Evaporation and Transpiration

Introduction

Wet washing dries on a line, puddles vanish after a shower and a glass of water left on a windowsill slowly empties — all without boiling. The water has evaporated into the air. Meanwhile, every tree and blade of grass is quietly releasing water vapour from its leaves, a process called transpiration . Together these two processes lift about half a million cubic kilometres of water into the atmosphere every year, powering the water cycle.

Core explanation

Evaporation in terms of particles. In a liquid, water molecules are close together but moving randomly at a range of speeds. Some molecules near the surface move fast enough to overcome the attractions of their neighbours and escape into the air as water vapour. This is evaporation . It happens at any temperature, not just at 100 °C, but only at the surface of the liquid.

Evaporation causes cooling. The molecules that escape are the most energetic ones. The molecules left behind have a lower average energy, so the liquid cools. This is why sweating cools your body and why wet skin feels cold in a breeze.

Evaporation versus boiling.

Evaporation Boiling --- --- Happens at any temperature Happens only at the boiling point Only at the surface Throughout the liquid, forming bubbles Slow Rapid

Factors that increase evaporation.

- Higher temperature — more molecules have enough energy to escape. - Larger surface area — more molecules are at the surface. - Wind (air movement) — carries away vapour, so escaped molecules are less likely to return. - Lower humidity — dry air has room for more water vapour.

Transpiration. Plants absorb water from the soil through their roots. It travels up the stem in tubes called xylem to the leaves. Leaves have tiny pores called stomata , mostly on the underside, which open to let carbon dioxide in for photosynthesis. When they are open, water evaporates from the moist surfaces inside the leaf and diffuses out as vapour. This loss of water vapour is transpiration , and it pulls a continuous stream of water up the plant.

Factors affecting transpiration. The same factors that speed up evaporation — warmth, wind and dry air — also increase transpiration. Bright light increases it too, because stomata open wider in light.

The scale. Evaporation from the oceans supplies most atmospheric water vapour. Over land, transpiration is very important: a large tree can release hundreds of litres of water in a single hot day, and forests such as the Amazon recycle much of their own rainfall.

Step-by-step reasoning

To predict whether washing will dry quickly:

1. Is it warm? Warmth gives molecules more energy, so evaporation is faster. 2. Is it windy? Wind removes vapour from around the cloth. 3. Is the air dry? Low humidity allows more vapour to enter the air. 4. Is the washing spread out? A larger surface area speeds drying. Warm, windy, dry weather with spread-out washing is ideal.

Visual explanation

Picture a close-up of a water surface: most molecules jiggle about, but a few at the top fly upwards into the air. Beside it, picture a leaf cut in half, with water vapour drifting out of a small open pore on its underside, while thin tubes carry water up from the roots.

Real-world analogy

Evaporation is like the fastest runners escaping from a crowded stadium. Only people with enough speed at the exit get out, and those left behind are, on average, the slower ones — just as the liquid left behind is cooler.

Real-world example

Traditional clay water pots, used in hot countries, keep drinking water cool without a fridge. Water seeps through the porous clay and evaporates from the outside surface, taking energy from the pot and the water inside.

Why?

Why does a puddle dry faster on a windy day than on a still day? Wind carries away the water vapour just above the puddle. Without wind, vapour builds up and some molecules fall back into the liquid, slowing overall evaporation.

Common misconception

"Water only turns into vapour when it boils." Evaporation happens at all temperatures below the boiling point; boiling is simply rapid vaporisation throughout the liquid at one temperature.

Worked example

Question: Two identical shirts are washed. One is hung outside on a warm, breezy day; the other is folded in a pile indoors. Explain which dries first.

Reasoning: The outdoor shirt has a higher temperature, moving air and a larger exposed surface area.

Answer: The shirt outside dries first, because warmth, wind and a larger surface area all increase the rate of evaporation.

Quick check

1. Through which structures do plants lose most of their water vapour? Answer: Stomata, the tiny pores mainly on the underside of leaves.

Exam focus

Use particle language: evaporation is the escape of the most energetic molecules from the surface. List the four factors (temperature, surface area, wind, humidity) and explain each, rather than just naming them. Make clear that transpiration is evaporation from plants.

Advanced insight

Evaporation absorbs a large amount of energy — about 2.3 kJ for every gram of water at room temperature. This energy is carried into the atmosphere with the vapour and released when it later condenses, so evaporation moves huge amounts of heat from the tropics towards the poles and from the surface into the upper air.

Summary

Evaporation is the escape of energetic molecules from a liquid's surface below its boiling point; it cools the liquid left behind. It is faster with higher temperature, larger surface area, wind and low humidity. Transpiration is evaporation of water from plant leaves through stomata, affected by the same factors and by light. Both add water vapour to the air.

Practice questions

1. State two differences between evaporation and boiling. Answer: Evaporation happens at any temperature and only at the surface; boiling happens at the boiling point and throughout the liquid. 2. Explain why evaporation makes a liquid cooler. Answer: The most energetic molecules escape, so the average energy, and therefore the temperature, of the remaining liquid falls. 3. Name two factors that increase the rate of transpiration. Answer: Any two of: higher temperature, wind, low humidity, bright light. 4. Why do plants lose water through stomata even though they need to conserve water? Answer: Stomata must open to let carbon dioxide in for photosynthesis, and water vapour escapes while they are open. 5. Explain why washing dries slowly on a humid day. Answer: The air already contains a lot of water vapour, so fewer molecules escape overall and more return to the cloth.