Question

Why do magnesium and manganese only liberate hydrogen gas with very dilute nitric acid?

Answer 1

Because nitric acid is highly oxidsing agent,
It directly convert hydrogen into water.

Answer 2

Heya user

Here's your answer

Magnesium and magnesium are metals
Then dilute nitric is acid
We know that,
Metal + Acid⇒Salt + Hydrogen(the gas is liberated)
Here though it is dilute nitric acid the hydrogen gas is liberated in all cases.

Have a great day ahead ^_^