Explain how sweat evaporation removes heat
Connect sweat glands, the skin surface and conditions affecting evaporation.
Section B13.3 · Homeostasis
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Recall hormone production and blood transport, respiration, liver storage and electrical impulses along neurones.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What change of state is central to cooling?
What you will learn
- Trace sweat to the surface
- Explain evaporation and humidity
Explain how sweat evaporation removes heat
| Feature | Meaning |
|---|---|
| Delivery | Sweat gland → duct → surface pore |
| Change of state | Liquid water → water vapour |
| Energy source | Heat from skin and body |
| Humidity | Can limit net evaporation and cooling |
Trace sweat to the surface
When body temperature rises, sweat glands can secrete more sweat through ducts to pores on the skin surface. The secreted liquid contains water. Secretion puts liquid at the surface, but the important cooling step is its evaporation: liquid water changes into water vapour and transfers to the surrounding air.
Evaporation requires energy, taken as heat from the skin and nearby body tissues. This removes heat from the body and helps lower temperature towards its regulated level. Sweating therefore needs more than the phrase water comes out: name evaporation and the source of the energy used in the change of state.
Explain evaporation and humidity
Humid air already contains more water vapour, reducing the water-vapour gradient under otherwise similar conditions and limiting net evaporation. Liquid dripping away is not the same evaporation process. The amount secreted does not automatically equal the amount evaporated or establish an exact cooling rate. Air movement and environmental conditions can also influence transfer.
Use a safe non-living demonstration if comparing evaporation, such as equal model surfaces with controlled starting temperatures, exposed area and air conditions. Do not induce overheating or dehydration in people. The syllabus explanation concerns the cooling mechanism; it does not provide a medical hydration or temperature-management plan.
Sources for this lesson
Pause and explain
Why can humidity limit evaporative cooling?
Worked example
Explain why evaporation, rather than secretion alone, cools skin.
Show the worked solution
- Sweat reaches the surface and its water evaporates.
- The evaporation energy comes from skin/body heat.
Answer Sweat water evaporates using heat from the skin and body, removing heat and helping lower body temperature.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Explain how sweating contributes to cooling.
Give me a hint
Include gland, surface, evaporation and energy source.
Compare my reasoning
- Glands deliver sweat to the skin surface.
- Water evaporates using heat from skin/body tissues.
Sweat reaches the surface; its water evaporates using heat from the skin and body, removing heat and helping lower temperature.
Look for these in your work
- Trace delivery to surface
- Explain energy transfer in evaporation
Compare otherwise similar sweat-covered surfaces in dry and humid air.
Give me a hint
Consider the water-vapour gradient.
Compare my reasoning
- Humid air reduces the gradient for net evaporation.
- Dry air can allow more evaporation and associated heat removal.
The humid condition can limit net evaporation and cooling compared with the otherwise similar dry condition.
Look for these in your work
- Link humidity to evaporation
- Keep other conditions similar
Design a safe model comparison that isolates air humidity rather than overheating learners.
Give me a hint
Use non-living surfaces and matched variables.
Compare my reasoning
- Use equal non-living wet surfaces with matched starting temperature and area.
- Keep air movement and other conditions matched while varying humidity.
Compare equal non-living wet surfaces at matched starting temperature, area and air movement, varying humidity and recording temperature change; do not induce human overheating.
Look for these in your work
- Control relevant variables
- Use a safe non-living model
Common mistakes
- Calling secretion alone the full cooling explanation.
- Equating dripping liquid with evaporated water.
What can you explain now?
Why can humidity limit evaporative cooling?
Compare with the explanation
It reduces net evaporation under similar conditions
A smaller water-vapour gradient limits transfer into air.
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Make it stick
Explain the evaporation mechanism and evaluate why secretion alone does not measure cooling.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.