Questions
1. Which of the following are matter?
Answer:
Chair, air, smell, almonds, cold, lemon water, smell of perfume
These have mass and occupy space, so they are matter.
Love, hate, thought are emotions or feelings — not matter.
2. Give reasons for the following observation:
The smell of hot sizzling food reaches you several metres away, but to get the smell from cold food you have to go close.
Answer:
Particles of matter are constantly moving. In hot food, the particles move faster and spread out quickly due to increased kinetic energy. So, the aroma reaches far. In cold food, particles move slowly, so the smell doesn’t travel far.
3. A diver is able to cut through water in a swimming pool. Which property of matter does this observation show?
Answer:
This shows that particles of matter have space between them. The diver can move through water because water particles are loosely packed and can be pushed aside.
4. What are the characteristics of the particles of matter?
Answer:
- Particles of matter have space between them.
- Particles of matter are continuously moving.
- Particles of matter attract each other.
5.
(a) The mass per unit volume of a substance is called density.
Formula: Density = Mass / Volume
(b) Arrange in order of increasing density:
Air < Exhaust from chimneys < Cotton < Water < Honey < Chalk < Iron
6.
(a) Tabulate the differences in characteristics of states of matter:
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Shape | Fixed | Not fixed | Not fixed |
| Volume | Fixed | Fixed | Not fixed |
| Compressibility | Negligible | Very less | High |
| Rigidity | Rigid | Not rigid | Not rigid |
| Fluidity | Cannot flow | Can flow | Can flow easily |
(b) Comment upon:
- Rigidity: Solids are rigid; liquids and gases are not.
- Compressibility: Gases are highly compressible; liquids slightly; solids almost incompressible.
- Fluidity: Liquids and gases can flow; solids cannot.
- Filling a gas container: Gases completely fill the container.
- Shape: Solids have fixed shape; liquids and gases take shape of the container.
- Kinetic energy: Lowest in solids, more in liquids, highest in gases.
- Density: Usually highest in solids, lower in liquids, lowest in gases.
7. Give reasons:
(a) A gas fills completely the vessel in which it is kept.
Answer:
Gas particles move freely and have negligible force of attraction, so they spread and fill the container.
(b) A gas exerts pressure on the walls of the container.
Answer:
Gas particles move randomly and collide with container walls, exerting pressure.
(c) A wooden table should be called a solid.
Answer:
It has a definite shape, volume, and is rigid—properties of a solid.
(d) We can easily move our hand in air but to do the same through a solid block of wood we need a karate expert.
Answer:
Air particles are loosely packed with lots of space between them, allowing movement. Wood particles are tightly packed, resisting movement.
8 . Liquids generally have lower density than solids. But ice floats on water. Find out why.
Answer:
Ice has a cage-like structure due to hydrogen bonding, creating more empty spaces. This makes ice less dense than liquid water, so it floats.
9. Convert the following temperature to Celsius scale:
Formula:
°C = K − 273
- (a) 300 K = 27°C
- (b) 573 K = 300°C
10. What is the physical state of water at:
- (a) 250°C – Vapour (since it is well above the boiling point of water, 100°C)
- (b) 100°C – Both liquid and vapour (this is the boiling point, where both states coexist during phase change)
11. For any substance, why does the temperature remain constant during the change of state?
Answer:
During a change of state, the heat energy supplied is used to overcome the forces of attraction between the particles, not to increase their kinetic energy. Hence, the temperature remains constant.
12. Suggest a method to liquefy atmospheric gases.
Answer:
By increasing pressure and lowering the temperature, atmospheric gases can be liquefied. This reduces the kinetic energy of gas molecules and brings them closer together.
13. Why does a desert cooler cool better on a hot dry day?
Answer:
Because on a hot dry day, the rate of evaporation is higher. More water evaporates from the cooler’s pads, taking away more heat and cooling the air more effectively.
14. How does the water kept in an earthen pot (matka) become cool during summer?
Answer:
Water seeps out through the pores of the earthen pot and evaporates. This evaporation requires heat, which is taken from the water inside, making it cool.
15. Why does our palm feel cold when we put some acetone or petrol or perfume on it?
Answer:
These substances evaporate quickly. During evaporation, they absorb heat from our palm, making it feel cold.
16. Why are we able to sip hot tea or milk faster from a saucer rather than a cup?
Answer:
A saucer has a larger surface area, so the tea or milk cools faster due to quicker evaporation.
17. What type of clothes should we wear in summer?
Answer:
We should wear light-colored, loose, and cotton clothes in summer because they reflect heat and absorb sweat, helping it evaporate and cool the body.
Exercises
1. Convert the following temperatures to the Celsius scale:
(a) 293 K = 293 − 273 = 20°C
(b) 470 K = 470 − 273 = 197°C
2. Convert the following temperatures to the Kelvin scale:
(a) 25°C = 25 + 273 = 298 K
(b) 373°C = 373 + 273 = 646 K
3. Give reason for the following observations:
(a) Naphthalene balls disappear with time without leaving any solid because they directly change from solid to gas (sublimation).
(b) We can smell perfume from a distance because its particles diffuse quickly through air.
4. Arrange in increasing order of force of attraction between particles:
Oxygen < Water < Sugar
5. Physical state of water at:
(a) 25°C — Liquid
(b) 0°C — Solid and Liquid (in equilibrium)
(c) 100°C — Liquid and Gas (in equilibrium)
6. Justify:
(a) Water at room temperature is a liquid:
- It has a definite volume but no definite shape.
- The intermolecular force is moderate, allowing flow.
(b) Iron almirah is solid at room temperature:
- It has a fixed shape and volume.
- Strong intermolecular forces keep the particles tightly packed.
7. Why is ice at 273 K more effective in cooling than water at the same temperature?
Because ice absorbs extra heat (latent heat of fusion) to melt into water, thus drawing more heat and cooling more effectively.
8. What produces more severe burns, boiling water or steam?
Steam, because it carries latent heat of vaporization in addition to its temperature.
9. Name A, B, C, D, E, F in the diagram:
- A = Fusion (Melting)
- B = Vaporization (Boiling)
- C = Condensation
- D = Solidification (Freezing)
- E = Sublimation
- F = Deposition