When you heat water to bring it to boiling,bubbles first form on the bottom of the pot.

That’s because, at first, the bottom of the potis the only place where it’s hot enough to

heat the water to the boiling point andchange it to a gas. But once the entire pot of

water is at the boiling point (100°C), andbubbles form throughout the bulk of thewater. Evaporation, on the other hand,occurs only at the surface of the water

LOCATION OF THE CHANGE TO A GAS

DIFFERENCE BETWEEN

BOILING & EVAPORATION

Set a tall glass of water on a shelf, and measurethe height of the water every few days for 2 week

and keep a table showing the changing level

In contrast, you can boil water in the same glassand watch the water level lower in a matter of

minutes, rather than the days and weeks it takesfor water to evaporate.

A liquid will evaporate at anytemperature above freezing. In contrast,

boiling occurs only when the liquidreaches a certain temperature, which wecall the boiling point. The boiling point

of water at sea level is 100°C (212°F)

TEMPERATURE REQUIRED

Evaporation is a slower processand boiling is faster.

SPEED

TRY THIS

While water is boiling, its temperature remains constant at 100°C. A rolling boil doesn’t make the water any hotter than a medium boil.

On the other hand, evaporation of water will cool the water—and any surface that thewater is evaporating off of.

That’s why if you wait to dry off after you step outof the shower, you feel cold. The evaporatingwater molecules carry away heat from your skin.

TEMPERATURE CHANGE OF THE LIQUID

Boiling usually requires an external source of energy, such as the burner under the pot of

water in which you’re boiling your eggs.

Evaporation uses the energy already in the liquid. No additional source of energy is required and the water

does not need to reach the boiling point to evaporate. As we’ve seen, water will evaporate at room temperature.

This implies that evaporation, but not boiling, is a naturalprocess. Water on your hair that you just washed will

evaporate w/o you doing anything special. Just wait, and itdries. But boiling does not usually happen naturally.

We have to deliberately heat the liquid to get it to boil.

SOURCE OF ENERGY

Evaporation does not involve the formation ofbubbles. When liquid evaporates, individualmolecules leave the liquid and become part ofthe air. During evaporation, there's a nettransfer of water molecules from the liquid inthe glass to the air. This process does notinvolve any bubbles.

In contrast, when you boil water, the liquid ischanging to a gas so rapidly that bubbles ofwater vapor form.

Try this experiment yourself to see the bubblesBring a pot of water to a rolling boil. Put a lid onit for about 20 seconds, and then remove the lidand look at the inside of it. It’s covered withdrops of water from those bubbles thatcondensed on the cooler lid (Figure 3).

BUBBLES

FACTORS AFFECTING RATE

OF EVAPORATION

WHAT CAN WE DO TO MAKE WATER EVAPORATE FASTER?

HEAT WIND HUMIDITY SURFACE

AREA

HEAT

Clothes drying on a clotheslinewill dry faster on a summer day

than in winter

WIND

If you step out of a swimming pool whenthe wind is blowing, you feel colder

because the wind causes the water toevaporate faster from your skin, carrying

away heat energy from your skin faster, leaving your skin colder

HUMIDITY

Water evaporates faster whenthe air is dry. When the air is dry,there are fewer water moleculesin the air returning to the liquid,so the water evaporates faster

SURFACE

AREA

Increasing surface area will providemore surface from which water canevaporate. So a wet towel will dryfaster if it’s spread out rather than

left folded or bunched up

Done by: BlueTree Education Learning Centre