Fill in the following blanks with suitable words :
The acidic or basic nature of a substance is tested by using an ……….

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks us to fill in the blank regarding how to test the acidic or basic nature of a substance.

2. **Identifying the Key Concept**: To determine whether a substance is acidic or basic, we use a specific type of substance known as an "indicator."

3. **Defining an Indicator**: An indicator is a substance that changes color or shows a visible sign when it comes into contact with an acid or a base.

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