Oxides

A metal oxide with acid gives a salt and water only — no gas, no fizzing. The trap is copying the metal reaction: Zn + HCl fizzes and releases hydrogen, but ZnO + HCl merely dissolves the solid to a colourless solution. Hydrogen or oxygen from ZnO + HCl is impossible — an oxide has no hydrogen to give. For an observation write "solid dissolves, no bubbles".

"Describe what is observed" is marked on visible changes, not names of products. For CuO + dilute H₂SO₄ write "the black solid dissolves to give a blue solution"; "copper(II) sulfate is formed" scores zero — it names a product. For any basic oxide + acid: the solid dissolves; the solution may turn coloured (blue for Cu²⁺); no gas forms.

Amphoteric and neutral are opposites. Neutral oxides (CO, NO, H₂O) react with neither acids nor alkalis; amphoteric oxides (ZnO, Al₂O₃) react with both. "Zinc oxide is neutral because it reacts with both and they cancel" is wrong and scores zero. Correct: ZnO reacts with acids to a salt and water (as a base) and with alkalis to a salt and water (as an acid).

Carbon monoxide (CO) is a neutral oxide — it does not react with acids or alkalis and does not form an acidic solution. Candidates wrongly call it acidic because carbon also forms CO₂, which is acidic; but CO and CO₂ are different oxides with different behaviour. The other neutral oxides to remember are water (H₂O) and nitrogen monoxide (NO).

Wrong ionic charges break oxide equations. Copper, magnesium, zinc and calcium are 2+, so their chlorides are CuCl₂, MgCl₂, ZnCl₂, CaCl₂ (not CuCl or CaCl), and each needs 2HCl. Aluminium is 3+: Al₂O₃ has two Al, so it needs 6HCl to give 2AlCl₃ + 3H₂O. Sodium is 1+, so Na₂O gives 2NaCl. Set the formula from the ion charge first, then balance.