Expert: George Maxwell - 7/11/2007

Question
Does the addition of heat to a substance automatically change its mass? And also, after iodine is heated, solid iodine is sometimes found on surfaces some distance away from the heat source. Why is this?
Cheers

Answer
Hi, and thank you for your question.

Heating up a substance is a physical process, and so it doesn't directly affect the mass of the substance. The only time that mass will change is if the substance is heated to the point where it boils, decomposes or reacts with the air around it.

If a substance boils, the boiling liquid will appear to lose mass as the vapour escapes from the container.

If a substance decomposes, it will appear to lose mass if it breaks down in to gaseous products which drift away from the container.

If a substance reacts with air when it is heated (e.g. it burns) then it will often GAIN mass; the substance and oxygen combine together, and so the product weighs more than the starting material: for example:
1 gram of magnesium + 1 gram of oxygen -> 2 grams of Magnesium Oxide.

In every case, all you're doing is moving mass from one place to another, you're not creating or destroying it.

Iodine is an interesting substance, in that when you heat it, it doesn't melt like most substances, it goes straight from a solid to a gas. This process is called sublimation, and happens because of the unusual crystal structure of iodine. In your example, you heat iodine, and turn it in to a vapour. This vapor drifts away from the heat source and hits the cooler surroundings where it sublimes straight back to a soild, so you end up with little piles of crystals surrounding your experiment. Carbon Dioxide is another common substance which sublimes: blocks of solid carbon dioxide (very cold!) sublime straight in to gas when heated. That's why solid carbon dioxide is sometimes called "Dry Ice": it doesn't melt to make a liquid like normal ice.
I hope this is useful to you, but please let me know if there's anything else you'd like to know.
Thanks again.