How do you account for the following?
Hydrochloric acid can be concentrated by conc. `H_2SO_4` but not by phosphorous pentoxide (or) quick lime.

Explanation

To account for why hydrochloric acid (HCl) can be concentrated by concentrated sulfuric acid (H₂SO₄) but not by phosphorus pentoxide (P₂O₅) or quick lime (CaO), we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Hydrochloric Acid (HCl)**:
- Hydrochloric acid is a strong acid that is often found in a dilute aqueous solution.

2. **Role of Concentrated Sulfuric Acid (H₂SO₄)**:
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