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How long does it take for 100 mL of water to evaporate under a lamp? What is the evaporation speed for water?
Question Date: 2018-03-28
Answer 1:

Hello! This question is a bit complicated and I actually cannot answer it without knowing a bit more about the environment that the experiment took place in. There are a lot of factors that affect the evaporation speed of water , but before we get to that, let’s try to understand what evaporation is.

Evaporation is the process where liquid water slowly turns into water vapor. It is a little different than boiling, because the water doesn’t have to be really hot (212 Fahrenheit) to evaporate, but it does to boil. Evaporation has to do with the slow process of individual water molecules escaping from the liquid source. The actual mechanism for this is a bit complicated so we’ll just simplify it and say that it requires heat and is a slow process.

So now to tackle the question: what factors influence evaporation? The most basic variable we need to know is: how hot is the lamp? The lamp is the heat source that will cause the water to evaporate as the water gains energy. The hotter the lamp, the faster the water will evaporate. In addition to knowing the temperature of the lamp, we need to know the temperature of the water. If the water was warm initially, it will need less heat energy to evaporate, whereas if the water was cold, it will need more energy. We also need to know how much wind is above the water that we are trying to evaporate. This seems a little odd so let’s think about why wind might be a factor. I mentioned that evaporation happens when individual water molecules escape the liquid, so if wind is blowing, it makes it much easier for single molecules to be swept away from the liquid. Wind speeds up evaporation! The last factor that we will consider is the air humidity.

Humidity is a measurement of how much water is in the air. If there is a lot of water in the air (high humidity) the air is already crowded with water so it is harder for water to evaporate. When the air is dry (low humidity), evaporation is much quicker. So in summary the following factors will speed up evaporation: warm water, hot lamp, high wind, and low humidity. Thanks for the question!

Answer 2:

That depends on many different things. What is the temperature of the air without the lamp? How powerful is the lamp? How far away is the lamp? What is the vessel you are holding the water in? How humid is the air? What altitude are you doing this experiment?

Answer 3:

This question is actually impossible to answer definitely with only the information given. The rate of evaporation depends on a huge number of factors , from the temperature of water (presumably the lamp will increase the temperature, but by how much?), the exposed surface area (all of the water in a very wide pan will evaporate more quickly than that in a thin straw, all else being equal), the purity of the water ( dissolved substances increase the boiling point ), the humidity of the air (higher humidity = higher water content in the air already, and therefore slower evaporation).

One equation to estimate the rate can be found here, but this is purely empirical and, as stated, is only an estimation.

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