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Acidic, Basic, Neutral and Amphoteric Oxides Explained

Chemistry · Periodic Classification Of Properties · NEET

Metal oxides (like Na2O, CaO) are basic. Non-metal oxides (like CO2, Cl2O7) are acidic. A few in-between oxides such as Al2O3, ZnO, BeO and PbO react with BOTH acids and bases, so they are amphoteric. Some oxides (CO, NO, N2O) do not react with acid or base, so they are neutral. Memory hook: "Metal = Mom (basic), Non-metal = Sour (acidic), Al-Zn-Be-Pb-Sn = the middle kids (amphoteric)."
Nature of Oxides Across Period 3metallic character decreases, non-metallic character increasesNa2O, MgOBASICAl2O3AMPHOTERICSO3, Cl2O7ACIDICmetal oxidesmiddlenon-metal oxidesNeutral oxides (CO, NO, N2O, H2O) form no salt
Across Period 3, oxides shift from basic (Na2O, MgO) to amphoteric (Al2O3) to acidic (SO3, Cl2O7) as the element becomes less metallic. Neutral oxides like CO and NO sit outside this scale because they form no salt.

Your doubts, answered

Is Al2O3 acidic, basic or amphoteric?

Al2O3 is amphoteric. This means it reacts with both acids and bases. With acid: Al2O3 + 6HCl -> 2AlCl3 + 3H2O (acts as a base). With a strong base: Al2O3 + 2NaOH -> 2NaAlO2 + H2O (acts as an acid). NEET loves testing Al2O3 as amphoteric in match-the-column questions.

What is the simple list of amphoteric oxides I must remember for NEET?

Remember these: BeO, Al2O3, ZnO, PbO, PbO2, SnO, SnO2, Ga2O3, Sb2O3, As2O3, and Cr2O3. A quick memory chain is 'Be-Al-Zn-Pb-Sn' (the metals near the metal/non-metal line). If an oxide is in this list, it reacts with both acids and bases.

Which oxides are neutral and why?

CO, NO, N2O and H2O are neutral oxides. They do not react with acids or bases to form salts. So they are neither acidic nor basic. Do NOT confuse them: CO is neutral but CO2 is acidic; NO/N2O are neutral but NO2 and N2O5 are acidic. NEET often traps you by mixing CO with CO2.

Why are metal oxides basic but non-metal oxides acidic?

Metals are electropositive (they give electrons easily), so their oxides are ionic and release OH- ions in water, making them basic (e.g. Na2O + H2O -> 2NaOH). Non-metals are electronegative, so their oxides are covalent and form acids in water, making them acidic (e.g. CO2 + H2O -> H2CO3). So the character of the oxide follows the metallic/non-metallic character of the element.

How does oxide character change across a period and down a group?

Across a period (left to right), metallic character decreases, so oxides change from basic -> amphoteric -> acidic (e.g. Na2O basic, Al2O3 amphoteric, Cl2O7 acidic in period 3). Down a group, metallic character increases, so oxides become MORE basic (e.g. in Group 2: BeO is amphoteric, but CaO and BaO are strongly basic).

For the same element, why does the higher oxide become more acidic?

When one element forms many oxides, the oxide with the HIGHER oxidation state is more acidic. Example for manganese: MnO is basic, Mn2O7 is strongly acidic. Same for nitrogen: N2O/NO neutral, N2O3 and N2O5 acidic. Reason: higher positive charge pulls electrons harder, making the O-H bond release H+ more easily.

⚠️ The NEET trap
✗ CO2 and CO are both acidic because both come from carbon.
✓ CO2 is acidic (forms carbonic acid), but CO is a NEUTRAL oxide (it does not form a salt with acid or base). Character depends on the oxide, not just the element.
🧠 NEET pairs CO with CO2 to trick you. Remember: CO = neutral, CO2 = acidic. Same trap for NO/N2O (neutral) vs NO2/N2O5 (acidic).

Real NEET questions

NEET 2020

Match the oxides with their nature. Column-I: (a) CO, (b) BaO, (c) Al2O3, (d) Cl2O7. Column-II: (i) Basic, (ii) Neutral, (iii) Acidic, (iv) Amphoteric.

A · (a)-iii; (b)-iv; (c)-i; (d)-ii
B · (a)-iv; (b)-iii; (c)-ii; (d)-i
C · (a)-i; (b)-ii; (c)-iii; (d)-iv
D · (a)-ii; (b)-i; (c)-iv; (d)-iii ✓
Solution: CO is neutral (a-ii). BaO is an s-block metal oxide, so it is basic (b-i). Al2O3 reacts with both acids and bases, so it is amphoteric (c-iv). Cl2O7 has chlorine in its highest oxidation state (+7), so it is acidic (d-iii). Correct option is D.
NEET 2023 (Phase 2)

The correct sequence containing a neutral, an acidic, a basic and an amphoteric oxide respectively is:

A · NO, CO2, ZnO, CaO
B · NO, CO2, CaO, ZnO ✓
C · NO, ZnO, CO2, CaO
D · ZnO, NO, CaO, CO2
Solution: Classify each: NO = neutral, CO2 = acidic, CaO = basic (s-block metal), ZnO = amphoteric. The required order neutral, acidic, basic, amphoteric is NO, CO2, CaO, ZnO. Correct option is B.
NEET 2018

Which one of the following oxides is most acidic in nature?

A · BaO
B · BeO ✓
C · MgO
D · CaO
Solution: Down Group 2, metallic character increases, so basic character of oxides increases: BeO < MgO < CaO < BaO. Therefore BeO is the least basic / most acidic (it is in fact amphoteric), while the rest are basic. Correct option is B.

Solved Periodic Classification Of Properties NEET PYQs

Try the real previous-year questions from this chapter — each with the answer and a full solution.

See all 28 Periodic Classification Of Properties NEET PYQs ›
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Frequently asked

Is ZnO acidic or basic?

ZnO is amphoteric. It reacts with acids (ZnO + 2HCl -> ZnCl2 + H2O) and with strong bases (ZnO + 2NaOH -> Na2ZnO2 + H2O). It is one of the most tested amphoteric oxides in NEET.

Which is the most acidic oxide among common examples?

Oxides where the element is in its highest oxidation state are most acidic, such as Cl2O7, Mn2O7 and SO3. For a fixed group, the smallest/most non-metallic member gives the more acidic oxide.

Is water acidic, basic or neutral?

Pure water (H2O) is a neutral oxide. It is neither acidic nor basic and has a pH of about 7.

Are all Group 1 and Group 2 oxides basic?

Almost all are basic (Na2O, K2O, CaO, BaO). The main exception is BeO, which is amphoteric because beryllium is small and less metallic than the rest of Group 2.

Why does this topic matter for NEET?

Oxide nature appears almost every year, usually as a match-the-column or 'find the correct sequence' question. It is easy marks if you memorise the amphoteric list and the neutral oxides, and remember the period/group trend.

Related concepts:
Ionic Character Trends in Hydrides and HalidesMetallic, Non-metallic Character and Reactivity TrendsWhat Is Electronegativity and Its TrendsPeriodicity of Valence and Oxidation StatesWhat Is the Diagonal Relationship? (Li-Mg, Be-Al)
Practise Chemistry for NEET:
Chemistry MCQs (free, chapter-wise)Periodic Classification Of Properties PYQs