Three interchangeable states of water. Changes of state: melting, freezing, boiling, evaporation, condensation. Role of evaporation and condensation in the water cycle.
Part A — Multiple choice (Booklet A style · 2 marks each)
Question 1 — MCQ (2 marks)
Booklet AChanges 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 entire liquid at 100°C. A puddle disappearing on a warm day is evaporation, not boiling.
A student heats a block of ice. The graph below shows how the temperature changes over time. At which point on 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
During a change of state, temperature stays constant even though heat is being added — all the energy goes into breaking bonds between particles, not raising temperature. The flat section at 0°C (Q) = melting; the flat section at 100°C (R) = boiling.
On a cold morning, the inside of a car windscreen is covered with water droplets. A student says the water came from inside the car.
(a) Name the change of state that produced the water droplets. (1 mark)
(b) Explain, using your knowledge of the water cycle, how the water droplets formed on the windscreen. (2 marks)
(a)Condensation
(b)Water vapour is present in the air inside the car (from passengers breathing). When this warm, moist air comes into contact with the cold windscreen surface, it loses heat energy and cools down. The water vapour then changes state from gas to liquid, forming tiny water droplets on 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, and (2) what causes it to condense — the cold surface. "The air cooled" alone is insufficient; you must link the cold surface to the cooling of the water vapour.
The diagram below shows a model of the water cycle set up in a plastic container. The container holds water at the bottom. An ice pack is placed on top. 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 — heat energy from sunlight causes water molecules at the surface to change from liquid to water vapour (gas).
(b)The water vapour rises and comes into contact with the cold surface of the ice pack. It loses heat energy and cools, causing it to condense — 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 (oceans, lakes) / clouds form in the atmosphere rather than on a flat surface / rain falls over a large area / the water cycle involves much larger quantities of water / the real water cycle also involves runoff and ground absorption.
Evaporation ↑ · Condensation ↓ — the water cycle in miniature
For (b), don't just write "it condenses because it is cold." The full mark-earning answer must state: (1) water vapour rises and meets the cold surface, and (2) it loses heat / cools, and (3) therefore condenses into liquid droplets. Two of these three elements are needed for 2 marks.
A student investigates factors that affect the rate of evaporation. She sets up three dishes of water and measures how much water is left after 2 hours. The table shows her results.
All dishes started with 100 mL of water.
(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 higher temperature AND wind." Explain why comparing Dish 1 and Dish 3 alone cannot support this conclusion. (2 marks)
(d) Predict the amount of water remaining if a Dish 4 was set up at 25°C with wind present. Explain your prediction. (1 mark)
(a)Temperature (25°C vs 35°C). Wind was kept the same (no wind in both).
(b)Wind increases the rate of evaporation. Dish 3 (wind present) had less water remaining (41 mL) than Dish 2 (no wind) at the same temperature (35°C).
(c)Comparing Dish 1 and Dish 3 is not a fair test — two variables changed at the same time (both temperature and wind). It is impossible to tell which factor — temperature, wind, or both — caused the greater evaporation. To draw a valid conclusion about wind alone, only wind should be changed while temperature is kept the same.
(d)Between 62 mL and 85 mL — likely around 70–75 mL. At 25°C, evaporation is slower than at 35°C. Wind speeds up evaporation compared to Dish 1 (no wind, 85 mL), so Dish 4 should have less water than Dish 1 but more than Dish 2.
Higher temp → more evaporation | Wind → more evaporation
The "two variables changed" trap (c) is one of the most commonly tested scientific inquiry concepts in PSLE Booklet B. Whenever two things differ between two setups, no valid conclusion can be drawn about either factor alone. Always check: is only ONE thing different between the two setups being compared?
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