Which of the following method is not used in preparing a base?

Options

A. burning of metal in air
B. adding water to a metal oxide
C. reaction between an acid and base
D. heating metal carbonates

Explanation

To determine which method is not used in preparing a base, let's analyze each option step by step.

### Step 1: Analyze the first method - Burning of metal in air
- When a metal (e.g., sodium) is burned in air, it reacts with oxygen to form a metal oxide (e.g., Na2O).
- Metal oxides are typically basic in nature. Therefore, this method can be used to prepare a base.

**Hint:** Remember that burning metals in air produces metal oxides, which are basic.

### Step 2: Analyze the second method - Adding water to metal oxide
- When a metal oxide (e.g., MgO) is added to water, it reacts to form a metal hydroxide (e.g., Mg(OH)2).
- Metal hydroxides are also basic. Thus, this method can be used to prepare a base.

**Hint:** Adding water to metal oxides produces metal hydroxides, which are bases.

### Step 3: Analyze the third method - Reaction between an acid and a base
- The reaction between an acid and a base is known as a neutralization reaction.
- This reaction produces salt and water, not a base. Therefore, this method cannot be used to prepare a base.

**Hint:** Neutralization reactions between acids and bases produce salt and water, not bases.

### Step 4: Analyze the fourth method - Heating metal carbonate
- When a metal carbonate (e.g., calcium carbonate) is heated, it decomposes to form a metal oxide (e.g., CaO) and carbon dioxide.
- Metal oxides are basic in nature, so this method can also be used to prepare a base.

**Hint:** Heating metal carbonates yields metal oxides, which are basic.

### Conclusion
After analyzing all the methods, we find that the third method, which involves the reaction between an acid and a base, is the one that is not used in preparing a base. Therefore, the correct answer is option 3.

**Final Answer:** The method that is not used in preparing a base is the reaction between an acid and a base.