Why an amino acid is usually solid at room temperature.

Explanation

### Step-by-Step Solution

1. **Understanding the State of Matter**:
- To determine why amino acids are usually solid at room temperature, we first need to recognize that solids have a melting point that is higher than room temperature (approximately 25°C or 298 K).

2. **High Melting Point of Amino Acids**:
- Amino acids typically exhibit high melting points. This characteristic is crucial in understanding their solid state at room temperature.

3. **Formation of Zwitterions**:
- Amino acids exist in a unique form known as zwitterions. A zwitterion is a molecule that contains both positive and negative charges but is overall neutral. For amino acids, this occurs due to the presence of both an amino group (-NH2) and a carboxyl group (-COOH).

4. **Structure of Zwitterions**:
- In the zwitterionic form, the amino group can gain a proton (H+) to become -NH3+, while the carboxyl group can lose a proton to become -COO-. This results in the structure:
- NH3+ - C - H - R - COO-
- Here, R represents the side chain that varies among different amino acids.

5. **Intermolecular Forces**:
- The presence of these positive and negative charges in zwitterions leads to strong intermolecular forces (ionic interactions) between the molecules. These forces are significantly stronger than the van der Waals forces present in many other organic compounds.

6. **Conclusion**:
- Due to the strong intermolecular forces resulting from the zwitterionic structure, amino acids have high melting points, which is why they are usually solid at room temperature.

### Final Answer
Amino acids are usually solid at room temperature because they exist predominantly in the form of zwitterions, which exhibit high melting points due to strong intermolecular forces between the charged species in the zwitterionic form.

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