Q1. Convert the following temperatures to the Celsius scale.

(a) 293 K
(b) 470 K

Solution:
We know:

Celsius (°C)=Kelvin (K)−273\(\text{Celsius (°C)} = \text{Kelvin (K)} - 273\) 

• (a) 293K−273=20∘C293K - 273 = \(20^{\circ}C\)
• (b) 470K−273=197∘C470K - 273 = \(197^{\circ}C\)

Q2. Convert the following temperatures to the Kelvin scale.

(a) 250°C
(b) 373°C

Solution:
We know:

Kelvin (K)=Celsius (°C)+273\(\text{Kelvin (K)} = \text{Celsius (°C)}\) + 273 

• (a) 250+273=523K250 + 273 = 523K
• (b) 373+273=646K373 + 273 = 646K

Q3. Give a reason for the following observations.

(a) Naphthalene balls disappear with time without leaving any solid.
(b) We can get the smell of perfume sitting several metres away.

Solution:

• (a) Naphthalene undergoes sublimation, i.e., it changes directly from solid to vapour without passing through the liquid state.
• (b) Perfume is volatile and evaporates easily. Its vapour particles mix with air and spread by diffusion, allowing the smell to be detected metres away.

Q4. Arrange the following substances in increasing order of forces of attraction between the particles: water, sugar, oxygen

Solution:

• Oxygen (gas): weakest forces of attraction.
• Water (liquid): moderate forces of attraction.
• Sugar (solid): strongest forces of attraction.

Order: Oxygen < Water < Sugar

Q5. What is the physical state of water at—

(a) 25°C
(b) 0°C
(c) 100°C

Solution:

• (a) At 25°C → Liquid
• (b) At 0°C → Solid (ice) and liquid (water) can coexist.
• (c) At 100°C → Liquid (water) and gas (steam) can coexist.

Q6. Give two reasons to justify:

(a) Water at room temperature is a liquid.
(b) An iron almirah is a solid at room temperature.

Solution:

• (a) Water at room temperature is a liquid
  1. Room temperature (≈25°C) lies between 0°C and 100°C, the range in which water exists as a liquid.
  2. Water molecules are held together by hydrogen bonding, giving it fluidity but no fixed shape.
• (b) An iron almirah is a solid
  1. Iron has a definite shape and volume, with rigid particles closely packed.
  2. It requires very high temperature to change its state, so it remains solid at room temperature.

Q7. Why is ice at 273 K more effective in cooling than water at the same temperature?

Solution:
Ice at 273 K absorbs additional latent heat of fusion while melting, apart from heat absorbed to rise in temperature. This extra heat absorption makes ice more effective for cooling compared to water at 273 K.

Q8. What produces more severe burns, boiling water or steam?

Solution:
Steam produces more severe burns than boiling water because steam contains additional latent heat of vaporization. When steam condenses on skin, it releases this extra energy, causing deeper burns. 

Q9. Identify the changes of state in the following stages:

• Stage A: Solid → Liquid (Melting/Fusion)
• Stage B: Liquid → Gas (Vaporization/Boiling)
• Stage C: Gas → Liquid (Condensation)
• Stage D: Liquid → Solid (Freezing)
• Stage E & F: No change in physical state (temperature changes within the same state).