## Topic outline

• ### Matter in Our Surroundings

#### Q49. A diver is able to cut through water in a swimming pool. Which property of matter does this observation show?

Ans. A diver is able to cut through water in a swimming pool. This shows that the particles (or molecules) in water are not tightly packed, they are somewhat loose, having spaces between them.

#### Q50. Liquids generally have lower density as compared to solids. But you must have observed that ice floats on water. Find out why?

Ans. Density of ice is less than density of water. The low density of ice is due to its structure.  The molecules in ice make a cage like structure with lot of vacant spaces; this makes ice float on water.

#### Q51. Why is honey more viscous than water?

Ans. The force of attraction between the particles of honey is more than the force of attraction between the particles of water. The particles are strongly bonded in honey than water. Thus, honey is more viscous than water.

#### Q52. What is meant by ‘diffusion’? Give one example of diffusion in gases.

Ans. Intermixing of particles of two different types of matter on their own is called diffusion. Example: When perfume is sprayed in one part of a room, it spreads to the rest through diffusion.

#### Q53. What does the diffusion of gases tell us about their particles?

Ans. The forces of attraction between the particles are minimum in gases and the particles move about randomly at high speed. Due to the movement of particles with high speed gas diffuses most rapidly.

#### Q54. When salt is dissolved in water, there is no increase in the volume. Which characteristic of matter is illustrated by this observation?OrWhy does the level of water not change when salt is dissolved?

Ans. When we dissolve salt in water, the particles of salt get into the spaces between particles of water. Hence, there is no increase in the volume. This shows that there is enough space between particles of matter.

#### Q55. What is the physical state of water at: a. 250ºC b. 100ºC?

Ans. (a) At 250°C, water exists in the gaseous state.

(b) At 100°C, water can exists as both liquid and gas. At this temperature, water starts changing from its liquid states to its gaseous (vapour) state.

#### Q56. Why does the temperature remain constant during the melting of ice even though heat is supplied continuously?OrWhy does temperature remain constant during melting of ice?

Ans. The temperature remain constant during the melting of ice even though heat is supplied continuously because this heat gets used up in changing the state by overcoming the forces of attraction between the particles.

#### Q57. Which contains more heat energy, water at 100°C or steam at 100°C?

Ans. Particles in steam, that is, water vapour at 373 K (100°C) have more energy than water at the same temperature. This is because particles in steam have absorbed extra energy in the form of latent heat of vaporisation.

#### Q58. Name the three states of matter. Give one example of each.

Ans. Matter around us exists in three different states–

a. solid - chalk

b. liquid - water

c. gas - air

#### Q59. Just a few crystals of potassium permanganate can colour a large volume of water (about 1000 L). Which characteristic of particles of matter is illustrated by this observation?

Ans. This shows that there must be millions of tiny particles in just one crystal of potassium permanganate, which keep on dividing themselves into smaller and smaller particles. Ultimately a stage is reached when the particles cannot divide further into smaller particles.

#### Q60. Why does the temperature remain constant during the boiling of water even though heat is supplied continuously?OrWhy does temperature remain constant during boiling of water?

Ans. The temperature remain constant during the boiling of water even though heat is supplied continuously because this heat gets used up in changing the state by overcoming the forces of attraction between the particles.