Q.Illustrate how copper metal can give different products on reaction with HNO3.

Step 1: Copper with dilute nitric acid.

Here nitrogen is reduced from $+5$ (in $HNO_3$) to $+2$ (in NO), a colourless gas that turns brown in air ($2NO + O_2 \rightarrow 2NO_2$).

Step 2: Copper with concentrated nitric acid.

Here nitrogen is reduced only to $+4$ (in $NO_2$, reddish-brown gas), since concentrated $HNO_3$ is a less selective, more vigorous oxidiser and the reaction proceeds without the deeper reduction seen with the dilute acid.

Step 3: Common feature. …

Write the two separate balanced equations for Cu with dilute HNO3 (product NO) and concentrated HNO3 (product NO2), noting the …

• A very common error is mixing up which acid gives which oxide of nitrogen — dilute gives NO, concentrated gives NO₂ (not the reverse).
• Do not write H₂ as a product — HNO₃ is oxidising, so it never releases hydrogen gas with metals the way a non-oxidising acid (like dilute H₂SO₄) would. …