43. Factors affecting the rate of evaporation

Flashcard No.8-

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Factors affecting the rate of evaporation

Evaporation happens when a liquid particle gets enough kinetic energy to break its liquid bonds and escape as a gas. This leads to cooling of the remaining liquid particles as each each particle that evaporates takes away some energy from the rest of the 'pack'.

So what factors affect the rate of evaporation - how can we speed up evaporation?

Firstly, the higher the temperature, the faster the particles move around and therefore the more likely they are to have enough energy to break their liquid bonds. So a higher temperature means a faster rate of evaporation.

Secondly, a movement of air (such as a breeze) over the liquid will increase the rate of evaporation. This is because particles that evaporate tend to form a dense vapour (gas) layer over the liquid. Many of these 'escapees' may collide with the liquid to form liquid bonds again.

A breeze is a movement of air particles that can collide with this layer of vapourised particles, effectively 'scooping' them away from the liquid - and reducing the likelihood of them 'falling' back into the liquid.

The third factor which will increase the rate of evaporation is the dryness of the surrounding air. What would be easier: drying your washing on a hot humid day, or on a hot dry day? A humid day means there is a lot of water vapour (gas) particles in the surrouding air. These particles may condense (become a liquid) onto your washing making it slightly wetter. Increasing the dryness of the surrounding air will therefore increase the rate of evaporation.

Finally, a liquid with a higher surface area will have a faster rate of evaporation. Would you expect your washing to dry easily if it is bundled up in a heap? It would dry much faster if you hang it out neatly on the line so that there is more surface area exposed to a breeze. This is because there is more chance that liquid molecules can escape as a gas into the surrounding air.

GCSE Physics Keywords: Rate of evaporation, Temperature, Breeze, Humidity, Water vapour, Condense, Surface area

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