A student adds 4 mL of acetic acid to a test tube containing 4 mL of distilled water. He then shakes the test tube and leaves it to settle. After about 10 minutes he observes :

Acetic acid ($CH_3COOH$) is highly miscible in water in all proportions due to the formation of hydrogen bonds between the water molecules and the polar carboxylic group of acetic acid. When 4 mL of acetic acid is mixed with 4 mL of water and shaken, they mix completely to form a single, homogeneous, clear, and colourless solution. No separate layers or precipitates are formed. Therefore, option D is correct.

• Options A and B are incorrect because water and acetic acid do not form separate layers (they are completely miscible, unlike oil and water).
• Option C is incorrect because no chemical reaction leading to an insoluble precipitate occurs.