An old penny rusting, remember you can’t undo a chemical change
Explanation:
If a sample is impure, it means it contains multiple substances, this means that when you test it in a flame test, it will come up with a mixed color meaning you wont be able to tell what ions are in the substance.
People had asked this many times and that is why they came up with methods and standards that will answer these type of questions. You can look it up in the NIST or the National Institute for Standards and Technology.
In general when you heat up a liquid such as water its volume "increases" as the heat energy causes the water molecules to move leading to more space between molecules causing it to occupy a larger volume.
hope that helps
