Water that you find in nature is not necessarily clean or pure; you would not want to drink pond water, for example. There are ways to clean water, though, and you can try this experiment at home!
First, take a clean jar with a lid and fill it with water from a lake, pond, stream, or any other source of water outside your home. Put the lid on until it is time for the next step. What do you notice about the water?
To clean the water, you will need something called alum. Alum is a hydrated double sulfate of aluminum and potassium, used in dyeing and tanning. If you don’t have this at home, it can be found in the spice aisle in the grocery store.
Put 2 tablespoons of alum in your water, close the lid tightly, and shake it up. Then let the water sit for a few hours.
When you look at your jar again, what do you see?
You may notice that the dirt gets pulled to the bottom. In a real water treatment plant, the added alum forms clumps with the dirt and pulls it down to the bottom of the basin.
To learn more about water and water pollution, check out one of these books.