Before you start this project with your kids, refresh their basic knowledge about water: Where does water come from? Why can’t we drink saltwater? Why is our water at home pure and who knows what a water filtration system is? Now, show your kids how to filter saltwater and produce their very own drinkable freshwater with this easy and fun purifying experiment.
This activity teaches desalinization (the kids will have fun pronouncing that one!), an important concept that has increasingly relevant real-world applications. You should have everything you need at home already to make this saltwater filter.
Gather the supplies:
- 1 large glass bowl or container
- 1 smaller glass, bowl, or container that fits within and is shorter than the larger container
- Salt (cooking or table salt is fine)
- Cling film
- A small but heavy object, such as a rock (it should not be heavy enough to break through the clingfilm when placed on top)
Science Experiment Instructions
This activity is a great outdoor activity but can easily be done indoors as well. Begin by making your saltwater. The salt will dissolve faster overheat, so you can do this in a pot on the stove. Keep mixing and adding more salt until no more salt will dissolve and the water is cloudy.
Wait for the water to cool a little, and then carefully pour the saltwater into the larger bowl/container.
Place the smaller container in the center of the larger, water-filled one, making sure that it sits centrally and on the bottom. If it floats, you may need to reduce the level of the saltwater until it sits steadily. No water should come into the smaller container.
Cover the large outer bowl with cling film, and then place your heavy object on top of the plastic, in the center. It should be directly over the opening of the smaller container.
Finally, leave your water filter in the direct sunlight, preferably outdoors in a hot place. If outdoors is not possible, leave it by a sunny window instead.
For the best results, start your experiment in the morning, so that the filter has enough time (several hours) to work.
Discover the Results
After removing the plastic wrap, you should see that the saltwater has somewhat evaporated and the drips have collected at the lowest point on the underside of the rock, then fallen into your central container.
Give the water in the central container a taste! It should be fresh and good enough to drink.
Through the process of evaporation, the H2O particles become steam and rise up, leaving the heavier salt particles behind. This is water distillation and is a method of water purification.
This video is a great clip to show your kids. It explains what evaporation is and how salt is made. You can also watch this before you start with the experiment.
Don’t Have All Day?
Instead of using glass containers and leaving them in the sun, use a large cooking pot and a smaller boil-safe mug and repeat the experiment on the stove. Remember to replace the cling film with foil, and be very careful when handling hot elements.
To extend our science experiment a little more, you can test the density of saltwater versus freshwater. Make two equally sized jars of water; one with salt, and one with fresh. Place two small potatoes, or two pieces of potato, into the jars and see what happens. The potato should float on the saltwater due to its higher density, whereas it should sink in the freshwater. You can compare this to floating easily in the ocean or sea, and being much less buoyant in a swimming pool or freshwater lake.