Question

1. A cold glass is left sitting outside on a hot day. Soon, water droplets form on the outside of the glass. Describe the events that must occur for the water droplets to form on the glass.
2. A glass of water sitting in direct sunlight evaporates over time. Explain this phase change in terms of the types of heat transfer that occur.

Answer 1

1. As long as the glass is colder than the water vapor's dew point, it will condense to a liquid form that forms on the outside of the glass and since surface of the glass is a lot cooler than the surrounding air. Warm moist air carries invisible molecules of water. When air gets colder it compresses (and if air gets hotter it expands, its volume increases). As the air cools, at a certain point the water molecules are unable to stay in the air as invisible 'gas' (water vapour), and they change their state from gas to liquid. 'Liquid' is a more condensed form than a gas. The molecules are closer together in liquids than they are in their gas form/state. If warm moist air meets the side of an icy glass, the water vapour in that bit of air 'condenses' into a liquid (water) on the side of the glass.

2. During evaporation, energetic molecule leaves liquid phase lowering average energy of other liquid molecules. Remaining to absorb energy from surrounding getting enough energy to enter into the gas phase. Evaporation depends on a number of factors like surface area, intermolecular attractions and kinetic energy of particles. Since the kinetic energy of a molecule is proportional to its temperature, evaporation proceeds more quickly at higher temperatures. As the faster-moving molecules escape, the remaining molecules have lower average kinetic energy, and the temperature of the liquid, thus, decreases.

Answer 2

Answer:

For 1: The air outside the cold glass is hot and it gets converted into water droplets due to the process of condensation.

For 2: The phase change occurs from liquid state to gaseous state and heat is transferred by the process of radiation.

Explanation:

• For 1:

When a cold glass is left outside on a hot day, the air outside the glass is hot and when this hot air comes in contact with the cold glass, it starts to form water droplets by the process of condensation.

Condensation is a process in which gaseous state starts to convert into liquid state when temperature is decreased.

$Gas\rightleftharpoons Liquid$

• For 2:

When a glass is left sitting in the direct sunlight and the water (liquid phase) on the surface evaporates into gaseous phase.

$Liquid\rightleftharpoons Gas$

Here, heat is transferred by the process of radiation which is defied as the process in which heat is transferred whenever there is direct transfer of heat through space. Here, sunlight is transferred from the Sun to the glass on Earth.