Acidic and basic oxides
Why an acid can dissolve rust
1. Acidic and basic oxides: from zero
2. Acid vs rust
Rust is an iron oxide. Dilute hydrochloric acid can dissolve it. Iron metal fizzes in this acid, giving hydrogen gas.
Pick the right answer
Predict: will rust fizz and give hydrogen gas in the acid, like iron metal?
- A. Yes, it fizzes, because rust contains iron
- B. No, it dissolves without giving a gas
- C. No, nothing happens to rust in an acid
الشرح
Rust dissolves in the acid without giving off hydrogen gas. Rust is an iron oxide, not free iron metal: the acid's hydrogen combines with the oxide's oxygen to form water, and a salt forms with it.
3. Basic oxides
Basic oxide
A metal oxide that reacts with an acid to give a salt and water.
CuO + H2SO4 → CuSO4 + H2O
- Metal → Metal oxide (basic oxide) (+ oxygen)
- Metal oxide (basic oxide) → Salt + water (+ acid)
A basic oxide acts like a base. So it neutralizes the acid: salt and water, no gas.
4. Acidic oxides
Acidic oxide
A non-metal oxide that dissolves in water to give an acid.
SO3 + H2O → H2SO4
Basic oxides
Most metal oxides: Na₂O, MgO, CuO
Acidic oxides
Most non-metal oxides: CO₂, SO₃
5. Classify and predict
Sort into groups
Basic oxide or acidic oxide?
K₂OCaOFeOSO₂P₂O₅
Fill in the blanks
A basic oxide reacts with an acid to give a salt and _____.
الشرح
The oxide's O and the acid's H form water.
6. Summary
- Most metal oxides are basic oxides: basic oxide + acid → salt + water.
- Most non-metal oxides are acidic oxides: they dissolve in water to give acids.
- Rust is an iron oxide, a basic oxide, so an acid dissolves it.
Can you explain why an acid dissolves copper (II) oxide but gives no hydrogen gas?
Reveal
CuO + H2SO4 → CuSO4 + H2O
Copper (II) oxide is a basic oxide, not free metal: it neutralizes the acid to give a salt and water. The acid's H joins the oxide's O to form water, so no gas is given off.