What does residue mean in science terms?

Answer 1

literally - "left behind". In chemistry - usually refers to what doesn't evaporate from a sample left at room temperature for a long time, or if the sample is heated to another specific temperature (like in fractional distillation). Elements and compounds that have low boiling points tend to evaporate quickly, higher boiling points take longer and require more heat, so they get left behind. Some do not boil at all (sort of - usually enough heat will cause a chemical reaction and the result may have a boiling point).

Can also refer to amino acids in Biology.

In agriculture, the soil and parts of the plant that are not usually used or sold.

In math, the remainder in modular arithmetic.