Sodium hydroxide is

Options

A. a base
B. an acid
C. an alkali
D. both base as well as alkali

Answer

### Step-by-Step Solution:

1. **Definition of Sodium Hydroxide**: Sodium hydroxide is a chemical compound with the formula NaOH. It is commonly known as lye or caustic soda.

2. **Classification**: Sodium hydroxide is classified as a base. Bases are substances that can accept protons (H+) or donate electron pairs in reactions.

3. **Physical Properties**: Sodium hydroxide has distinct physical properties:
- It has a bitter taste.
- It feels soapy to the touch.

4. **Reactivity**: Sodium hydroxide reacts with acids in a neutralization reaction. When it reacts with hydrochloric acid (HCl), it produces sodium chloride (NaCl) and water (H2O):
\[ \text{NaOH} + \text{HCl} \rightarrow \text{NaCl} + \text{H}_2\text{O} \]

5. **pH Level**: Sodium hydroxide is a strong base and has a pH greater than 7 when dissolved in water. The pH scale ranges from 0 to 14, where values below 7 indicate acidity and values above 7 indicate basicity.

6. **Alkali Definition**: Sodium hydroxide is also classified as an alkali because it is a soluble base. Alkalis are bases that dissolve in water to produce hydroxide ions (OH⁻).

7. **Ionization in Water**: When sodium hydroxide is dissolved in water, it dissociates into sodium ions (Na⁺) and hydroxide ions (OH⁻):
\[ \text{NaOH} \rightarrow \text{Na}^+ + \text{OH}^- \]
This increases the concentration of hydroxide ions in the solution.

8. **Conclusion**: Therefore, sodium hydroxide is a base that is soluble in water, and it is classified as an alkali due to its ability to produce hydroxide ions in solution.