Evaporation, specific heat and latent heat — GCSE Physics practice questions

Question 1

During soldering, a small drop of molten solder on a circuit board cools and becomes a solid.

What is the name of this change of state?

Accepted Answer

D: solidification. The solder starts as a liquid and ends as a solid, so the change is solidification (freezing). Melting is the opposite change; condensation and evaporation involve a gas.

Question 2

A student puts 100 cm³ of water in each of four dishes in the same room. The dishes have different diameters. A fan blows air across three of the dishes, and one of these has a lid. The diagram shows the dishes.

From which dish does water evaporate fastest?

: Four dishes side by side, labelled P, Q, R and S from left to right, each holding the same volume of water. P is wide and shallow with still air. Q is narrow, so its water is deep; arrows labelled wind blow across it. R is wide and shallow with wind arrows across it. S is wide and shallow, covered by a lid, with wind arrows across the lid.

From which dish does water evaporate fastest?

Accepted Answer

C: R. The volumes are equal, so the wide dishes have the largest surface area of water. Of the wide dishes, R and S have air blown across them, but S has a lid, so the vapour cannot escape into the room. R has both the large surface and moving air, so its water evaporates fastest.

Question 3

A glass contains 0.40 kg of lemonade at 30 °C. A 0.050 kg ice cube at 0 °C is added and stirred in. Treat the lemonade as water.

specific heat capacity of water = 4200 J / (kg °C)
specific latent heat of fusion of ice = 3.3 × 10⁵ J / kg

Assume that no energy is exchanged with the glass or the surroundings.

What is the final temperature of the drink?

Accepted Answer

B: 18 °C. Cooling the lemonade all the way to 0 °C would release 0.40 × 4200 × 30 = 50 400 J, more than the 0.050 × 3.3 × 10⁵ = 16 500 J needed to melt the ice, so all the ice melts. Then 0.40 × 4200 × (30 − θ) = 16 500 + 0.050 × 4200 × θ, which gives 50 400 − 16 500 = 1890 θ and θ = 18 °C.