The Rusty Iron is Heavier Than Non-Rusty Iron Experiment

This experiment is very easy to do in a classroom, but it does take a few days before you get the results. Iron is a metal that easily begins to rust when exposed to moisture and oxygen. We probably all have seen rusty iron at some point in our lives. Because we tend to see rusty iron with holes in it or pit marks we would tend to think it weighs less, but it actually weighs more. It is not as strong and pieces do break off causing the whole unit from where it broke from to weigh less.

To check this out you will need two large test tubes, some steel wool (without soap) some cotton and a good scale. The best kind of scale would be one that can weigh things to grams and possibly to a tenth of a gram like a triple beam balance or one of the new electronic balances. You must weigh the two test tubes and get an accurate measurement of each. Label one A and the other B. (If you use tape to write on, do the labeling first and then weigh the test tubes. Record the weights of each test tube.

Now you will need to dropper 3 drops of water into ONE test tube. The drops should be equal in size. Now you need to weigh two equal amounts of steel wool. You can have enough steel wool to go about 1/2 way into each test tube or you can simply weigh out about 15 grams to go into each test tube. The amount of steel wool in each test tube must be equal. (The amount depends a bit on the size of the test tubes). Don't pack the steel wool into the test tubes tightly, it should remain loose to allow air and moisture to get all the way through easily. Now put a very small bit of cotton in the top of each test tube. (Again equal amounts of cotton for each test tube.) This should only be enough to seal the top so that nothing can fall into the test tubes or out of the test tubes.

You now have equal amounts of steel wool and 3 drops of water in ONE test tube. The weight of each tube should again be measured. The test tube with the 3 drops of water should weigh more, but it will not be much heavier. A good scale should register the difference. After a few days the water will evaporate and the test tube that had the 3 drops of water should now weigh less except that it has caused some of the steel wool to rust. (You should be able to see rusty steel wool through the glass of the test tube.) As the steel wool rusts it combines with oxygen in the air making iron oxide (rust) which weighs more than iron. Because none of the rusty steel wool can fall off on the ground you have all the iron you started with, but now some has changed to iron oxide.

When you compare the two test tubes now the one that had the drops of water (now evaporated) will be heavier than the one that had no water. Sometimes you may have to wait a full week or more, but you should be able to get the data needed to prove that rust weighs more. You might be able to accelerate the process if you place the test tubes in a sunny area to speed up the evaporation.

I have had many automobiles that have been old and rusty and have become lighter not heavier as pieces have fallen off, but scientifically speaking they got heavier first.