Q.Select the correct one: Which is most acidic oxide
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What Are Acidic and Basic Oxides? Let's Start With What You Already Know
You know that lemon juice is acidic and soap is basic. You also know that when you burn something — wood, coal, magnesium — you get an ash or a powder. That powder is an oxide (a compound of the element with oxygen).
Now here's the key question: What happens when you dissolve that oxide in water? Does the solution turn acidic or basic? That single question is the entire idea behind acidic and basic oxides.
The Intuition: It's About the Element's "Personality"
Think of elements as having a personality:
- Metals (like sodium, calcium, iron) are "basic" in nature. When their oxides dissolve in water, they produce a basic (alkaline) solution.
- Non-metals (like carbon, sulfur, nitrogen) are "acidic" in nature. When their oxides dissolve in water, they produce an acidic solution.
This is not a random rule — it follows from how metals and non-metals bond with oxygen. Metals give electrons easily, so their oxides tend to accept protons (behave as bases). Non-metals share electrons more equally, so their oxides tend to donate protons (behave as acids).
The Precise Definitions
Acidic Oxide
An oxide that reacts with water to form an acid, or reacts with a base to form a salt and water.
Examples:
- CO2+H2O→H2CO3 (carbonic acid — weak acid, but still acidic)
- SO3+H2O→H2SO4 (sulfuric acid — strong acid)
- N2O5+H2O→2HNO3 (nitric acid)
Basic Oxide
An oxide that reacts with water to form a base, or reacts with an acid to form a salt and water.
Examples:
- Na2O+H2O→2NaOH (sodium hydroxide — strong base)
- CaO+H2O→Ca(OH)2 (calcium hydroxide — lime water)
- MgO+H2O→Mg(OH)2 (magnesium hydroxide — milk of magnesia)
Acidic oxide + water → acid
Basic oxide + water → base
The Big Picture: A Simple Table
| Type | Made from | Reacts with water to give | Example |
|---|---|---|---|
| Acidic oxide | Non-metals | Acid | CO2, SO2, SO3, NO2, P2O5 |
| Basic oxide | Metals | Base | Na2O, CaO, MgO, K2O, CuO |
Not all oxides fit neatly into these two boxes. Some are amphoteric (can act as both acid and base — e.g., Al2O3, ZnO). Some are neutral (no reaction with water or acids/bases — e.g., CO, NO). But for a first meeting, focus on the clear-cut cases above.
Why This Matters for Exams
In Indian board exams (CBSE, ICSE, state boards), you will be asked to:
- Classify given oxides as acidic or basic.
- Write balanced equations for their reactions with water.
- Predict the nature of an oxide based on the element's position in the periodic table. …
Among a series of oxides of the same element, acidic character increases as the oxidation state (and number of oxygen atoms) of the central atom increases, so the chlorine oxide with chlorine in its highes …
Acidity of an element's oxides rises with the oxidation state of that element; Cl in Cl2O7 is +7, the highest among the four, so Cl2O7 is the most acidic oxide (it is the anhydride of perchloric acid, HClO4, the strongest of the chlorine oxoacids).
Oxidation states of chlorine in the given oxides:
- Cl2O: Cl is +1
- Cl2O3: Cl is +3
- Cl2O5: Cl is +5
- Cl2O7: Cl is +7 …
- CBSE 2026Set sz1 markMCQQ.Select the correct one: Which is most acidic oxide(a) Cl2O(b) Cl2O3(c) Cl2O5(d) Cl2O7
›Reveal solutionSolution
Acidity of an element's oxides rises with the oxidation state of that element; Cl in Cl2O7 is +7, the highest among the four, so Cl2O7 is the most acidic oxide (it is the anhydride of perchloric acid, HClO4, the strongest of the chlorine oxoacids).
Oxidation states of chlorine in the given oxides:
- Cl2O: Cl is +1
- Cl2O3: Cl is +3
- Cl2O5: Cl is +5
- Cl2O7: Cl is +7 …
- CBSE 2025Set hz1 markMCQQ.Select the correct one: Which one of the following is most acidic?(a) Al2O3(b) Na2O(c) MgO(d) CaO
›Reveal solutionSolution
Among the four oxides, Na2O, MgO and CaO are basic oxides of strongly electropositive s-block metals, while Al2O3 is amphoteric and is the most acidic of the group.
