Physics Past Paper PDF 2023-24

Q uestions 1. Convert the following temperature to celsius scale: a. 300 K b. 573 K 2. What is the physical state of water at: a. 250oC b. 100oC ? 3. For any substance, why does the temperature remain constant during the change of state? 4. Suggest a method to liquefy atmospheric gases. 1.5 Evaporat...

Q uestions 1. Convert the following temperature to celsius scale: a. 300 K b. 573 K 2. What is the physical state of water at: a. 250oC b. 100oC ? 3. For any substance, why does the temperature remain constant during the change of state? 4. Suggest a method to liquefy atmospheric gases. 1.5 Evaporation Do we always need to heat or change pressure for changing the state of matter? Can you quote some examples from everyday life where change of state from liquid to vapour takes place without the liquid reaching the boiling point? Water, when left uncovered, slowly changes into vapour. Wet clothes dry up. What happens to water in the above two examples? We know that particles of matter are always moving and are never at rest. At a given temperature in any gas, liquid or solid, there are particles with different amounts of kinetic energy. In the case of liquids, a small fraction of particles at the surface, having higher kinetic energy, is able to break away from the forces of attraction of other particles and gets converted into vapour. This phenomenon of change of liquid into vapours at any temperature below its boiling point is called evaporation. 1.5.1 FACTORS AFFECTING EVAPORATION Let us understand this with an activity. You must have observed that the rate of evaporation increases with– an increase of surface area: We know that evaporation is a surface phenomenon. If the surface area is increased, the rate of evaporation increases. For example, while putting clothes for drying up we spread them out. an increase of temperature: With the increase of temperature, more number of particles get enough kinetic energy to go into the vapour state. a decrease in humidity: Humidity is the amount of water vapour present in air. The air around us cannot hold more than a definite amount of water vapour at a given temperature. If the amount of water in air is already high, the rate of evaporation decreases. an increase in wind speed: It is a common observation that clothes dry faster on a windy day. With the increase in wind speed, the particles of water vapour move away with the wind, decreasing the amount of water vapour in the surrounding. 1.5.2 HOW M AT T E R Take 5 mL of water in a test tube and keep it near a window or under a fan. Take 5 mL of water in an open china dish and keep it near a window or under a fan. Take 5 mL of water in an open china IN DOES EVAPORATION CAUSE COOLING? Activity _____________ 1.14 dish and keep it inside a cupboard or on a shelf in your class. Record the room temperature. Record the time or days taken for the evaporation process in the above cases. Repeat the above three steps of activity on a rainy day and record your observations. What do you infer about the effect of temperature, surface area and wind velocity (speed) on evaporation? In an open vessel, the liquid keeps on evaporating. The particles of liquid absorb energy from the surrounding to regain the energy lost during evaporation. This absorption of energy from the surroundings make the surroundings cold. 9 O U R S U R R O U N D I N GS Rationalised 2023-24