Evaporation vs Boiling: What's the Difference?

A puddle on the pavement disappears by lunchtime, and it never got anywhere near 100 °C. So if water has to reach 100 °C to become a gas — how did the puddle manage it?

The short answer: evaporation happens only at a liquid's surface, at any temperature, as fast-moving molecules escape one at a time. Boiling happens throughout the liquid, only at the boiling point, when bubbles of vapour can form inside it. Both are vaporisation; they just differ in where and when.

Quick comparison at a glance

Feature	Evaporation	Boiling
Where it happens	Surface only	Throughout the whole liquid
Temperature	Any temperature below boiling point	Only at the boiling point
Speed	Slow and gradual	Rapid
Bubbles?	No	Yes — vapour bubbles rise and burst
Energy source	Heat drawn from the surroundings	Heat supplied continuously
Effect on the liquid	Cools it down	Temperature stays constant
Everyday example	A puddle drying; sweat on skin	A kettle at full boil

What evaporation is

In any liquid, molecules have a range of energies. Most are middling, but a few are moving much faster than average. If one of those fast molecules happens to be at the surface and is heading the right way, it can break free of its neighbours and escape as vapour.

Two consequences follow, and both are worth remembering:

Evaporation works at any temperature. It doesn't need the whole liquid to reach the boiling point — it only needs a handful of surface molecules to be lucky. That's how a puddle dries at 15 °C and how wet clothes dry on a line in winter.

Evaporation cools what's left behind. The molecules that escape are the fastest ones. Removing them lowers the average kinetic energy of everything remaining — which is the temperature. This is why sweating cools you, why a wet hand feels cold in a breeze, and why a clay pot of water stays cool in the shade.

Evaporation speeds up with higher temperature (more fast molecules), larger surface area (more escape routes), moving air (vapour is carried away before it returns) and lower humidity.

What boiling is

Boiling starts when the liquid's vapour pressure equals the external pressure pushing down on it. At that point a bubble of vapour can form inside the liquid without being instantly crushed — so vaporisation stops being a surface event and happens everywhere at once.

That's what the bubbles are: pockets of gaseous water, forming at the bottom of the pan where the heat is, then rising and bursting at the surface. (The tiny bubbles that appear well before the boil are just dissolved air coming out of solution — not steam.)

Because boiling needs that pressure condition, the boiling point isn't a fixed property of the liquid alone — it depends on pressure:

• Water boils at 100 °C at sea level (1 atm).
• In Denver, about 1,600 m up, it boils near 95 °C.
• On the summit of Everest it boils at roughly 71 °C — which is why food takes far longer to cook there.
• In a pressure cooker, raised pressure pushes the boiling point to around 120 °C, cooking food faster.

And while a liquid boils, its temperature stays put. All the incoming energy is spent as latent heat of vaporisation — about 2,260 J per gram for water — separating molecules rather than speeding them up.

How to tell them apart

Three questions, in order:

1. Are there bubbles inside the liquid? Bubbles of vapour mean boiling. No bubbles means evaporation.
2. Is it happening at one specific temperature? Boiling only at the boiling point; evaporation at any temperature.
3. Is the whole liquid involved, or just the top? Throughout means boiling; surface only means evaporation.

The bubble test is the quickest, and it's the one most exam mark schemes are looking for.

Worked examples

Decide before you read the answer.

• Wet clothes drying on a line at 18 °C — evaporation. Well below boiling; surface only.
• A kettle roaring away — boiling. Bubbles throughout at 100 °C.
• A drop of perfume disappearing from your wrist — evaporation.
• Water bubbling vigorously at 71 °C on Everest — boiling, at the lower boiling point set by low air pressure.
• Sea water turning into clouds — evaporation, at ordinary sea temperatures.
• Steam rising off a hot cup of coffee at 70 °C — evaporation, plus condensation of that vapour into visible mist as it cools.

Common mistakes to avoid

• Thinking a liquid must reach its boiling point to become a gas. Evaporation happens at any temperature — a liquid never has to reach its boiling point. The puddle proves it.
• Calling the first small bubbles "boiling." Those are dissolved air escaping. Boiling proper is vapour bubbles forming and surviving all the way up.
• Saying "steam" for the white cloud you see. Steam is an invisible gas. The white plume above a kettle is tiny liquid droplets — vapour that has already condensed in cooler air.

FAQ

What is the main difference between evaporation and boiling?
Evaporation happens only at the surface and at any temperature; boiling happens throughout the liquid and only at its boiling point, when vapour bubbles can form inside it.

Can water evaporate without boiling?
Yes. Fast-moving molecules escape a liquid's surface at any temperature, which is how puddles dry and washing dries on a line without ever reaching 100 °C.

Why does evaporation cool a liquid?
The molecules that escape are the fastest ones. Removing them lowers the average kinetic energy of the molecules left behind, and average kinetic energy is what temperature measures.

Why does water boil at a lower temperature on a mountain?
Boiling begins when a liquid's vapour pressure matches the surrounding air pressure. Air pressure is lower at altitude, so that match happens at a lower temperature — about 71 °C on Everest.

The takeaway

Both processes turn liquid into gas, so the exam mark is never for "it becomes a gas" — it's for where and when. Evaporation: surface, any temperature, slow, cooling. Boiling: throughout, one specific temperature set by pressure, fast, temperature held constant. Say those two sentences and you've answered the question.

Background → [What Is a Phase Change?] and [What Are the States of Matter?] (sibling posts). What holds the molecules back → Hydrogen Bonds vs Van der Waals Forces. Why neither one is a chemical reaction → Physical vs Chemical Changes.

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