Why a Wet Cloth Feels Cold Even in Hot Sunshine
Learn how evaporation differs from boiling, what speeds evaporation up, why evaporation produces cooling, how impurities and pressure move the melting and boiling points, and why water expands when it freezes.
Why does wetting your arm make it feel cold on a hot day?
Because the water leaving your skin takes energy with it, and it takes that energy from your skin.
Notice what this does not require. The water is nowhere near , there is no flame, and the surroundings are hot rather than cold. Evaporation does not wait for boiling point — it happens at every temperature, and it always cools whatever it leaves behind.
This page covers the second part of the ICSE Class 8 Physics chapter on matter: how evaporation differs from boiling, what controls its rate, why it cools, how impurities and pressure shift the melting and boiling points, and the odd way water behaves on freezing.
Notice what this does not require. The water is nowhere near , there is no flame, and the surroundings are hot rather than cold. Evaporation does not wait for boiling point — it happens at every temperature, and it always cools whatever it leaves behind.
This page covers the second part of the ICSE Class 8 Physics chapter on matter: how evaporation differs from boiling, what controls its rate, why it cools, how impurities and pressure shift the melting and boiling points, and the odd way water behaves on freezing.
How is evaporation different from boiling?
Both turn a liquid into a gas, but almost everything else about them differs.
- Temperature. Evaporation occurs at all temperatures. Boiling occurs only at one fixed temperature, the boiling point — for water at normal atmospheric pressure.
- Where it happens. Evaporation is a surface process; only molecules at the free surface escape. Boiling happens throughout the whole bulk of the liquid, which is why bubbles form deep in the pan and rise.
- Speed. Evaporation is slow and quiet. Boiling is rapid and vigorous.
- Heat needed. Evaporation draws heat from the surroundings and the liquid itself. Boiling needs heat supplied continuously from an external source.
- Control. Evaporation cannot be switched off; boiling stops the moment you remove the flame.
Why only surface molecules escape in evaporation. Molecules in a liquid have a range of speeds, not one speed. A molecule at the surface that happens to be moving fast enough, and in the right direction, can overcome the intermolecular attraction of its neighbours and leave. A fast molecule deep in the liquid simply collides with the molecules above it and stays.
That is also why a covered glass of water barely evaporates: escaped molecules are trapped above the surface and are just as likely to fall back in.
- Temperature. Evaporation occurs at all temperatures. Boiling occurs only at one fixed temperature, the boiling point — for water at normal atmospheric pressure.
- Where it happens. Evaporation is a surface process; only molecules at the free surface escape. Boiling happens throughout the whole bulk of the liquid, which is why bubbles form deep in the pan and rise.
- Speed. Evaporation is slow and quiet. Boiling is rapid and vigorous.
- Heat needed. Evaporation draws heat from the surroundings and the liquid itself. Boiling needs heat supplied continuously from an external source.
- Control. Evaporation cannot be switched off; boiling stops the moment you remove the flame.
Why only surface molecules escape in evaporation. Molecules in a liquid have a range of speeds, not one speed. A molecule at the surface that happens to be moving fast enough, and in the right direction, can overcome the intermolecular attraction of its neighbours and leave. A fast molecule deep in the liquid simply collides with the molecules above it and stays.
That is also why a covered glass of water barely evaporates: escaped molecules are trapped above the surface and are just as likely to fall back in.
What makes evaporation faster?
Four things, and each one has a molecular reason.
Temperature. Higher temperature means greater molecular kinetic energy, so a larger fraction of surface molecules are moving fast enough to escape. Clothes dry faster in the sun than in the shade.
Surface area. More exposed surface means more molecules positioned to escape. Wet clothes spread out dry far faster than the same clothes left bundled up, and tea cools faster in a wide saucer than in a narrow cup.
Humidity. Humidity is the amount of water vapour already in the air. When the air is nearly saturated, escaped molecules return almost as fast as they leave, so the net rate falls. Clothes take much longer to dry in the monsoon than in the dry season — the temperature may be similar, but the air already holds a great deal of vapour.
Wind. Moving air carries escaped molecules away before they can return, keeping the air just above the surface dry. This is why a breeze dries clothes quickly, and why a fan cools you even though it blows air at room temperature — it is not chilling the air, it is speeding up the evaporation of your sweat.
The nature of the liquid matters too. Spirit or petrol on your palm feels far colder than water, and vanishes far faster, because its intermolecular forces are weaker — less energy is needed for a molecule to break free. Such liquids are called volatile.
Temperature. Higher temperature means greater molecular kinetic energy, so a larger fraction of surface molecules are moving fast enough to escape. Clothes dry faster in the sun than in the shade.
Surface area. More exposed surface means more molecules positioned to escape. Wet clothes spread out dry far faster than the same clothes left bundled up, and tea cools faster in a wide saucer than in a narrow cup.
Humidity. Humidity is the amount of water vapour already in the air. When the air is nearly saturated, escaped molecules return almost as fast as they leave, so the net rate falls. Clothes take much longer to dry in the monsoon than in the dry season — the temperature may be similar, but the air already holds a great deal of vapour.
