How a Filter Works
You will need: Blotting paper, paper clip, water and glass, sand, salt, sugar, ink.
When you turn on the faucet and fill a glass with water you must have noticed how clean and sparkling the water looks. This is because it has been filtered and chemically treated to remove impurities, for impure drinking water can cause illness.
When you stir sand into water it never dissolves, but remains suspended in the water. A filter can be used to remove suspended substances from liquids, but it cannot remove dissolved matter, such as salt.
You can make a simple filter by cutting a circle of blotting paper about 5 inches in diameter, and making a straight cut into this circle from the outside to the center, as shown in the diagram.
Form several shallow cones from the blotting paper, securing the ends with a paper clip. Place a cone in the top of a glass.
Now try the effect of your filter on a sand-water mixture; on a salt mixture; on a sugar mixture; on an ink-and-water mixture, etc. You must use a fresh filter cone each time, of course. It will be easy for you to discover which are the suspended substances and which have been fully dissolved.