## Topic outline

• ### Matter in Our Surroundings

#### Q73. What is matter? Give examples.

Ans. Everything in this universe is made up of material which scientists have named “matter”. The air we breathe, the food we eat, stones, clouds, stars, plants and animals, even a small drop of water or a particle of sand– each thing is matter.

#### Q74. What is the relation between Kelvin scale and Celsius scale of temperature?

Ans. Kelvin is the SI unit of temperature, 0°C C=273.16 K. To change a temperature on the Kelvin scale to the Celsius scale we have to subtract 273 from the given temperature, and to convert a temperature on the Celsius scale to the Kelvin scale you have to add 273 to the given temperature.

#### Q75. Why solid carbon dioxide is also known as dry ice?

Ans. Solid carbon dioxide is stored under high pressure. Solid CO2 gets converted directly to gaseous state on decrease of pressure to 1 atmosphere without coming into liquid state. This is the reason that solid carbon dioxide is also known as dry ice.

#### Q76. Why do gases have neither a fixed shape nor a fixed volume?

Ans. The forces of attraction between the particles are minimum in gases. In the gaseous state, the particles move about randomly at high speed. As a result, a gas does not have a definite shape or a definite volume. It will assume the shape and the volume of the container in which it is placed.

#### Q77. When a crystal of copper sulphate is placed at the bottom of a beaker containing water, the water slowly turns blue even without stirring. Why?

Ans. When a crystal of Copper Sulphate is placed in a beaker of water, the water slowly turns blue on its own, even without stirring because particles of matter intermix on their own with each other. They do so by getting into the spaces between the particles.

#### Q78. What is the physical state of water at -(a) 25°C (b) 0°C (c) 100°C

Ans. (a) At 25°C, water exists in the liquid state.

(b) At 0°C, water can exists as both solid and liquid. At this temperature, ice starts melting to form liquid water.

(c) 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.

#### Q79. How does perspiration or sweating help keep our body cool on a hot day?OrWhy do we feel cool after sweating?OrExplain how sweating helps to cool the body.

Ans. During summer, we perspire more because of the mechanism of our body which keeps us cool. We know that during evaporation, the particles at the surface of the liquid gain energy from the surroundings or body surface and change into vapour. The heat energy equal to the latent heat of vaporisation is absorbed from the body leaving the body cool.

#### Q80. For any substance, why does the temperature remain constant during the change of state?

Ans. The heat which is going into the substance but not increasing its temperature, is the energy required to change the state of the substance from solid to liquid. This is known as the latent heat. This heat energy is used up in changing the state of the substance by overcoming the force of attraction between its particles.

#### Q81. How can the evaporation of a liquid be made faster?OrHow can we increase the rate of evaporation?

Ans. Evaporation of a liquid can be made faster by:

a. Increasing the temperature.

b. Increasing the surface area of the liquid.

c. Lowering humidity.

d. Increasing wind speed.

#### Q82. What are the factors affecting evaporation?

Ans. Factors affecting evaporation are

a. temperature

b. surface area of the liquid

c. humidity

d. wind speed