Wind. Moving air carries escaped molecules away before they can return, keeping the air just above the surface dry. This is why a breeze dries clothes quickly, and why a fan cools you even though it blows air at room temperature — it is not chilling the air, it is speeding up the evaporation of your sweat.
The nature of the liquid matters too. Spirit or petrol on your palm feels far colder than water, and vanishes far faster, because its intermolecular forces are weaker — less energy is needed for a molecule to break free. Such liquids are called volatile.
Why does evaporation cause cooling?
Because the molecules that escape are the fastest ones, and removing the fastest members lowers the average.
Temperature is a measure of the average kinetic energy of the molecules. Only a high-energy molecule can break away from the surface. Once it is gone, the molecules left behind have a lower average energy — so the liquid's temperature falls.
The liquid then draws heat from whatever it touches to make up the loss. That is the object being cooled.
Everyday consequences.
- Sweating. The body produces sweat; as it evaporates it takes heat from the skin. On a humid day the sweat cannot evaporate, which is why the same temperature feels far more uncomfortable.
- An earthen pot. Water seeps through the porous clay wall and evaporates from the outside, cooling the water within. A glazed or steel pot cannot do this at all — the mechanism needs the pores.
- A wet cloth on the forehead lowers a fever patient's temperature by the same route.
- Spirit on the skin before an injection evaporates almost instantly, producing a sharp cold sensation.
- A desert cooler blows air across wet pads, combining wind with surface area.
What cooling does not mean. Nothing is being made cold by adding coldness. Heat is simply leaving with the escaping molecules, and heat only ever flows out of the object that is cooling.
Temperature is a measure of the average kinetic energy of the molecules. Only a high-energy molecule can break away from the surface. Once it is gone, the molecules left behind have a lower average energy — so the liquid's temperature falls.
The liquid then draws heat from whatever it touches to make up the loss. That is the object being cooled.
Everyday consequences.
- Sweating. The body produces sweat; as it evaporates it takes heat from the skin. On a humid day the sweat cannot evaporate, which is why the same temperature feels far more uncomfortable.
- An earthen pot. Water seeps through the porous clay wall and evaporates from the outside, cooling the water within. A glazed or steel pot cannot do this at all — the mechanism needs the pores.
- A wet cloth on the forehead lowers a fever patient's temperature by the same route.
- Spirit on the skin before an injection evaporates almost instantly, producing a sharp cold sensation.
- A desert cooler blows air across wet pads, combining wind with surface area.
What cooling does not mean. Nothing is being made cold by adding coldness. Heat is simply leaving with the escaping molecules, and heat only ever flows out of the object that is cooling.
How do impurities and pressure move the melting and boiling points?
An impurity lowers the melting point and raises the boiling point. Pressure raises the boiling point, and for ice it slightly lowers the melting point.
Impurities. Salt mixed with ice lowers its melting point well below , which is how a freezing mixture for making ice cream works, and why salt is spread on frozen roads to melt the ice. Salt added to water raises its boiling point above , so salted water boils a little later than pure water.
Pressure on boiling. Higher pressure makes it harder for molecules to escape the surface, so a higher temperature is needed — water boils above in a pressure cooker, which is why food cooks faster in one. On a high mountain the atmospheric pressure is lower, so water boils below , and rice takes far longer to cook because the water is simply not as hot.
Pressure on ice. Ice is unusual: increasing the pressure lowers its melting point, so ice under pressure melts. Press two ice cubes together hard and the contact surfaces melt; release the pressure and the water refreezes, welding the cubes into one lump. This is why a snowball can be squeezed into a firm ball.
Why ice behaves that way. Water expands on freezing, which is the reverse of nearly every other substance. Pressure resists that expansion, and so favours the liquid state.
The expansion, calculated. Water has a density of about and ice about . Take and use
So the volume grows by about in every litre — roughly nine per cent.
That is enough to burst a sealed water pipe in freezing weather, and it is also why ice is less dense than water and therefore floats.
Impurities. Salt mixed with ice lowers its melting point well below , which is how a freezing mixture for making ice cream works, and why salt is spread on frozen roads to melt the ice. Salt added to water raises its boiling point above , so salted water boils a little later than pure water.
Pressure on boiling. Higher pressure makes it harder for molecules to escape the surface, so a higher temperature is needed — water boils above in a pressure cooker, which is why food cooks faster in one. On a high mountain the atmospheric pressure is lower, so water boils below , and rice takes far longer to cook because the water is simply not as hot.
Pressure on ice. Ice is unusual: increasing the pressure lowers its melting point, so ice under pressure melts. Press two ice cubes together hard and the contact surfaces melt; release the pressure and the water refreezes, welding the cubes into one lump. This is why a snowball can be squeezed into a firm ball.
Why ice behaves that way. Water expands on freezing, which is the reverse of nearly every other substance. Pressure resists that expansion, and so favours the liquid state.
