NCERT Solutions Class 9 Science Chapter 1 – Matter in Our Surroundings

NCERT Solutions for Class 9 Science Chapter 1: Matter in Our Surroundings

This foundational chapter introduces the concept of matter, its states, and the interconversion between states. Understanding this chapter is essential as it forms the basis for chemistry concepts in higher classes.

In-text Questions and Answers

Page 3 Questions

Q1. Which of the following are matter? Chair, air, love, smell, hate, almonds, thought, cold, cold drink, smell of perfume.

Answer:

• Matter: Chair, air, almonds, cold drink, smell of perfume (they have mass and occupy space)
• Not Matter: Love, hate, thought, cold (these are feelings/sensations, not physical substances)

Q2. Give reasons for the following observations:

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

Answer: Naphthalene undergoes sublimation – it directly converts from solid to gaseous state without passing through liquid state. The naphthalene vapors diffuse into air, causing the balls to gradually disappear.

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

Answer: Perfume contains volatile substances that evaporate easily. The gaseous perfume molecules diffuse rapidly through air (gases have high rate of diffusion) and reach our nose even from a distance.

Page 6 Questions

Q1. What is the physical state of water at: (a) 25°C (b) 0°C (c) 100°C?

Answer:

• (a) At 25°C – Liquid (normal room temperature, water remains liquid)
• (b) At 0°C – Solid or Liquid (this is the melting/freezing point, both states can coexist)
• (c) At 100°C – Liquid or Gas (this is the boiling point at 1 atm, both states can coexist)

Q2. Give two reasons to justify that water at room temperature is a liquid.

Answer:

1. Fluidity: Water flows and takes the shape of the container it is poured into, which is a characteristic property of liquids.
2. Fixed Volume: Water has a definite volume but no definite shape. Unlike gases, it cannot be compressed easily, and unlike solids, it doesn’t have a fixed shape.

Exercise Questions

Q1. Convert the following temperatures to the Celsius scale: (a) 300 K (b) 573 K

Solution:

Formula: °C = K – 273

(a) 300 K = 300 – 273 = 27°C

(b) 573 K = 573 – 273 = 300°C

Q2. Convert the following temperatures to the Kelvin scale: (a) 25°C (b) 373°C

Solution:

Formula: K = °C + 273

(a) 25°C = 25 + 273 = 298 K

(b) 373°C = 373 + 273 = 646 K

Q3. Give reason for the following observations: (a) A diver is able to cut through water in a swimming pool.

Answer: Water is a liquid with weak intermolecular forces of attraction compared to solids. The molecules can move past each other easily. When a diver enters the water, the water molecules move aside, allowing the diver to cut through. This is due to the property of fluidity in liquids.

Comparison of States of Matter

Property	Solid	Liquid	Gas
Shape	Fixed	Not fixed	Not fixed
Volume	Fixed	Fixed	Not fixed
Compressibility	Negligible	Very less	Highly compressible
Diffusion	Very slow	Slow	Very fast
Intermolecular Force	Maximum	Less than solid	Minimum

Key Takeaways

• Matter is anything that has mass and occupies space
• Matter exists in three states: solid, liquid, and gas
• States can be interconverted by changing temperature or pressure
• Melting point: solid → liquid; Boiling point: liquid → gas
• Sublimation: direct conversion from solid to gas
• Evaporation is a surface phenomenon that causes cooling
• Latent heat is absorbed/released during change of state without temperature change

Key Concepts You Must Know

Matter in Our Surroundings is one of the foundational chapters of Class 9 Science. It introduces students to the physical nature of matter — how it exists, how it changes, and what happens when conditions like temperature and pressure change.

States of Matter

Matter exists in three common states: solid, liquid, and gas. Each state has distinct characteristics based on the arrangement and movement of particles:

• Solid: Particles are tightly packed in a regular arrangement. Solids have definite shape and volume. They are incompressible.
• Liquid: Particles are loosely arranged and can slide past each other. Liquids have definite volume but no fixed shape — they take the shape of the container.
• Gas: Particles are far apart and move freely in all directions. Gases have neither definite shape nor definite volume. They are highly compressible.

Changes of State

Matter can change from one state to another when temperature or pressure changes:

• Melting (Fusion): Solid → Liquid (e.g., ice melting at 0°C)
• Freezing (Solidification): Liquid → Solid (e.g., water freezing at 0°C)
• Vaporisation/Boiling: Liquid → Gas (e.g., water boiling at 100°C)
• Condensation: Gas → Liquid (e.g., water vapour forming dew drops)
• Sublimation: Solid → Gas directly (e.g., camphor, dry ice, iodine crystals)
• Deposition: Gas → Solid directly

Important Definitions

• Latent Heat of Fusion: The heat energy required to change 1 kg of a solid into liquid at its melting point, without any change in temperature. For ice, it is 3.34 × 10⁵ J/kg.
• Latent Heat of Vaporisation: The heat energy required to change 1 kg of liquid to vapour at its boiling point. For water, it is 22.5 × 10⁵ J/kg.
• Evaporation: A surface phenomenon where liquid converts to gas below its boiling point. Evaporation causes cooling — this is why we feel cool after applying a spirit/alcohol to the skin.
• Diffusion: The mixing of particles of one substance with particles of another due to random motion. Rate of diffusion is faster in gases and slower in solids.

Factors Affecting Evaporation

Evaporation is faster when:

• Temperature is higher (more particles have enough energy to escape)
• Surface area is larger (more particles exposed at the surface)
• Wind speed is higher (vapour particles are carried away, making room for more)
• Humidity is lower (dry air absorbs more moisture)

Important Questions for CBSE Board Exam

• Why does a gas exert pressure on the walls of its container?
• Why does ice at 0°C feel colder than water at 0°C?
• Why does a wet cloth dry faster on a hot windy day?
• What happens when pressure is increased on a gas? What about temperature?
• Define sublimation with two examples.
• Why is the smell of perfume noticed several metres away?
• Explain why cooling occurs during evaporation.

CBSE Exam Tips for This Chapter

• Remember the exact temperatures: ice melts at 0°C, water boils at 100°C
• Learn the direction of all state changes (solid→liquid→gas and reverse)
• The Latent Heat values (3.34 × 10⁵ and 22.5 × 10⁵) are frequently asked as fill-in-the-blank or reason-based questions
• Diagrams of the particle arrangement in three states can fetch full marks in 3-mark questions
• Evaporation vs Boiling: Both convert liquid to gas, but evaporation is a slow surface process below boiling point

Frequently Asked Questions (FAQs)

Q: What is the difference between evaporation and boiling?
A: Evaporation occurs at the surface of a liquid at any temperature below its boiling point. Boiling occurs throughout the liquid at its boiling point. Evaporation is a slow process; boiling is rapid.

Q: Why does ice feel colder than water even at the same temperature (0°C)?
A: Because ice absorbs extra heat (latent heat of fusion = 3.34 × 10⁵ J/kg) from our body to convert into water. This extra heat absorption makes ice feel colder.

Q: Why does liquid nitrogen (boiling point −196°C) boil at room temperature?
A: Room temperature (≈25°C) is far above liquid nitrogen’s boiling point (−196°C). At room temperature, liquid nitrogen has enough energy to rapidly convert to gas.

Q: What is dry ice and why is it called “dry”?
A: Dry ice is solid carbon dioxide (CO₂). It is called “dry” because it undergoes sublimation — it converts directly from solid to gas without passing through the liquid state, leaving no wet residue.