You said Cool the distillation flask and add 20 mL of 10% sodium hydroxide solution to dissolve the resulting cinnamic acid by forming a sodium salt. After the filtrate is cooled to room temperature, with stirring, (acidify with concentrated hydrochloric acid until the solution is acidic.) Cooling, suction filtration, wash the precipitate with a small amount of cold water, drying, to get the crude cinnamic acid.

This step in parentheses, it is mainly the method of alkali acid precipitation without acidification, how to precipitate cinnamic acid? Only acidic to ensure that cinnamic acid completely precipitated, but also to prevent it from becoming ionic form, we want the molecular form, that is, to prevent the reverse reaction, understand it