Question

an unknown white solid (X) has a melting point range 120-122C. benzoic acid is a white soild of 121-122C melting point. how can a student confirm the identity of the unkown solid (X)

Answer 1

The student need to perform an identification test for benzoic acid, tp confirm whether the unknown solid is benzoic acid or not.

1. First take some portion of the solid in a test tube(clean and dry).
2. Add a little amount of soda-lime(which is a NaOH-CaO mixture) to the test tube containing the unknown solid.
3. Thenheat the test tube mixture in a bunsen burner(always use test tube holder while heating a test tube).

If from the test tube mixture, white fumes are evolved, then the unknown solid can be confirmed as benzoic acid.

The reason is shown below :

So, the fumes generated because of the release of carbon dioxide.

Whether the fumes are carbon dioxide or not, can be tested as follows : hold a mildly flamed match stick near the mouth of the test tube, from where carbon dioxide is evolving. If the fumes extinguish the flame, then the student can confirm the identity of the evolving gas as carbon dioxide (P.S.: Carbon dioxide is used as fire extinguisher).