iGCSE Physics (0625) 2.3.4 Consequences of Thermal energy transfer Paper 3 -Exam Style Questions- New Syllabus

(a)(i)
For the correct answer:
Evaporation.

The process responsible for the puddles disappearing over the course of the school day is evaporation. This is the gradual conversion of a liquid into a gas or vapour that occurs at the surface of the liquid at temperatures below its boiling point. Unlike boiling, which takes place throughout the entire body of liquid at a fixed temperature, evaporation is a slow and continuous surface process that can happen at any temperature. Because the air is not saturated with water vapour and there is warmth from sunlight, the water molecules at the puddle’s surface are constantly escaping into the surrounding air, and over several hours the entire puddle disappears in this way.

(a)(ii)
For the correct answer:
At the surface, the more energetic molecules escape from the liquid by overcoming the intermolecular forces holding them in the liquid, turning from liquid into gas/vapour.

Within any liquid, the molecules are in constant random motion but do not all possess the same kinetic energy — there is a continuous distribution of speeds. At the surface of the puddle, some molecules happen to be moving fast enough in an upward direction to overcome the attractive intermolecular forces that bind them to the rest of the liquid. These high-energy molecules break free and escape into the air above as water vapour. Because it is specifically the most energetic molecules that leave, the average kinetic energy of the remaining molecules in the puddle decreases, which is why evaporation also causes the liquid to cool. Over time, more and more molecules escape in this manner until the puddle has completely evaporated.

(b)
For the correct answer:
Suggestion 1: Surround the container with an insulating material such as foam, lagging, or cotton wool — this reduces heat loss by conduction.
Suggestion 2: Add a lid to the top of the container — this reduces heat loss by convection and evaporation.

The walls of a simple cardboard cup allow thermal energy to conduct outward directly through the material into the cooler surroundings. Wrapping the container in a thick layer of insulating material such as foam or cotton wool introduces a poor conductor between the hot liquid and the outside air, greatly reducing the rate of heat loss by conduction. Additionally, the open top of the container allows hot air and steam to rise and escape freely — this is convection — and also permits evaporation of the hot liquid from its surface, both of which carry significant amounts of thermal energy away. Fitting a well-insulated lid seals the top, trapping the warm air and preventing both convective heat loss and evaporative cooling simultaneously, making it the second highly effective improvement to the design.