In a laboratory experiment, a fermenting aqueous solution of glucose and yeast produces carbon dioxide gas and ethanol. The solution was heated by burning natural gas in a Bunsen burner to distill the ethanol that formed in the flask. During the distillation, the ethanol evaporated and then condensed in the receiving flask. The flame of the burner was kept too close to the bottom of the flask and some of the glucose decomposed into a black carbon deposit on the inside of the flask. During this experiment the following changes occurred. Which of these changes involved a physical change and not a chemical change?
1. evaporation of ethanol
2. condensation of ethanol
3. formation of a carbon deposit inside the flask
4. formation of carbon dioxide gas from glucose
5. burning of natural gas
6. formation of ethanol from glucose by yeast

The first and second process imply a change of phase, which is essentially a physical change because the matter change from liquid to gas and from gas to liquid respectively and it can ve reversed.

The options 3 to 6 are related to a chemical reaction because the outcomes of the process are different compounds than the ones we had at the beginning. The main reaction that implies process 3 to 6 is represented by:

C6H12O6 + impurities → 2 C2H5OH + 2 CO+ C

For option 5. burning of natural gas , we have:

Natural gas(Mainly a mix of CH4 and C2H10) +O2→COn+H2O

Therefore, justo 1 and 2 are a physical change