Water Cycle & States of Matter

Three interchangeable states of water. Changes of state: melting, freezing, boiling, evaporation, condensation. Role of evaporation and condensation in the water cycle.

evaporationcondensationmelting & freezingwater cycle

P5 Standard Science · Theme: Cycles · Topic: Cycles in Matter and Water (Water)

Changes of state — key facts

MeltingSolid → Liquid at 0°C

FreezingLiquid → Solid at 0°C

EvaporationLiquid → Gas (any temp)

BoilingLiquid → Gas rapidly at 100°C

CondensationGas → Liquid (cooling)

Water cycle roleEvaporation ↑ · Condensation ↓

Part A — Multiple choice (Booklet A style · 2 marks each)

Question 1 — MCQ (2 marks) Booklet A Changes of state

A puddle of water disappears on a warm sunny day. Which change of state best describes this?

ACondensation — water vapour turns into liquid water

BEvaporation — liquid water turns into water vapour

CMelting — ice turns into liquid water

DFreezing — liquid water turns into ice

Evaporation occurs at the surface of a liquid at any temperature — not just at 100°C. Boiling is rapid evaporation throughout the liquid at 100°C. A puddle disappearing on a warm day is evaporation, not boiling.

Question 2 — MCQ (2 marks) Booklet A Heat gain · change of state

A student heats a block of ice steadily. The graph below shows how the temperature of its contents changes over time. During which part of the graph is the ice melting?

AP — temperature is rising from −10°C

BQ — temperature stays constant at 0°C

CR — temperature stays constant at 100°C

DS — temperature is rising above 100°C

The melting point of ice is 0°C and the boiling point of water is 100°C. While a change of state is taking place, the temperature stays constant even though heat is still being gained — the heat gained is used to change the state rather than to raise the temperature. The flat part at 0°C (Q) is melting; the flat part at 100°C (R) is boiling.

Part B — Structured questions (Booklet B style)

Question 3 — Structured (3 marks) Booklet B Condensation · everyday context

On a cold morning, the inside of a car windscreen is covered with water droplets.

(a) Name the change of state that produced the water droplets. (1 mark)

(b) A student says that the water came from the air inside the car. Do you agree? Explain how the water droplets formed on the windscreen. (2 marks)

(a)Condensation

(b)Yes, I agree. There is water vapour in the air inside the car (for example, from the passengers breathing out). When this water vapour comes into contact with the cold windscreen, it loses heat and cools. It then changes from a gas to a liquid, forming water droplets on the inside of the glass.

Water vapour (gas) → cools on cold surface → liquid water droplets

The 2-mark explanation needs two things: (1) a source of water vapour in the air inside the car, and (2) the cold windscreen causing that water vapour to lose heat and condense. "The air cooled" on its own is not enough — you must link the cold surface to the cooling of the water vapour.

Question 4 — Structured (4 marks) Booklet B Water cycle · evaluating a model

The diagram below shows a model of the water cycle set up in a closed plastic container. The container holds water at the bottom. An ice pack is placed at the top of the container. The container is placed near a window in sunlight.

(a) What process causes water vapour to rise from the water surface? (1 mark)

(b) Explain why water droplets form on the underside of the ice pack. (2 marks)

(c) This model represents the water cycle. Identify ONE way in which this model is different from the actual water cycle on Earth. (1 mark)

(a)Evaporation — the water at the surface gains heat from the sunlight and changes from liquid water to water vapour (gas).

(b)The water vapour rises and comes into contact with the cold surface of the ice pack. It loses heat and cools, so it condenses — changing from water vapour (gas) to liquid water droplets.

(c) One differenceAccept any one: in the real water cycle the Sun heats large bodies of water such as seas and lakes / the water vapour condenses on cold air high in the sky instead of on a cold pack / the water falls as rain over a very large area / much larger amounts of water are involved.

Evaporation ↑ · Condensation ↓ — the water cycle in miniature

For (b), don't just write "it condenses because it is cold." The two marks are awarded for two separate ideas: (1) the water vapour rises and meets the cold surface of the ice pack, and (2) it loses heat and cools, so it condenses into liquid water droplets. Both ideas must appear.

Question 5 — Structured (5 marks) Booklet B Rate of evaporation · fair test

A student investigates factors that affect the rate of evaporation. She sets up three identical dishes, each containing 100 mL of water, and measures how much water is left in each dish after 2 hours. The table shows her results.

(a) What is the changed variable between Dish 1 and Dish 2? (1 mark)

(b) State the conclusion that can be drawn from comparing Dish 2 and Dish 3. (1 mark)

(c) A student says: "Dish 3 had the most evaporation because it had both a higher temperature AND wind." Explain why comparing Dish 1 and Dish 3 alone cannot support this conclusion. (2 marks)

(d) Dish 4 is set up in the same way at 25°C with wind present. State, with a reason, how the amount of water remaining in Dish 4 would compare with that in Dish 1. (1 mark)

(a)Temperature (25°C compared with 35°C). Wind was kept the same (no wind in both dishes).

(b)Wind increases the rate of evaporation. At the same temperature of 35°C, Dish 3 (wind present) had less water remaining (41 mL) than Dish 2 (no wind, 62 mL).

(c)Comparing Dish 1 and Dish 3 is not a fair test — two variables were changed at the same time (both temperature and wind). It is not possible to tell whether the temperature, the wind, or both caused the greater evaporation. To draw a valid conclusion about wind alone, only the wind should be changed while the temperature is kept the same.

(d)Dish 4 will have less water remaining than Dish 1 (less than 85 mL). Only the wind is different between Dish 1 and Dish 4, and wind increases the rate of evaporation, so more water will evaporate from Dish 4.

Higher temp → more evaporation | Wind → more evaporation

The "two variables changed" trap (c) is one of the most commonly tested scientific inquiry ideas in Booklet B. Whenever two things differ between two set-ups, no valid conclusion can be drawn about either factor on its own. Always check: is only ONE thing different between the two set-ups being compared?

Why (d) compares Dish 4 with Dish 1, not Dish 2. Dish 1 and Dish 4 differ in wind only, so a firm prediction can be made. Dish 2 and Dish 4 differ in both temperature and wind, so the results cannot tell you which of the two would have more water left.