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:
- Fluidity: Water flows and takes the shape of the container it is poured into, which is a characteristic property of liquids.
- 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
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