When litmus paper is exposed to vinegar, it usually turns red, indicating that the vinegar is acidic. This is because vinegar contains acetic acid, which is a weak acid. When the litmus paper comes into contact with the acetic acid, the acid molecules react with the dye in the paper, causing it to change colour. Litmus paper is commonly used as an indicator to determine the pH levels of substances, with red indicating an acidic environment, blue indicating a basic environment, and purple indicating a neutral environment.