Q1. Convert the following temperature to the Celsius scale.

(a) 293 K

Answer: 293 K = (293 – 273)° Celsius = 20° Celsius

(b) 470 K

Answer: 470 K = (470 – 273 )° Celsius = 197° Celsius

Q2. Convert the following temperature to kelvin scale.

(a) 25° Celsius

Answer: 25° Celsius = (25 + 273) K = 298 K

(b) 373° Celsius

Answer: 373° Celsius = (373 + 273) K= 646 K

Q3. Give reason for the following observation :

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

Answer: Because at room temperature Naphthalene balls undergo sublimation and directly changes into vapour state without leaving any solid. It directly converted in gaseous form, from solid without undergoing the intermediate state i.e. liquid state.

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

Answer: Because perfume contains a volatile solvent which scatters quickly in the air, becoming less concentrated over a distance.

Q4. Arrange the following substances in increasing order of forces of attraction between the particles – Water, Sugar, Oxygen.

Answer: Oxygen (gas) < Water (liquid) < Sugar (solid).

Q5. What is the physical state of water at–

(a) 25° Celsius

Answer: At 25° Celsius water in its liquid form

(b) 0° Celsius

Answer: At 0° Celsius water is in both solid and liquid form because  0° Celsius is the freezing point of water.

(c) 100° Celsius

Answer: At 100° Celsius water is in both liquid and gaseous form (water vapour) because 100° Celsius is the boiling point of water.

Q6. Give two reason to justify–

(a) Water at room temperature is liquid.

Answer: The freezing point of water is 0° Celsius and Boiling point of water is 100° Celsius.

At room temperature water is in its liquid state because we observe-

• Water flows at this temperature
• It has a fixed volume that it takes the shape of its container

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

Answer: The melting point of iron is more than the room temperature. Melting point is about higher than 1538° C  and Boiling is  2862° C. Room temperature is about 20°-25° C.

Hence iron almirah is solid at room temperature because we observe

• It has definite shape and volume like a solid at room temperature
• It is rigid as solid at room temperature.

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

Answer: We know that when steam will be cooled it will change into water. Water cooling will be more in that case in which there is more removal of heat from the system.

Ice at 273 K will absorb heat energy or latent heat from the medium to overcome the fusion to become water. Water does not absorb this extra heat from the medium. Hence the cooling effect of ice is more than the water at the same temperature.

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

Answer: Steam (at 100° C) produces more severe burns because it is an exothermic reaction that releases a high amount of heat which it had consumed during vaporization. It posses additional latent heat energy.

 So, when steam falls on the skin and condense to produce water it gives out  22.5 * 105 J/Kg more heat than boiling water at the same temperature.

Q9. Name A, B, C, D, E and F in the following diagram showing the change in its state.

Answer:

A – Melting or Liquefaction or fusion

B – Evaporation or Vaporization

C – Condensation

D – Solidification

E – Sublimation

F – Sublimation