Classify the phase‑change scenarios according to the direction of the heat flow in each case.

Question

1. Heat is transferred from the water to the surroundings
2. Heat is transferred to the water from the surroundings
3. No net transfer of heat occurs

1. a cloud of steam condenses into liquid water
2. a block of ice sublimates into water vapor
3. a block of ice melts
4. a puddle of water evaporates
5. a puddle of water freezes into ice

Answer 1

When working through these problems, think about the heat in terms of movement of the molecules of water.

1. Heat is transferred from the water to the surroundings --- water molecules slowing down.
2. Heat is transferred to the water from the surroundings --- water molecules heating up.

1. a cloud of steam condenses into liquid water - Heat is transferred from the water to the surroundings. Steam has more molecules moving around than liquid water. Therefore the molecules had to slow down, thereby transferring heat to the surroundings.
2. a block of ice sublimates into water vapor - Heat is transferred to the water from the surroundings --- water molecules are heating up
3. a block of ice melts - Heat is transferred to the water from the surroundings. The water molecules are basically not moving. Therefore heat has to be introduced to the molecules, which causes the melting.
4. a puddle of water evaporates - Heat is transferred to the water from the surroundings. The water molecules are moving in the liquid state. However, when more heat is introduced to the liquid, the molecules move more rapidly and evaporate.
5. a puddle of water freezes into ice - Heat is transferred from the water to the surroundings --- water molecules slowing down as they go from a liquid state to a solid state. .