Neutralize the Oxides
React HCl with metal oxides and hydroxides
What this lesson covers
Why it matters
The insoluble metal oxides and hydroxide shown here will not dissolve in water — but watch them vanish into clear salt solutions when hydrochloric acid attacks.
The idea in plain words
Analyze the stoichiometry: One ZnO unit requires two HCl molecules to balance the oxygen and hydrogen into water.
Predict first
What products form when a basic oxide reacts with an acid?
Acids neutralize bases (including oxides) to produce only a salt and water. No gases are evolved.
- Salt and water — correct
- Salt and hydrogen gas
- Salt and carbon dioxide
- Water and hydrogen gas
What you do
Add HCl to each solid. Note how the black CuO and white ZnO dissolve, while the white Mg(OH)2 also reacts. Identify the salts formed.
Check yourself
Which equation correctly represents the reaction of Zinc Oxide with Hydrochloric Acid?
Zinc has a valency of 2, forming ZnCl₂. Two HCl molecules are needed to provide two Cl⁻ ions and two H⁺ ions to form one H₂O molecule with the O²⁻ from ZnO.
What is the observable result when Copper(II) Oxide reacts with HCl?
Black CuO dissolves to form copper(II) chloride (CuCl₂), giving a blue-green (greenish-blue) solution. No gas is produced.
Why is the reaction between Mg(OH)₂ and HCl classified as neutralization?
Neutralisation is H⁺ from the acid combining with OH⁻/O²⁻ from the base to give a salt and water: Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O.
- ZnO + HCl → ZnCl + H₂O
- ZnO + 2HCl → ZnCl₂ + H₂O — correct
- ZnO + 2HCl → ZnCl₂ + H₂
- ZnO + HCl → ZnCl₂ + H₂O
- A blue-green (greenish-blue) solution forms — correct
- A colourless solution forms
- Bubbles of gas appear
- The solid remains black
- It produces a salt and water — correct
- It produces hydrogen gas
- It releases heat only
- It changes the pH to 7 only