An element which can exist as a positive ion in acidic solution and also as a negative ion in basic solution is said to be…..

Explanation

To solve the question, "An element which can exist as a positive ion in acidic solution and also as a negative ion in basic solution is said to be…", we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks about an element that can form a positive ion in acidic conditions and a negative ion in basic conditions.

2. **Identifying Ions in Solutions**:
- In acidic solutions, the common positive ion is **H⁺** (hydrogen ion).
- In basic solutions, the common negative ion is **OH⁻** (hydroxide ion).

3. **Concept of Amphoteric Substances**:
- An element that can react with both acids and bases is termed **amphoteric**. This means it can behave as an acid (donating H⁺ ions) in acidic solutions and as a base (accepting H⁺ ions or donating OH⁻ ions) in basic solutions.

4. **Examples of Amphoteric Elements**:
- Common examples of amphoteric elements include **Zinc (Zn)** and **Aluminium (Al)**. These elements can form positive ions in acidic solutions and can also form negative ions in basic solutions.

5. **Chemical Reactions**:
- For example, Aluminium oxide (Al₂O₃) can react with acids to form Al³⁺ ions and with bases to form Al(OH)₄⁻ ions.
- Reaction with acid:
\[ \text{Al}_2\text{O}_3 + 6 \text{H}_2\text{SO}_4 \rightarrow 2 \text{Al}_2(\text{SO}_4)_3 + 3 \text{H}_2\text{O} \]
- Reaction with base:
\[ \text{Al}_2\text{O}_3 + 2 \text{NaOH} + 3 \text{H}_2\text{O} \rightarrow 2 \text{NaAl(OH)}_4 \]

6. **Conclusion**: Therefore, the answer to the question is that the element is said to be **amphoteric**.

### Final Answer:
An element which can exist as a positive ion in acidic solution and also as a negative ion in basic solution is said to be **amphoteric**.