaporisation (liquid to gas) are crucial for phase changes at constant temperature.

Measurable Quantities and Units

Temperature: Kelvin (K)
Length: Metre (m)
Mass: Kilogram (kg)
Weight: Newton (N)
Volume: Cubic metre (m³)
Density: Kilogram per cubic metre (kg m⁻³)
Pressure: Pascal (Pa)