Science Buddies: "Ask an Expert"

I need an experts opinion on how cleaning salt water works. I put regular tap water and salt in two conainers. I then placed a cup in the middle of the containers and covered them with plastic wrap. I put one container outside in the sun and the other inside under a light bulb. Can you explain how this process cleans the water and which one, sunlight or the light bulb, works the best and why?

This is often taught in wilderness classes! Let's take a look at your setup, step by step. You have a large container of water (either salt or tap). What would happen if you just put a container of water outside in the sun (or near a window)? You'd expect the water to evaporate, right? Most of the time, there's nothing left in the bowl when the water is gone, but that would only be for clean water. Now let's take a look at the salt water. Have you ever let salt water from the beach evaporate on your skin or from a bowl? The salt is actually left behind! Since salt's boiling temperature is much much higher than the water, the water is able to evaporate, but the salt is just left, still a solid, in the bowl. Although the salt is dissolved in the water (you can't see it, it is bound to the water molecules), it starts to precipitate out of the water-salt solution (it goes from being dissolved in a solution to being a solid that is not bound to the water molecules).

With the plastic wrap, instead of evaporating into the air, the water actually collects on the plastic, and it drips into the smaller bowl. The water in the middle small bowl should be clean, and in the big bowl, only salt and other impurities will be left.

I'm not sure whether sunlight or light bulb would work best. It depends on the power of the bulb and how sunny it is outside. (For this experiment, you would actually want to leave both of the bowls inside otherwise you will introduce other variables, such as the dryness outside and temperature of inside vs. outside, which would make the water evaporate faster or slower). If you've done the actual experiment, you can find out!

If you have time to do the experiment again, I would suggest putting equal amounts of water in each large bowl and then putting a couple of tablespoons of salt into them. Then, put on the small cup and plastic wrap. Set one next to a window, and the other next to a light bulb (in a dark room that has a similar temperature to the other bowl), then you can see what happens.

If you don't have time to do another experiment, I would talk about how you accidentally introduced other variables which may have affected the results (since you said you put one of them outside and one inside).

If you have more questions please let us know. Good luck!

I also forgot to mention, it helps to leave the plastic wrap a bit loose so you can push it slightly with your finger in the center so that the collected water drips straight down into the smaller bowl instead of collecting all over the place on the wrap and going back into the larger bowl. You might also take a newspaper and put it under each of the large bowls to act as an insulator (you don't want heat to leak out of the water and into a ceramic countertop, for example).