How do you measure somthing that is so light it won't show up on a scale?

Question

Its a science project

Answer 1

Use a balance.  You can build a very simple one, and it can be as accurate as your reference.  New, uncirculated coins make for a simple reference.

Answer 2

If it won't weigh by itself, your scale won't have enough resolution to measure the difference. The common solutions to this problem are 1) get a more sensitive balance, or 2) get more of the substance. If neither of those options is available, maybe there is some physical property other than mass that you can measure, such as chemical activity or ionization, radioactivity (innate or induced), optical density, vapor pressure, etc., from which you can infer the amount of substance present.

Answer 3

yes, your suggestion will work, but it will not read very accurately. If you are trying to weigh something that weighs an ounce and your scale reads only in pounds, then it won't work.

Best is to find a more sensitive scale.