To turn a liquid into a gas, energy is needed to pull its particles apart, even though the temperature does not rise during the change. This energy is the specific latent heat of vaporisation.
When sweat evaporates from your skin, it must draw this latent heat from somewhere, and the nearest source is your body. As the sweat carries that heat away as vapour, your skin loses energy and feels cooler. This is why a fan helps: moving air removes the humid layer and lets more sweat evaporate.
In Malaysia's humid weather the air already holds a lot of water vapour, so sweat evaporates slowly and cooling is less effective, which is why humid days feel so much more uncomfortable than dry ones at the same temperature.
In SPM you should explain evaporative cooling using latent heat, and note that the temperature stays constant while a substance changes state.
Common misconceptions
- Sweat cools you simply because it is a cool liquid -> The cooling comes from evaporation taking latent heat away, not from the sweat's own temperature.
- Temperature rises while sweat is boiling off -> During a change of state the temperature stays constant; the heat goes into the change.
- Sweating works just as well in humid air -> Humid air slows evaporation, so cooling is less effective on humid days.
The physics behind it
Turning a liquid into vapour without changing its temperature still costs energy, called the specific latent heat of vaporisation, L. The heat needed is Q = mL, where Q is in joules (J), m is the mass evaporated in kilograms (kg), and L is in joules per kilogram (J kg⁻¹). For water, L is about 2.26 × 10⁶ J kg⁻¹.
Suppose 10 g of sweat, that is 0.01 kg, evaporates from your skin. The heat it must absorb is Q = mL = 0.01 kg × 2.26 × 10⁶ J kg⁻¹ = 22600 J. That energy does not come from nowhere; the fastest-moving water molecules escape as vapour and carry it away, drawing 22600 J out of your skin and the blood beneath it.
Because the energy leaves your body, your skin cools. Notice that the sweat does not need to be cold to do this, and its temperature barely changes while it evaporates.
The cooling comes entirely from the latent heat carried off by the escaping vapour, not from the temperature of the liquid itself.
See it in daily life
Traditional Malaysian earthenware, such as a labu sayong or a clay water jar, keeps drinking water cool even without a refrigerator. The clay is slightly porous, so a thin film of water constantly seeps to the outer surface and evaporates into the air.
Each bit of water that evaporates must take its latent heat from the water still inside the pot. As gram after gram escapes as vapour, energy is steadily removed from the remaining water, and its temperature drops below that of the surroundings.
The pot works best in a breezy, shaded place, because moving air sweeps the vapour away and lets more water evaporate.
You feel the same effect when you step out of a swim and a gust of wind hits your wet skin, or when a damp cloth is laid on a feverish child's forehead. In every case, evaporating water pulls latent heat from whatever it is touching, leaving that surface cooler than before the water dried.
How this comes up in SPM
In Paper 2 this idea is examined with command words such as explain, describe, relate and state. You may be asked to explain evaporative cooling in terms of latent heat, or to describe factors that speed up evaporation, such as higher temperature, drier air, more surface area and moving air.
Within the same Heat chapter it sits directly beside specific heat capacity, where energy changes an object's temperature, in contrast with latent heat, where energy changes an object's state at constant temperature. It also connects to thermal equilibrium, since evaporative cooling is one way heat flows out of a warmer body.
A common task is to compare evaporation and boiling, and to relate the mass evaporated to the heat lost using Q = mL. Keep the unit J kg⁻¹ on L throughout your working, and remember to state clearly that temperature stays constant during a change of state.
That distinction between temperature change and state change is often where marks are won or lost.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)