Describe two different test tube reactions to identify the following organic compounds: propanal and benzoic acid.

There are several different answers one could make and an exam question would just require one for each. 

1) Propanal - from the ending "al" this is an aldehyde.

        - Add Tollens' reagent. Positive result: Silver mirror formed.

        - Add Fehling's / Benedict Solution. Positive result: Red precipitate formed.

        - Acidified potassium dichromate. Positive result: Orange to green (due to oxidation). 

2) Benzoic acid - You probably have not come across this in the lab at school. Nevertheless from the name you can work out that it is a carboxylic acid. 

        - Add Sodium carbonate/ Sodium hydrogen carbonate. Positive result: Effervescence is observed.