Rate of evaporation

Evaporation is the conversion of liquid to vapour without the boiling point necessarily being reached. In this experiment, students measure and compare the time taken for a drop of propanone to evaporate under a number of different conditions.

Propanone is highly flammable; ensure that there are no sources of ignition nearby. Pupils could be asked to devise their own experiment, in which case teachers must check the plans before practical work starts, or they could be told how to vary the conditions and exactly what to do – see the sample results table from the student sheet available for download below.

Equipment

Apparatus

• Eye protection
• Microscope slides, x2 or 3
• Access to warm water (see note 4 below)
• Dropper pipette
• Timer

Chemicals

• Propanone (HIGHLY FLAMMABLE, IRRITANT), few cm³

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection throughout.
• Propanone, CH₃COCH₃(l), (HIGHLY FLAMMABLE, IRRITANT) – see CLEAPSS Hazcard HC085A.
• Use warm water from a tap or a kettle. Do not use a naked flame to heat the water.

Procedure

1. Put a drop of propanone onto a microscope slide and time how long it takes to evaporate.
2. Change the conditions and repeat the experiment, ensuring that you record the conditions used and the time taken for each one. For example: warm the microscope slide by holding it in your hands, or by placing in warm water and then drying; spread the drop out with the tip of the dropper pipette; for a cool air flow, fan with a book or similar; for a warm air flow, blow across the drop.

Teaching notes

Students should be able to observe that warmth, air flow and spreading out the drop all increase the rate at which it evaporates.

Liquids evaporate below their boiling point. This is because as the particles move around and collide, some have more energy than the others allowing them to escape from the rest of the liquid as vapour. This results in the overall energy of the liquid (and therefore its temperature) decreasing.

This experiment lends itself well to being a planning exercise or alternatively there are some sample results tables available.