Evaporation Compared with Boiling

Introduction

A kettle of water on a hob and a puddle on a pavement both end up as water vapour, yet the two processes look completely different. The kettle roars and bubbles, while the puddle quietly shrinks. Both are changes from liquid to gas, but one is boiling and the other is evaporation . This page sets out the key differences, which are a favourite topic for examination questions.

Core explanation

What they have in common. Both evaporation and boiling are changes of state from liquid to gas. Both are physical changes, both need energy to overcome the attractions between particles, and both produce the same gas. Steam from a kettle and vapour from a puddle are both water molecules, H₂O.

Where it happens.

- Evaporation happens only at the surface of the liquid. Particles deep inside cannot escape because they are surrounded by other particles. - Boiling happens throughout the liquid. Particles everywhere in the liquid have enough energy to form gas, so pockets of vapour appear inside the liquid as bubbles , which rise and burst at the surface.

When it happens.

- Evaporation can happen at any temperature at which the substance is liquid. Only the fastest particles escape. - Boiling happens only at one temperature , the boiling point, for a given pressure. For pure water at normal atmospheric pressure this is 100 °C.

Rate. Evaporation is usually slow , because only a small fraction of particles at the surface escape. Boiling is fast , because gas forms throughout the liquid at once.

Temperature of the liquid. During boiling, the temperature stays constant at the boiling point, however strongly you heat, because all the energy supplied is used to overcome attractions. During evaporation, the liquid left behind tends to cool down , because the fastest particles leave and the average energy of those remaining falls.

Summary table.

Feature Evaporation Boiling --- --- --- Where Surface only Throughout the liquid Temperature Any, below boiling point Fixed boiling point Bubbles No Yes Speed Slow Fast Effect on liquid temperature Liquid cools Stays constant

Step-by-step reasoning

To decide whether a change is evaporation or boiling:

1. Check the temperature: is it at the boiling point? 2. Look for bubbles forming inside the liquid. 3. If there are bubbles at the boiling point, the liquid is boiling. 4. If the liquid is below the boiling point and is losing mass slowly with no bubbles, it is evaporating.

Visual explanation

Draw two beakers. In the first, show a few particle arrows leaving the top surface only. In the second, show round bubbles forming at the bottom and sides, growing as they rise, and bursting at the top. The first is evaporation; the second is boiling.

Real-world analogy

Evaporation is like people slipping quietly out of a theatre through the doors at the edge, one at a time, during a long performance. Boiling is like the moment the show ends, when everyone everywhere in the theatre stands up and leaves at once.

Real-world example

Pasta is cooked in boiling water, which stays at about 100 °C however high the hob is turned. A bowl of water left out to cool the next morning will have lost a little volume by evaporation, with no bubbles and at room temperature.

Why?

Why do bubbles form only at the boiling point? A vapour bubble inside a liquid is pushed on by the atmosphere and the liquid above it. The bubble can only survive and grow if the vapour inside pushes back at least as hard. This happens only when the liquid reaches its boiling point; below it, any tiny bubble of vapour collapses.

Common misconception

"The bubbles in boiling water are air or hydrogen." The large bubbles that form during boiling are water vapour. The small bubbles that appear early when heating cold water are dissolved air coming out of solution, which is a different thing.

Worked example

Question: A student leaves 50 cm³ of ethanol in an open dish at 20 °C. Ethanol boils at 78 °C. After an hour the volume has fallen to 40 cm³, and no bubbles were seen. Is this boiling or evaporation? Explain.

Reasoning: The temperature, 20 °C, is well below the boiling point of 78 °C, no bubbles formed, and the loss was slow.

Answer: Evaporation, because it happened at the surface, below the boiling point, without bubbles.

Quick check

1. Give one difference between where evaporation and boiling happen. Answer: Evaporation happens only at the surface, while boiling happens throughout the liquid.

Exam focus

Questions often ask for "two differences between evaporation and boiling". Safe answers are: surface versus throughout; any temperature versus a fixed boiling point; slow versus fast; no bubbles versus bubbles. Give both sides of each difference to earn the mark.

Advanced insight

Boiling occurs when the vapour pressure of the liquid equals the external pressure. Lowering the external pressure, as at high altitude, lowers the boiling point. Very clean liquids in smooth containers can sometimes be heated above their boiling point without bubbling, called superheating, which can lead to sudden, violent boiling. This is why laboratory liquids are heated gently with anti-bumping granules.

Summary

Evaporation and boiling both turn a liquid into a gas. Evaporation happens only at the surface, at any temperature below the boiling point, slowly and without bubbles, and cools the liquid. Boiling happens throughout the liquid at a fixed boiling point, quickly and with bubbles of vapour, while the temperature stays constant.

Practice questions

1. State two differences between evaporation and boiling. Answer: Evaporation happens only at the surface but boiling happens throughout; evaporation happens at any temperature but boiling happens only at the boiling point. 2. What gas is inside the large bubbles in boiling water? Answer: Water vapour (steam). 3. Why does the temperature of boiling water stay at 100 °C even when heated more strongly? Answer: The extra energy is used to overcome the attractions between particles to form gas, not to raise the temperature. 4. A liquid is losing volume slowly at 25 °C with no bubbles. Its boiling point is 65 °C. Name the process. Answer: Evaporation, because it is below the boiling point and there are no bubbles.