Answer:
I have two methods to distinguish between concentrated and dilute nitric acid
Method 1: Using magnesium/manganese.
Mg + 2HNO₃ (1%) → Mg(NO₃) + H₂
very very dilute
Mg + 4HNO₃ → Mg(NO₃)+ 2H₂O +2NO₂
concentrated
In the first reaction of magnesium with concentrated nitric acid, hydrogen is evolved.
In the second reaction of magnesium with very very dilute nitric acid, water is evolved.
Using the respective tests for hydrogen and water, we can test them.
Method 2: Using iron.
3 Fe + 8HNO₃→ 3Fe(NO₃)₂ + 4H₂O + 2NO
dilute
Fe + 6HNO₃→ Fe(NO₃)₃ + 3H₂O + 3NO₂
concentrated
In the first case of method 2,
Fe(NO₃)₂ ⇆ Fe²⁺ + (NO₃) ⁻ Iron with valency 2+ is formed
In the second case of method 2,
Fe(NO₃)₃ ⇄ Fe³⁺ + (NO₃) ⁻ Iron with valency 3+ is formed
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