Week 3 · Lesson 2 Changes in the States of Matter (B): reversible changes, sublimation and deposition, boiling and evaporation, what speeds evaporation · 3 lessons
3
Evaporation or boiling, and what speeds evaporation
Science Prep 2 Term 1, Unit 1: pp. 15–20
GoalTell evaporation from boiling, and explain what makes evaporation faster.
New words
evaporation · a liquid changing to a gas at its surface, at any temperaturehumidity · how much water vapour is in the airsurface area · the size of the part of a liquid that is open to the air
Explanation
Boiling and evaporation both turn a liquid into a gas, but they are different. Boiling happens at the boiling point, all through the liquid, and makes bubbles. Evaporation happens at any temperature below the boiling point, only at the surface, and makes no bubbles: a wet cloth dries without ever boiling. Boiling is not just very fast evaporation. In evaporation, some particles near the surface have enough energy to escape. The rate of evaporation rises with higher temperature (more particles have enough energy), a larger exposed surface (more particles at the surface), lower humidity (less vapour is already in the air) and moving air (it carries the vapour away). That is why washing dries fastest in warm, dry, breezy sunshine. In hot, humid weather sweat evaporates slowly, so we feel uncomfortable. Evaporation, condensation, freezing and melting together make up the water cycle. Instant coffee is dried by spraying strong extract into hot, dry air: the tiny drops have a huge surface, so they dry in a moment.
Boiling
Evaporation
Temperature
only at the boiling point
any temperature below it
Where
all through the liquid
at the surface only
Bubbles
yes
no
Worked examples
Three equal wet cloths are left to dry: one in the shade, one in the sun and one in front of a fan. The graph shows the invented times. Sun and moving air both make evaporation faster, because the particles gain more energy and the vapour is carried away.
The same amount of water is put in a wide plate and in a narrow glass. After a day, the plate has less water. The wide plate has a larger exposed surface, so more particles escape each second.
In a fair test of drying speed, the independent variable is the thing we change (sun or shade), the dependent variable is the thing we measure (time to dry), and the controlled variables stay equal (cloth size, amount of water).
Class activity
Fair drying test, with a grown-up. Take two equal paper towels, wet each with the same amount of water (for example 5 spoons). Put one in the sun and one in the shade, and change nothing else. After the same time, feel which is drier and describe what you see. Write the independent, dependent and controlled variables. Do not predict the result before you look.
Practice
1
Give two differences between boiling and evaporation.
Boiling is at the boiling point, through the liquid, with bubbles; evaporation is at any lower temperature, only at the surface, with no bubbles.
2
Name three things that make evaporation faster.
For example higher temperature, larger surface, drier air, moving air (any three).
3
Why do we feel uncomfortable in hot, humid weather?
The air already holds much vapour, so sweat evaporates slowly.
4
Why is instant coffee dried by spraying fine drops into hot, dry air?
Tiny drops have a very large surface and the hot, dry air makes evaporation fast.
