What happens when magnesium or manganese is treated with very dil HNO3​

Metals also react with nitric acid, but hydrogen gas is not evolved, because nitric acid is a strong oxidising agent. So, it oxidises the hydrogen to water and itself gets reduced to any nitrogen oxide. However, magnesium and manganese react with dilute nitric acid to evolve hydrogen gas.

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acid rain does not usually kill trees directly. instead, it is more likely to weaken the trees by damaging their leaves, limiting the nutrients available to them, or poisoning them with toxic substances slowly released from the soil. the main atmospheric pollutants that affect trees are nitrates and sulphates.