Baking soda `(NaHCO_3)` is a compound because

Options

A. the constituents retain their properties.
B. the constituents can be separated by physical methods.
C. the constituents are combined chemically.
D. it is heterogeneous in nature.

Explanation

To determine why baking soda (NaHCO₃) is classified as a compound, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Definition of a Compound**:
A compound is a substance that is made up of two or more elements that are chemically combined in a fixed ratio. This means that the elements lose their individual properties and form a new substance with different properties.

2. **Analyzing Baking Soda (NaHCO₃)**:
Baking soda consists of sodium (Na), hydrogen (H), carbon (C), and oxygen (O). These elements are chemically combined in a specific ratio: one sodium atom, one hydrogen atom, one carbon atom, and three oxygen atoms.

3. **Chemical Combination**:
In baking soda, the elements are not just mixed together; they are combined through chemical bonds. This means that the properties of the individual elements (sodium, hydrogen, carbon, and oxygen) are not retained in the compound. Instead, baking soda has its own unique properties.

4. **Homogeneity**:
Compounds are homogeneous in nature, meaning that they have a uniform composition throughout. Baking soda appears as a single substance, and its composition is the same no matter where you sample it.

5. **Conclusion**:
Therefore, baking soda (NaHCO₃) is a compound because its constituents (sodium, hydrogen, carbon, and oxygen) are combined chemically in a definite ratio, resulting in a substance with distinct properties.

### Final Answer:
Baking soda (NaHCO₃) is a compound because its constituents are combined chemically.

---