What do bottled water and cloud have in common?
They both involve a process called evaporation.
Evaporation is an important step in the water cycle.
Warm water evaporates and turns into vapor.
In bottled water factories, water is boiled and vapors are collected to make the product.
In nature, drops of vapor gather to form clouds in the sky.
Try this experiment to see how evaporation happens.
- warm water
- plastic food wrap
- a small, slightly weighty object (you can use things like a stuffed toy)
- food coloring (optional)
- a large bowl
- a small cup (make sure it's shorter than the large bowl)
- adult supervision
- Pour warm water into the large bowl.
- Add food coloring if you want to see how the distilled water turns out different.
- Put the empty small cup in the middle of the water.
- Wrap the mouth of the large bowl with food wrap.
- Place the small object on the wrap so that it is slightly depressed right above the small cup. But make sure the cling wrap doesn't touch the rim of the small cup.
- Set the bowl under the sun and wait.
- After a while, you should start seeing drops of water appear under the plastic wrap.
- Note the color of the water collected in the small cup.
When the vapor reaches the wrap which is colder, the vapor condenses and becomes water drops.
The water drops stick to the food wrap due to adhesion, water’s tendency to stick to objects.
The small item on the food wrap creates a slope for the water droplets to slide down and eventually drop into the small cup.
What the small cup collects is distilled water. This process of using evaporation to purify water is called distillation.
Commercial distilled water is made by boiling the water and condensing the steam.
Distillation can be used to purify water.
Most impurities, such as minerals, have melting points or boiling points much higher than water’s. Therefore, only water evaporates into water vapor leaving the impurities behind.
That is why in our experiment, the collected water is clear even though we put blue food coloring in the water at the beginning.
Although distillation is the main step in purifying water, our homemade distillation setup by itself cannot be used to make safe drinking water (unless drinking water is used as the “impure” water to start with, in which case you can safely drink away). Some contaminants such as pesticides have lower boiling points than water’s.
These volatile organic compounds (VOCs) become gases before water boils off and can become a part of the collected water.
Extra steps need to be taken to remove those VOCs before the distilled water is safe for consumption.