The expansion, calculated. Water has a density of about and ice about . Take and use
So the volume grows by about in every litre — roughly nine per cent.
That is enough to burst a sealed water pipe in freezing weather, and it is also why ice is less dense than water and therefore floats.
Exam tip
Exam tip: naming the fastest molecules, not just the heat
Why does evaporation cause cooling? needs three linked steps for full marks: only the fastest molecules escape, so the average kinetic energy of those left falls, so the temperature falls and heat is drawn from the surroundings. Stopping at heat is taken away loses the reason.
When a question asks for differences between evaporation and boiling, answer in pairs — temperature, location, rate, heat source — never a list of facts about only one of them.
For rate of evaporation, give the factor and its direction: higher humidity slows evaporation because escaped molecules return. Naming humidity alone is half an answer.
Remember the two directions of the pressure effect: pressure raises the boiling point of water but lowers the melting point of ice. Only ice-like substances that expand on freezing behave this second way.
And keep the units honest — for temperature, for density — and state that boiling point figures are for normal atmospheric pressure.
When a question asks for differences between evaporation and boiling, answer in pairs — temperature, location, rate, heat source — never a list of facts about only one of them.
For rate of evaporation, give the factor and its direction: higher humidity slows evaporation because escaped molecules return. Naming humidity alone is half an answer.
Remember the two directions of the pressure effect: pressure raises the boiling point of water but lowers the melting point of ice. Only ice-like substances that expand on freezing behave this second way.
And keep the units honest — for temperature, for density — and state that boiling point figures are for normal atmospheric pressure.
Did you know
Why does a frozen lake stay liquid underneath?
Because water is the wrong way round.
Almost every substance contracts and grows denser as it solidifies, so its solid form sinks. If water did the same, ice would form at the surface of a lake and immediately sink to the bottom, and fresh surface water would freeze and sink in turn, until the whole lake was solid ice from bed to surface.
Instead the nine per cent expansion makes ice less dense than the water beneath, so it stays at the top as a floating sheet. Ice is also a poor conductor of heat, so that sheet acts as a blanket, slowing the loss of heat from the water below.
The result is liquid water under the ice all winter — which is the only reason fish survive in a lake whose surface has frozen solid.
Almost every substance contracts and grows denser as it solidifies, so its solid form sinks. If water did the same, ice would form at the surface of a lake and immediately sink to the bottom, and fresh surface water would freeze and sink in turn, until the whole lake was solid ice from bed to surface.
Instead the nine per cent expansion makes ice less dense than the water beneath, so it stays at the top as a floating sheet. Ice is also a poor conductor of heat, so that sheet acts as a blanket, slowing the loss of heat from the water below.
The result is liquid water under the ice all winter — which is the only reason fish survive in a lake whose surface has frozen solid.
Key takeaways
Evaporation, boiling and change of state: quick revision
- Evaporation happens at all temperatures, only at the surface, slowly, using heat from the surroundings. Boiling happens at one fixed temperature, throughout the bulk, rapidly, and needs heat supplied continuously.
- Only a surface molecule moving fast enough can overcome the intermolecular attraction and escape; a fast molecule deep inside just collides with its neighbours.
- Evaporation is faster with higher temperature, larger surface area, lower humidity and more wind — and faster still for a volatile liquid such as spirit.
- Cooling by evaporation: the fastest molecules leave, the average kinetic energy of the rest falls, the temperature drops, and heat flows in from whatever the liquid touches — sweating, earthen pots, a wet cloth, a desert cooler.
- An impurity lowers the melting point and raises the boiling point — salt on icy roads, salted water boiling later.
- Pressure raises the boiling point (a pressure cooker) and lowers it at low pressure (rice on a mountain).
- Pressure lowers the melting point of ice, so ice melts under pressure and refreezes when released — two cubes weld together.
- Water expands about nine per cent on freezing: occupies as water and as ice, from with and . Hence burst pipes, and floating ice.
Attempt some questions on this now — the three-step reason for cooling by evaporation is asked in almost every paper.
- Only a surface molecule moving fast enough can overcome the intermolecular attraction and escape; a fast molecule deep inside just collides with its neighbours.
- Evaporation is faster with higher temperature, larger surface area, lower humidity and more wind — and faster still for a volatile liquid such as spirit.
- Cooling by evaporation: the fastest molecules leave, the average kinetic energy of the rest falls, the temperature drops, and heat flows in from whatever the liquid touches — sweating, earthen pots, a wet cloth, a desert cooler.
- An impurity lowers the melting point and raises the boiling point — salt on icy roads, salted water boiling later.
- Pressure raises the boiling point (a pressure cooker) and lowers it at low pressure (rice on a mountain).
- Pressure lowers the melting point of ice, so ice melts under pressure and refreezes when released — two cubes weld together.
- Water expands about nine per cent on freezing: occupies as water and as ice, from with and . Hence burst pipes, and floating ice.
Attempt some questions on this now — the three-step reason for cooling by evaporation is asked in almost every paper.