Give one use of quaternary ammonium salts.

Explanation

**Step-by-Step Solution:**

1. **Understanding Quaternary Ammonium Salts**: Quaternary ammonium salts are compounds that contain a nitrogen atom bonded to four carbon-containing groups (alkyl or aryl groups) and carry a positive charge. The general structure can be represented as R₄N⁺X⁻, where R represents the carbon chains and X is the counter ion.

2. **Structure Representation**: A typical quaternary ammonium salt can be visualized with a nitrogen atom (N) at the center, bonded to four alkyl groups (for example, CH₃, CH₂, etc.). Since nitrogen usually forms three bonds, the fourth bond creates a positive charge, necessitating a counter ion to balance the charge.

3. **Identifying Uses**: One of the primary uses of quaternary ammonium salts is as surfactants. Surfactants are compounds that lower the surface tension between two substances, such as oil and water, making them effective in cleaning applications.

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