2. Which of the following is good evidence that an unknown sample is a pure
substance.
A
After leaving a liquid to settle, it separates.
B
After leaving a liquid to settle, it does not separate.
С
After evaporating a liquid, there is nothing left behind.
D
After evaporating a liquid, a white powder is left behind.
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С. After evaporating a liquid, there is nothing left behind

Explanation:

In chemistry, a pure substance is that substance that has a definite chemical composition. It cannot be separated using physical means as the chemical bonds that forms it have to be broken. Pure substances include elements and compounds.

According to this question, a good evidence that an unknown sample is a pure substance would be that: After evaporating a liquid, there is nothing left behind. This means that after using physical separation technique, THE PURE SUBSTANCE COULD NOT BE SEPARATED, hence, nothing is left behind.