Page 3 – Questions and Answers

Question 1

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

Answer:

Matter is anything that has mass and occupies space.

✔ Matter:

• Chair

• Air

• Almonds

• Lemon water

• Smell of perfume

✘ Not matter:

• Love

• Hate

• Thought

• Cold

Explanation:
Chair, air, almonds, lemon water and perfume smell have mass and occupy space.
Love, hate, thought and cold are feelings or sensations, so they do not have mass or volume.

---

Question 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 continuously moving.

• In hot food, particles have more kinetic energy, so they move faster and diffuse quickly in air.

• In cold food, particles move slowly, so the smell spreads slowly.

Therefore, the smell of hot food reaches us from far away, while cold food must be smelled from nearby.

Question 3

A diver is able to cut through water in a swimming pool. Which property of matter does this observation show?

Answer:

This observation shows that particles of matter have spaces between them.

Explanation:
Water particles are not tightly packed. There is space between them, so the diver can move through water easily.

---

Question 4

What are the characteristics of the particles of matter?

Answer:

The particles of matter have the following characteristics:

1. Particles of matter are very small
   They are too small to be seen with naked eyes.

2. Particles of matter have spaces between them
   This allows substances to mix or dissolve in each other.

3. Particles of matter are continuously moving
   They possess kinetic energy and show diffusion.

4. Particles of matter attract each other
   There is a force of attraction between particles which keeps them together.

---

Page 6 – Questions and Answers

---

Question 1

The mass per unit volume of a substance is called density.

$$\text{Density}=\frac{\text{Mass}}{\text{Volume}}$$

Arrange the following in order of increasing density:
air, exhaust from chimneys, honey, water, chalk, cotton and iron.

Answer:

Increasing order of density (lowest to highest):

Air < Exhaust from chimneys < Cotton < Water < Honey < Chalk < Iron

Explanation:

• Air has the least density.

• Exhaust gases are slightly denser than air.

• Cotton contains air trapped in it, so its density is low.

• Water is denser than cotton.

• Honey is denser than water.

• Chalk is a solid and denser than liquids.

• Iron is a metal and has the highest density among these.

---

Question 2 (a)

Tabulate the differences in the characteristics of states of matter.

Answer:

---

Question 2 (b)

Comment upon the following: rigidity, compressibility, fluidity, filling a gas container, shape, kinetic energy and density.

Answer:

• Rigidity: Solids are rigid, liquids and gases are not.

• Compressibility: Gases are highly compressible, liquids slightly, solids almost not.

• Fluidity: Liquids and gases can flow, solids cannot.

• Filling a gas container: Gases completely fill the container due to large spaces between particles.

• Shape: Solids have fixed shape; liquids and gases do not.

• Kinetic energy: Least in solids, more in liquids, maximum in gases.

• Density: Highest in solids, lower in liquids, least in gases.

---

Question 3

Give reasons:

(a) A gas fills completely the vessel in which it is kept.

Answer:
Gas particles have very large spaces between them and high kinetic energy, so they spread out and fill the entire vessel.

---

(b) A gas exerts pressure on the walls of the container.

Answer:
Gas particles move randomly at high speed and collide with the walls of the container, exerting pressure.

---

(c) A wooden table should be called a solid.

Answer:
A wooden table has a fixed shape, fixed volume, and is rigid, which are properties of solids.

---

(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 have large spaces and weak attraction, so the hand moves easily.
Wood particles are closely packed with strong forces, so great force is required.

Question 4

Liquids generally have lower density as compared to solids. But you must have observed that ice floats on water. Find out why.

Answer:

Ice floats on water because:

• Ice has a lower density than water

• During freezing, water expands due to formation of an open structure

• This makes ice lighter than liquid water

Therefore, ice floats on water.

---

Page 9 – Questions and Answers

---

Question 1

Convert the following temperature to Celsius scale:

(a) 300 K

(b) 573 K

Answer:

To convert Kelvin to Celsius:

$${}^{\circ }C=K-273$$

• (a) 300 K = 300 − 273 = 27°C

• (b) 573 K = 573 − 273 = 300°C

---

Question 2

What is the physical state of water at:

(a) 25°C

(b) 100°C

Answer:

• (a) At 25°C: Water is in liquid state

• (b) At 100°C: Water is in gaseous state (steam)

(At 100°C, water reaches its boiling point and changes into vapour.)

---

Question 3

For any substance, why does the temperature remain constant during the change of state?

Answer:

During a change of state:

• The heat supplied is used to overcome the force of attraction between particles

• It does not increase the kinetic energy of particles

• Therefore, the temperature remains constant

This absorbed heat is called latent heat.

Question 4

Suggest a method to liquefy atmospheric gases.

Answer:

Atmospheric gases can be liquefied by:

• Applying high pressure

• Lowering the temperature

These conditions bring gas particles closer and convert them into liquid state.

---

Page 10 – Questions and Answers

---

Question 1

Why does a desert cooler cool better on a hot dry day?

Answer:

A desert cooler cools better on a hot dry day because:

• The air is dry (low humidity)

• Water evaporates faster

• Evaporation absorbs more heat from surroundings

• This causes greater cooling effect

---

Question 2

How does the water kept in an earthen pot (matka) become cool during summer?

Answer:

• The earthen pot has tiny pores on its surface

• Water slowly seeps out and evaporates

• Evaporation absorbs heat from the water inside

• As a result, the water inside the pot becomes cool

---

Question 3

Why does our palm feel cold when we put some acetone or petrol or perfume on it?

Answer:

• Acetone, petrol and perfume evaporate quickly

• They absorb heat from our palm

• Loss of heat makes the palm feel cold

This is due to evaporation causing cooling.

---

Question 4

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

• Evaporation occurs faster

• Heat is lost quickly

• The liquid cools faster than in a cup

Therefore, we can sip it easily from a saucer.

---

Question 5

What type of clothes should we wear in summer?

Answer:

We should wear cotton clothes in summer because:

• Cotton absorbs sweat

• Sweat evaporates easily

• Evaporation removes heat from the body

• This keeps us cool and comfortable