Science 7–10 · Year 8

Leaf surface temperature: transpiration as evaporative cooling

Science understanding: Biological sciences

PracticalLow risk

The idea

Evaporation removes heat from the surface it leaves, so a transpiring leaf runs cooler than one whose stomata are sealed, the same physics that makes sweating cool skin.

What you need

A potted plant with several similar leaves still attached (the SAPS investigation uses living plants with leaves on the stem)
Hand-held infrared thermometer without a laser pointer (the SAPS notes say laser-pointer models are not recommended for schools and should not be used)
Petroleum jelly and a cotton bud
A lamp or a sunny spot, a notebook for paired readings

How to do it

Choose six similar leaves at the same height, facing the same way. Read the upper surface temperature of each with the infrared thermometer held 1 cm from the leaf at 90 degrees, as the SAPS notes advise, and record it.
Coat the whole lower surface of three of the leaves with petroleum jelly to seal the stomata; leave the other three untreated.
Place the plant under the lamp or in the sun. After 20 minutes read the upper surface of all six leaves again, taking three readings per leaf.
Calculate the mean temperature of treated and untreated leaves and the difference.
Repeat the readings at 40 minutes.
Compare with the class: pool all treated and untreated means.

What you should see

Sealing the lower surface stops evaporation through the stomata, so the sealed leaves are expected to warm relative to the untreated leaves. In the SAPS sample data from 31 Salvia leaves, the mean upper-surface temperature rose from 13.2 °C to 16.8 °C after the lower surface was sealed, a mean rise of 3.6 °C (standard deviation 2.8 °C), and 2 of the 31 leaves read cooler. Any change in the untreated leaves shows how much of the rise comes from time under the lamp rather than sealing, and pooling the class towards the 30 or more leaves SAPS recommends gives a reliable mean. The learner knows it worked when the three readings on a leaf agree closely and the pooled class difference has the same sign as their own.

What changes

What you change: Whether the lower surface is sealed with petroleum jelly
What you measure: Upper leaf surface temperature (°C)
What you keep the same: Same plant
Leaf size and position
Light source and distance
Time under the lamp
Thermometer distance and angle

Common misconceptions

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

Leaves are the same temperature as the air.
Water loss is a flaw in plant design; it also cools the leaf and drives transport.
Sweat cools because it is cold; it cools by evaporating.

Safety card

Low riskLearners carry it out

Hazards

Hot lamp
Plant allergies or irritant sap

Controls

Lamp on a stand; do not touch the globe
Check for allergies first; gloves for anyone who reacts to plants; avoid plants with stings or prickles

Note

No hazardous chemicals; record a RiskAssess risk assessment for the activity as school procedure requires.

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)SC4-LIV-01 SC4-WS-03 SC4-WS-06
Australian Curriculum v9AC9S8U02AC9S8I02AC9S8I05

Sources

The pages the author read to write this activity.

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