Class 9 - Chemistry - Matter In Our Surroundings

Question 1:

Convert the following temperatures to the Celsius scale.

(a) 300 K                  (b) 573 K

Answer:

Using the formula: - T (°C) = T (K) - 273.15

• 300K = (300 - 273)

= 270C

• 573 K = ( 573 – 273)

= 3000C

Question 2:

Convert the following temperatures to the Kelvin scale.

(a) 25°C                        (b) 373°C

Answer:

Using the formula: - T (°C) = T (K) - 273.15

Kelvin is an SI unit of temperature, where 00C =273.16K (approx. 273K)

• 250C = (25 + 273)

= 298K

• 3730C = ( 373 + 273)

= 646K

Question 3:

Give 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.

Answer:

• Naphthalene balls disappear with time without leaving any solid, because naphthalene balls exhibit the property of sublimation.
• That is, it changes from solid state to gas easily. So evaporation of naphthalene balls takes place directly to the vapour state leaving no solid.
• We can get the smell of perfume sitting several metres away because perfume contain volatile solvent
• which vaporises very fast and its vapours diffuse faster into air. As a result the smell of perfume can reach people sitting several metres away.

Question 4:

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

Answer:

As sugar is a solid therefore the force of attraction between the particles is very strong.

Water is a liquid therefore the force of attraction are weaker as compared to solids (sugar).

Oxygen is a gas therefore the force of attraction between the particles is weakest.

Therefore the increasing order of attraction between the particles will be:-

Oxygen < Water < Sugar

Question 5:

What is the physical state of water at —?

(a) 25°C            (b) 0°C                   (c) 100°C

Answer:

• Water at 250C is in liquid state.
• At 00C water can exist as both solid and liquid.
• At this temperature when the heat becomes equal to latent heat of fusion,
• the solid form of water changes to latent heat of vaporisation; i.e. ice (solid) changes to water (liquid).

Question 6:

Give two reasons to justify—

(a) Water at room temperature is a liquid.

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

Answer:

• Water at room temperature is a liquid as water has no shape but has definite volume that is why it occupies the shape of the container.
• At room temperature it flows because of its fluidity property.
• An iron Almirah is a solid at room temperature as it is a solid at room temperature because of which it is rigid and has definite shape.

Question 7:

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

Answer:

Ice at 273K has less energy than water even though both are at the same temperature.

Water possesses the additional latent heat of fusion at the same temperature in the form of extra energy.

Question 8:

What produces more severe burns, boiling water or steam?

Answer:

Steam produces more severe burns than boiling water because steam has more energy than boiling water, in the form of latent heat of vaporization.

Question 9:

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