Task 4: Processes that influence the speed of evaporation

The speed of evaporation depends not only on the chemical composition of a liquid but also on outer conditions. Pupils work in groups and agree what their work procedure will be.

For every group, the teacher prepares 14 slides, 2 droppers, a beaker or a glass, a container with ice or cold ethanol, a candle and ethanol. To raise the attractiveness and for easier observation, it is suitable to dye ethanol with food colouring.

The higher the temperature of the surroundings, the faster the liquid evaporates. Pupils simultaneously place a drop of ethanol on the first and the second slide by using the dropper. They carefully heat up the first slide over the flame of the candle and compare the speed of evaporation of the drop on the first slide and the drop on the second slide.

When the influence of the lower temperature of surroundings on the speed of evaporation is proved, pupils simultaneously place a drop of ethanol on the first and the second slide by using the dropper. They carefully place the first slide on an ice cube or the container with ice and compare the speed of evaporation of the drop on the first slide and the drop on the second slide.

The higher the temperature of a liquid, the faster the evaporation is. Pupils suck ethanol into one of the droppers and warm it up in hands for at least one minute. Due to this, the temperature of ethanol increases approximately up to 37 °C. After that they suck ethanol into the other dropper as well. Using ethanol from both droppers they simultaneously place a drop on the slides or the unabsorbent mat. The second drop has the room temperature. Therefore the drop of heated ethanol evaporates faster. When proving the influence of the lower temperature of the liquid on its evaporation, the teacher brings ethanol from the refrigerator as the first sample. In this case, the second drop evaporates faster.

Another way to speed up the evaporation of a liquid is to extend the surface, which will the liquid evaporate from, as much as possible. Pupils simultaneously make two drops of ethanol and they spread one of the drops over the slide with the use of the dropper. The drop with a larger, unobstructed surface will evaporate faster. This is used when drying wet clothes – it is necessary to shake them out first and spread them over an area as large as possible. In the same way, we generally do not make a ponytail but let the hair loose when drying it.

Pupils again place two drops of ethanol on two slides. They carefully blow on the first one so that the drop is not blown away from the slide. The drop, which the current of air flows over, evaporates faster. The same as the washing hung in the wind or hair dried by cold air will dry faster rather than when the air is not moving at all.

Pupils repeat the same procedure of placing two drops of ethanol on two slides. They cover the first slide with the beaker and leave the other one uncovered. The drop, which is not closed under the beaker, evaporates more slowly.