Science and Technology K–6 · Year 4

Where does the water on a cold can come from? Finding the dew point

Science understanding: Earth and space sciences

Practical, model not builtLow risk

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The idea

Air holds invisible water vapour, and when it touches a surface cold enough it condenses into drops, at a temperature called the dew point that a learner can measure with a can, ice and a thermometer.

What you need

1 shiny metal can, dry on the outside
tap water at room temperature, 150 mL
ice cubes
digital thermometer
kitchen balance reading to 0.1 g if available
a dry cloth

How to do it

Weigh the dry, empty can. Half-fill it with room-temperature water and weigh again.
Stand the thermometer in the water. Add one ice cube at a time and stir, watching the outside of the can.
The moment a mist of tiny drops appears on the outside, read the water temperature: that is the dew point of the air in the room.
Once the can is misted, add enough ice to keep it cold and weigh it at once. Then add nothing more, wait 5 minutes and weigh it again with its drops (do not wipe). The gain between these two weighings is water that came from the air.
Record the room temperature and, if there is a hygrometer, the humidity.
Repeat on a humid day and a dry day and compare the dew points.

What you should see

Drops appear when the can drops to the dew point, which in a 25 degree room at 50 percent humidity is 13.9 degrees Celsius; on a humid day the dew point is higher and drops appear sooner. Between the two weighings the can gains a small mass, a fraction of a gram, so a balance reading to 0.1 g is needed; it is water that was in the air and was not in the can. The nearer the dew point is to the room temperature, the more humid the air.

What changes

What you change: the day's humidity
What you measure: the dew point temperature
What you keep the same: can
stirring
starting water

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

The can is leaking or sweating.
Cold makes water (the water was already in the air as vapour).
Dry air has no water in it at all.

Safety card

Low riskLearners carry it out

Hazards

sharp can rim

Controls

tape the rim
wipe spills

Note

Risk assessment before the lesson using Primary RiskAssess or the school's own template.

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 and Technology K-6 Syllabus (2017), current, taught until 2026; the 2017 syllabus has no water-cycle content; evaporation and condensation fall under this Stage 2 changes-of-state outcome ('recognise that a change of state can be caused by adding or removing heat'); code read from the syllabus document on 22 September 2026ST2-6MW-S
Science and Technology K-6 Syllabus (2017), current, taught until 2026; code read from the syllabus document on 22 September 2026ST2-1WS-S
Science and Technology K-6 Syllabus (2024), implemented from 2027; NESA's timeline is 2026 plan and prepare and 2027 start teaching, and schools may choose to implement it during 2026; code read from the outcomes page on 22 September 2026ST2-DAT-01
Australian Curriculum v9AC9S4U02AC9S4H01AC9S4I03

Sources

The pages the author read to write this activity.

All Concept Studio activities