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Oxides: from bases to acids across one row

Your guide: Miss SanaCounts atoms the way you count rupees: carefully.

The problem

Sodium and phosphorus both sit in period 3, and both burn in oxygen. Drop sodium oxide in water and you get a strong base. Drop phosphorus oxide in water and you get an acid. Same row, same oxygen, opposite results.

And some oxides refuse to pick a side. Zinc oxide dissolves in an acid and in a base. The whole topic is one slide across the table: basic on the left, acidic on the right, and a middle that is both.

Metal oxides are basic: they give bases in water. Non-metal oxides are acidic: they give acids in water. Oxides that behave as both are amphoteric. Basic and acidic oxides react with each other to give salts.

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Notes, short questions and MCQs

Read the full notes: key terms, model answers and MCQs with answers

The problem

Sodium and phosphorus both sit in period 3, and both burn in oxygen. Drop sodium oxide in water and you get a strong base. Drop phosphorus oxide in water and you get an acid. Same row, same oxygen, opposite results.

And some oxides refuse to pick a side. Zinc oxide dissolves in an acid and in a base. The whole topic is one slide across the table: basic on the left, acidic on the right, and a middle that is both.

Metal oxides are basic: they give bases in water. Non-metal oxides are acidic: they give acids in water. Oxides that behave as both are amphoteric. Basic and acidic oxides react with each other to give salts.

Key terms

Basic oxides
The oxides of the alkali and alkaline earth metals, except beryllium, are basic and contain O2− ions. O2− cannot exist alone in water: it immediately takes a proton from water and forms OH−.
Acidic oxides
The oxides of non-metals such as carbon, nitrogen, phosphorus and sulphur are acidic. They generally dissolve in water to give acidic solutions.
Amphoteric oxides
Oxides of less electropositive elements such as BeO, Al2O3, Bi2O3 and ZnO act as bases towards strong acids and as acids towards strong bases.
Oxidation state rule
The oxidation state of the metal also matters: the acidity of its oxide increases with increasing oxidation state. For manganese: MnO < Mn2O3 < MnO2 < Mn2O7.

Short questions with model answers

  1. Q1. Sodium and phosphorus are in the same period, yet Na2O is basic and P2O5 is acidic. Give the reason.

    • Sodium is a metal; phosphorus is a non-metal.
    • Na2O + H2O → 2NaOH
    • Phosphorus oxide dissolves in water to give an acidic solution.

    Na is a metal, so its oxide is basic; P is a non-metal, so its oxide is acidic.

  2. Q2. Show with equations that zinc oxide is amphoteric.

    • ZnO + H2SO4 → ZnSO4 + H2O
    • ZnO + 2NaOH + H2O → Na2[Zn(OH)4]

    It reacts with both an acid and a base, so it is amphoteric.

  3. Q3. Arrange MnO, Mn2O3, MnO2 and Mn2O7 by acidity, using oxidation states.

    • MnO: x − 2 = 0 → +2 · Mn2O3: 2x − 6 = 0 → +3
    • MnO2: x − 4 = 0 → +4 · Mn2O7: 2x − 14 = 0 → +7
    • MnO < Mn2O3 < MnO2 < Mn2O7

    MnO < Mn2O3 < MnO2 < Mn2O7

Common mistakes

  • ✗ Writing that every metal oxide is basic.

    ✓ Oxides of less electropositive metals, such as BeO, Al2O3, Bi2O3 and ZnO, are amphoteric. ZnO being amphoteric is a true/false item in the exercise.

  • ✗ Applying “more basic down the group” to transition metal oxides too.

    ✓ That trend is for main-group metals (BeO < … < BaO). The book says the reverse trend is observed in transition metal oxides.

  • ✗ Saying the oxide ion, O2−, stays in water as it is.

    ✓ O2− has a high affinity for protons and cannot exist alone in water. It takes a proton from water at once and forms OH−, which is why the solution is basic.

MCQs

  1. 1. Metals form ______ oxides and non-metals form ______ oxides.

    1. (a) acidic, basic
    2. (b) basic, acidic
    3. (c) amphoteric, basic
    4. (d) neutral, acidic
    Show answer

    (b) Metals: basic oxides (bases in water). Non-metals: acidic oxides (acids in water).

  2. 2. True or false: zinc oxide is an example of an amphoteric oxide.

    1. (a) True
    2. (b) False
    Show answer

    (a) True: it reacts with H2SO4 and with NaOH.

  3. 3. Which of these oxides is the most basic?

    1. (a) BeO
    2. (b) MgO
    3. (c) CaO
    4. (d) BaO
    Show answer

    (d) BaO: basicity of main-group metal oxides increases down a group (BeO < MgO < CaO < SrO < BaO).

  4. 4. Which manganese oxide is the most acidic?

    1. (a) MnO
    2. (b) Mn2O3
    3. (c) MnO2
    4. (d) Mn2O7
    Show answer

    (d) Mn2O7: manganese is +7, the highest oxidation state, and acidity rises with oxidation state.

  5. 5. Na2O + SO3 → ?

    1. (a) NaOH
    2. (b) Na2SO4
    3. (c) H2SO4
    4. (d) Na2SO3
    Show answer

    (b) A basic oxide and an acidic oxide react to give a salt: sodium sulphate, Na2SO4.

Quick revision

  • Metal oxides are basic, non-metal oxides acidic, and oxides like BeO, Al2O3, Bi2O3 and ZnO amphoteric.
  • Across period 3: Na2O, MgO (basic) → Al2O3 (amphoteric) → P4O10, SO3, Cl2O7 (acidic).
  • Main-group oxides get more basic down a group; transition oxides the reverse; higher oxidation state means a more acidic oxide.