How do you identify a pure substance?

Answer 1

1. Check the melting point or boiling point of the substance. E.g. it is known that the boiling point of water is 100 degree celcius. It a sample of water boils are 101 degree celcius, the sample is impure.

2. Check if melting or boiling occur over a range of temperatures. E.g. if water boils from 101 to 103 degree celcius, then it is impure because pure water boil exactly at 100 degree celcius.

3. Perform paper chromatography. If the sample has only one spot on the chomatogram then it is pure substance.