At `pH=4`, glycine exists as:

Options

A. `H_(3)overset(o+)(N)-CH_(2)COoverset(Θ)(O)`
B. `H_(3)overset(o+)(N)-CN_(2)COOH`
C. `H_(2)NCH_(2)COOH`
D. `H_(2)NCH_(2)COoverset(Θ)(O)`

Explanation

To determine the form of glycine at pH 4, we need to analyze its structure and behavior in relation to pH. Glycine is an amino acid with both an amino group (-NH2) and a carboxylic acid group (-COOH).

### Step-by-Step Solution:

1. **Understanding Glycine Structure**:
Glycine (NH2CH2COOH) has two functional groups: an amino group (NH2) and a carboxylic acid group (COOH).

2. **Identifying pH Conditions**:
At pH 4, the environment is acidic. This means that there are excess H⁺ ions present in the solution.

3. **Protonation of Amino Group**:
In acidic conditions, the amino group of glycine can accept a proton (H⁺) to become positively charged (NH3⁺).

4. **Dissociation of Carboxylic Acid Group**:
The carboxylic acid group (COOH) can lose a proton (H⁺) to become negatively charged (COO⁻). However, at pH 4, the carboxylic acid group is predominantly in its protonated form (COOH) because the pKa of the carboxylic acid group of glycine is around 2.34, which means it remains protonated at pH 4.

5. **Formation of Zwitterion**:
As a result, at pH 4, glycine exists predominantly as a zwitterion, which is a molecule that has both a positive charge (from the protonated amino group) and a negative charge (from the deprotonated carboxylic acid group). The overall charge of the zwitterion is neutral.

6. **Conclusion**:
Therefore, at pH 4, glycine exists as an amphoteric zwitterion, which is a form that can act as both an acid and a base.

### Final Answer:
At pH 4, glycine exists as an amphoteric zwitterion.