How can a substance gain weight by just sitting out in a lab?

Answer 1

by absorbing moisture especially if it is hygroscopic

Answer 2

Everyone has lept all over the the "water" culprit already. I won't contribute much to that line of thought other than say. "Water absorbtion would be the first thing I would look for."

One thing that you didn't specify in your question was what the substance was. Their are a lot of other chemicals out thier that would leach material from the atmosphere like carbon dioxide, oxogen, nitrogen or some other local airborn substance.

You also didn't specify how much 'weight' was gained. Perhaps the gravity around your scale was amped up a touch over night? (I'M KIDDING! YOU GUYS STOP THROWING THINGS AT ME! OW! THAT WAS A STAPLER!) Another possiblilty if we are looking at a very small change in mass would be dust.

Hope this helps.

Answer 3

Everyone else is right. Water absorption is the culprit.
A good example is Copper Sulphate. In its common form, it is hydrated and is a blue granular powder, (like fine blue sugar). If you heat it in a crucible over a bunsen burner for about 10 minutes, the water will be lost from around the molecules and it will become a very pale green, almost white, fine powder. Leave it for about 30 minutes and as it cools it gathers water again, to become its most common, hydrated form.
A great demonstation, really.

Hope this helps.

Answer 4

In a closed system it cannot gain weight (or mass), however, in a lab it can absorb water or grow if it is a plant or something organic.

Answer 5

By oxidation (addition of oxygen or other electronegative elements) a metallic substance can gain weight.

Answer 6

By absorbing moisture from the air.

Answer 7

water absorption?