🌙 Switch to night

19.3 Oxides – Binary compounds with oxygen

Basic • Acidic • Amphoteric • Neutral oxides

19.3 Oxides

Binary compounds of elements with oxygen are called oxides. Oxygen shows an oxidation state of -2 in these oxides. Metal oxides are commonly basic and amphoteric while non-metallic oxides are acidic in nature. Both basic and acidic oxides are formed when metals and non-metals are heated respectively in the presence of air or oxygen. Metal oxides are typically ionic compounds in which electrons are transferred from metals to oxygen. Non-metal oxides are covalent compounds where electrons are shared between non-metals and oxygen atom.

(i) Basic Oxides

Oxygen reacts with metals to give oxides which when dissolved in water produce hydroxides. They change red litmus blue. When treated with acids they give salts.

O₂(g) + 4Na(s) → 2Na₂O(s)
Na₂O(s) + H₂O(l) → 2NaOH(aq)
O₂(g) + 2Ca(s) → 2CaO(s)
CaO(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l)
O₂(g) + 2Cu(s) → 2CuO(s)
CuO(s) + 2HCl(aq) → CuCl₂(aq) + H₂O(l)

(ii) Acidic Oxides

Oxygen combines with non metals (S, C, N) to give acidic oxides. These oxides react with water to give acids which turn blue litmus red. Acidic oxides react with bases to give salts.

O₂(g) + S(s) → SO₂(g) (Sulphur dioxide)
SO₂(g) + H₂O(l) → H₂SO₃(l) (Sulphurous Acid)
O₂(g) + C(s) → CO₂(g)
CO₂(g) + H₂O(l) → H₂CO₃(aq) (Carbonic Acid)
SO₂(g) + 2NaOH(aq) → Na₂SO₃(aq) + H₂O(l)
CO₂(g) + Ca(OH)₂(aq) → CaCO₃(s) + H₂O(l)

(iii) Neutral Oxides

Neutral oxides are those oxides which on contact with water produce neither an acid nor a base. Their aqueous solutions have no action on blue or red litmus paper. Examples are carbon monoxide (CO), nitric oxide (NO) and nitrous oxide (N₂O).

(iv) Amphoteric Oxide

Amphoteric oxides are usually formed when oxygen reacts with less electropositive metals. These oxides behave both as an acid and a base. Zinc oxide and aluminium oxide behave as bases in the presence of an acid and as acids in the presence of an alkali. These oxides are insoluble in water and have no action on litmus paper.

O₂(g) + 2Zn(s) → 2ZnO(s)
3O₂(g) + 4Al(s) → 2Al₂O₃(s)
ZnO(s) + 2HCl(aq) → ZnCl₂(aq) + H₂O(l)
Al₂O₃(s) + 6HCl(aq) → 2AlCl₃(aq) + 3H₂O(l)
ZnO(s) + 2NaOH(aq) + H₂O(l) → Na₂[Zn(OH)₄](aq) (Sodium zincate)
Al₂O₃(s) + 2NaOH(aq) + 3H₂O(l) → 2Na[Al(OH)₄](aq) (Sodium aluminate)

Interesting Information: Water is an amphoteric substance. It can act as both an acid and a base, depending on the other substance it reacts with.

📌 19.3 Quick Check (solutions)

1. Why is lead oxide called an amphoteric oxide? Because it reacts with both acids (like HCl) to form salt and water, and with bases (like NaOH) to form sodium plumbite. It shows both basic and acidic behavior.

2. Why do non-metals form covalent oxides? Non-metals have high electronegativity and tend to share electrons with oxygen rather than transfer them completely, forming covalent bonds. Examples: CO₂, SO₂, NO₂.

🧠 Memorization & tips

🧪 Basic oxides
Metal oxides (Na₂O, CaO, CuO) → with water give base; with acids → salt + water. Turn red litmus blue.

☁️ Acidic oxides
Non-metal oxides (SO₂, CO₂) → with water give acids; with bases → salt + water. Turn blue litmus red.

⚖️ Amphoteric
ZnO, Al₂O₃, PbO react with both acid and alkali. Water (H₂O) is amphoteric!

😐 Neutral oxides
CO, NO, N₂O – no reaction with water, no effect on litmus.

📝 Key reactions
Na₂O + H₂O → NaOH; CaO + HCl → CaCl₂ + H₂O; SO₂ + NaOH → Na₂SO₃ + H₂O; ZnO + NaOH + H₂O → sodium zincate

📚 Teacher’s Lesson Planner (19.3 Oxides)

Session	Learning objectives	Activities & resources	Assessment
Day 1	Define oxides; oxidation state -2; classification into basic, acidic, amphoteric, neutral	Discuss binary compounds; show examples; introduce ionic vs covalent character	Classify given formulas: MgO, SO₃, CO, Al₂O₃
Day 2	Basic oxides: formation, reaction with water and acids, litmus test	Demonstrate with Na₂O, CaO, CuO equations; explain ionic nature	Write products: K₂O + H₂O, MgO + HNO₃
Day 3	Acidic oxides: formation (S, C), reaction with water and bases, litmus test	Show SO₂, CO₂ reactions; discuss covalent bonding; environmental aspects	Predict: SO₃ + H₂O, CO₂ + KOH
Day 4	Amphoteric oxides: ZnO, Al₂O₃ – reactions with acid and alkali; neutral oxides (CO, NO, N₂O)	Write dual behavior equations; explain no reaction for neutral oxides	Quick check 19.3; classify PbO, CO, NO
Day 5	Review all oxide types; interesting fact: water is amphoteric	Solve mixed examples; discuss real-life applications	10 MCQ quiz & key analysis

⏱️ Each session ~40-45 min. Use @everexams.com quiz for instant feedback.

📝 Self check (10 MCQs: Oxides)

Click on option — correct turns green, wrong turns red. Use submit to see score & key.