What is CH₃COOH commonly known as? 

Acetic acid 

The correct answer is option 3. Acetic acid.

Acetic acid, with the chemical formula \(CH_3COOH\), is a colorless liquid with a sour taste and a pungent odor. It is one of the simplest carboxylic acids and is commonly encountered in everyday life. Here's an explanation:

1. Chemical Structure: Acetic acid consists of two main parts: a carboxyl group \((-COOH)\) and a methyl group \((-CH_3)\). The carboxyl group consists of a carbon atom double-bonded to an oxygen atom and a single-bonded to a hydroxyl group \((-OH)\). The other carbon atom in acetic acid is attached to three hydrogen atoms, forming the methyl group.

2. Occurrence: Acetic acid is found in nature and is a major component of vinegar, which is typically a 5-8% solution of acetic acid in water. It is also produced by certain bacteria during the fermentation of carbohydrates. Additionally, acetic acid is used in the chemical industry for the synthesis of various organic compounds.

3. Properties: Acetic acid is highly soluble in water and miscible with many organic solvents. It is a weak acid, meaning it does not completely dissociate in water. In its pure form, acetic acid is corrosive and can cause burns upon contact with the skin or eyes. However, dilute solutions of acetic acid are commonly used in cooking, food preservation, and cleaning.

4. Uses: Acetic acid has a wide range of applications. In addition to its use in vinegar, it is utilized as a solvent, a preservative in food and beverages, a chemical reagent in the synthesis of pharmaceuticals and plastics, and as a cleaning agent in household and industrial settings.

5. Health Effects: Exposure to concentrated acetic acid can be hazardous, causing irritation to the skin, eyes, and respiratory system. Inhalation of acetic acid vapors can also lead to respiratory distress. However, dilute solutions of acetic acid, such as those found in vinegar, are generally safe for consumption and use.

In summary, acetic acid is a versatile compound with numerous industrial, culinary, and household applications. Its prevalence in nature and its chemical properties make it a valuable substance in various contexts.