Across a period, as nuclear charge increases and metallic character decreases, oxides become progressively less basic and more acidic:
Na2O (strongly basic) < MgO (basic) < Al2O3 (amphoteric) < SiO2 (acidic).
Na2O is the oxide of an alkali metal (Na) — strongly basic.
MgO and CaO are oxides of alkaline earth metals — basic, though CaO (from a more electropositive, larger metal) is even more basic than MgO. …
- CBSE 2023Set ANNUAL1 markMCQQ.Which one of the following is an amphoteric oxide?(a) MgO(b) Al2O3(c) K2O(d) CuO
›Reveal solutionSolution
Al2O3 is amphoteric because it reacts with both acids and bases; MgO and K2O are basic oxides, CuO is a basic oxide too.
An amphoteric oxide is one that can behave as both an acid and a base, i.e., it reacts with both acids and bases to give salt and water.
- MgO: basic oxide (reacts only with acids: MgO + 2HCl -> MgCl2 + H2O). …
- CBSE 2022Set ANNUAL1 markMCQQ.The atomic number of an element is 11. The nature of its oxide will be:(a) Acidic(b) Basic(c) Amphoteric(d) Neutral
›Reveal solutionSolution
The element with atomic number 11 is sodium; its oxide Na2O dissolves in water to give a basic (alkaline) solution.
An element with atomic number 11 is Sodium (Na), a highly electropositive Group 1 (alkali) metal. Metallic oxides of strongly electropositive elements are basic in nature, because they react with water to form hydroxides that release OH- ions: Na2O + H2 …
- CBSE 2022Set ANNUAL1 markMCQQ.In which of the following is Amphoteric oxide?(a) B2O3(b) ZnO(c) Na2O(d) SO2
›Reveal solutionSolution
ZnO is amphoteric (reacts with acid and base) — option (b).
Nature of oxides (NCERT Class 11 Chemistry, Periodicity):
- B₂O₃, SO₂: acidic oxides.
- Na₂O: basic oxide.
- ZnO: amphoteric — …
- CBSE 2022Set ANNUAL1 markMCQQ.In which of the following is neutral oxide?(a) CO(b) SnO2(c) ZnO(d) SiO2
›Reveal solutionSolution
CO is a neutral oxide — option (a).
Classification of oxides (NCERT Class 11 Chemistry, Periodicity):
- CO: neutral oxide — does not react with acids or bases to form salts.
- SnO₂, ZnO: amphoteric. …
- CBSE 2022Set ANNUAL1 markMCQQ.Which is maximum basic in the following?(a) Na2O(b) BaO(c) As2O3(d) Al2O3
›Reveal solutionSolution
Na₂O is the most basic oxide of the set — option (a).
Nature of oxides (NCERT Class 11 Chemistry, Periodicity): the more electropositive (metallic) the element, the more basic its oxide.
- Na₂O: strongly basic (alkali-metal oxide, gives NaOH with water).
- BaO: basic, but less so than Na₂O.
- Al₂O₃: amphoteric. …
- CBSE 2020Set ANNUAL1 markQ.Answer in one word/sentence: Write the nature of Al2O3.
›Reveal solutionSolution
Aluminium oxide (Al2O3) can behave as both an acid and a base, so it is classified as an amphoteric oxide.
Oxides can be classified by how they react with acids and bases:
- Basic oxides react only with acids (e.g. Na2O).
- Acidic oxides react only with bases (e.g. SO2, CO2).
- Amphoteric oxides react with BOTH acids and bases.
Aluminium oxide, Al2O3, reacts with acids to form aluminium salts and water:
Al2O3 + 6HCl -> 2AlCl3 + 3H2O
…
- CBSE 2018Set ANN1 markMCQQ.Which is the acidic oxide among the following?(a) Cl2O7(b) Na2O(c) Al2O3(d) CO
›Reveal solutionSolution
Non-metal oxides are acidic; Cl2O7 (of chlorine) is acidic, Na2O is basic, Al2O3 amphoteric, CO neutral. Answer: (a).
Nature of oxides varies across a period: metallic oxides are basic, non-metallic oxides acidic, and some (like Al2O3) amphoteric. …
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