Science 7–10 · Year 10
Water cycle in a box: energy that lifts water and cold that returns it
Science understanding: Earth and space sciences (NSW Stage 5 focus area: Environmental sustainability)
The idea
Evaporation takes about 2.4 kJ from the water for every gram it lifts into the air and condensation gives it back, so the water cycle is also one of the main ways energy moves from the ocean to the atmosphere.
Safety card
Hazards
- mains lamp near water
- hot lamp
Controls
- lamp stays outside the box on a stand, plugged into a power point with residual current protection
- wipe up water before touching the lamp; let the lamp cool before moving it
Note
No hazardous chemical and no flame or heating apparatus: a generic classroom risk assessment (CSIS 1.7 or RiskAssess) covers trips, spills, warm lamps and sharp edges.
What you need
- 2 clear plastic boxes or small aquariums with lids, about 30 cm x 20 cm x 20 cm (the UCAR water cycle model)
- 2 shallow dishes each holding 200 g of water at about 35 degrees C, weighed to 0.1 g, and a clay slope in each box (the UCAR mountain)
- 2 Petri dishes of crushed ice to sit on each lid above the slope
- 1 lamp (60 to 100 W) outside one box, shining in, as the Sun
- a balance reading to 0.1 g and a thermometer in each box
How to do it
- Weigh each dish of warm water, place it in its box beside the clay slope, close the lids and put the ice dishes on top.
- Switch on the lamp over box 1 only and run both boxes for 30 minutes.
- Record what forms under the ice on each lid and where the drops fall.
- Remove and re-weigh each dish; the loss is the mass evaporated (drops that fall on the slope are outside the dish).
- Calculate the energy the evaporated water carried away: mass in grams x 2.42 kJ per gram (the NIST value at 35 degrees C, the water's starting temperature).
- Repeat three times and compare the lamp and no-lamp means.
What you should see
Box 1 fogs under the ice within minutes and drops run down the slope; the lamp dish loses more mass than the dark-box dish. Each gram evaporated took about 2.42 kJ from the water (NIST: enthalpy of vaporisation 2,417.9 kJ/kg at 35 degrees C, rising to 2,441.7 kJ/kg at 25 degrees C and 2,453.5 kJ/kg at 20 degrees C as the water cools) and released about the same where it condensed on the cold lid; so if the lamp dish loses 5.0 g, about 12.1 kJ has moved from the water to the air and lid (12.3 kJ if the water had cooled to 20 degrees C). A 60 W lamp supplies 108 kJ in 30 minutes, only part of which reaches the water. The learner knows it worked when the lamp box loses more mass than the dark box in all three runs.
What changes
- What you change
- energy input (lamp on or off)
- What you measure
- mass of water evaporated in 30 minutes
- What you keep the same
- starting mass and temperature of the water
- box size
- amount of ice
- time
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- Water only evaporates when it boils; it evaporates at any temperature, faster when warmer.
- Clouds are made of water vapour; they are tiny liquid droplets or ice crystals formed where vapour condenses or freezes.
- The water cycle moves only water; it also carries energy from the surface into the air.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
- Science 7-10 Syllabus (2023), NSW Education Standards Authority (implemented from 2026; codes read at curriculum.nsw.edu.au on 22 and 23 September 2026)SC5-ENV-01SC5-WS-03SC5-WS-04
- Australian Curriculum v9AC9S10U04AC9S10I03AC9S10I04
Sources
The pages the author read to write this activity.