NDA Chemistry · Acids, Bases and Salts
Acid-Base Theory, Oxides and Electrolytes
The three definitions of acids and bases (Arrhenius, Bronsted-Lowry, Lewis), how oxides are sorted into acidic, basic, neutral and amphoteric, and why some solutions conduct electricity while others do not.
Why this matters
The foundation of the chapter — seven PYQs, mostly testing one fact each: name the concept behind a definition, count the Lewis acids in a list, pick the neutral oxide, or pick the non-conducting solution. Learn the three definitions and the oxide buckets and the rest of the chapter has a frame to hang on.
Concept 1 of 6
The three acid-base concepts: Arrhenius, Bronsted-Lowry and Lewis
Intuition
Chemists defined 'acid' and 'base' three times, each definition wider than the last. Arrhenius talks about H+ and OH- in water; Bronsted-Lowry talks about giving and taking a proton (H+); Lewis talks about giving and taking an electron pair. Each later definition contains the earlier one — a Lewis acid is the most general kind.
Definition
The three definitions of acids and bases:
- Arrhenius — an acid dissociates in water to give H+ (aq) ions; a base dissociates in water to give OH- (aq) ions. Limited to aqueous solutions.
- Bronsted-Lowry — an acid is a proton (H+) donor; a base is a proton acceptor. Works beyond water.
- Lewis — an acid is an electron-pair acceptor; a base is an electron-pair donor. The most general definition.
Worked example
Ammonia (NH3) reacts with boron trifluoride (BF3): the nitrogen lone pair fills boron's empty orbital. Classify NH3 and BF3 using the Lewis definition.
Practice this concept4 quick reps
From the bank · past-year question
Consider the following statement: "Acids are substances that dissociate in water to give hydrogen ions H+ (aq), and bases are substances that produce hydroxyl ions OH- (aq)."
The above statement is based on which one of the following concepts?
[Q81 · Apr · 2026]
Match the definition to the right concept
'Acids give H+ and bases give OH- in water' is the Arrhenius concept, not Bronsted-Lowry. Bronsted-Lowry is the proton-transfer (donor/acceptor) one; Lewis is the electron-pair one.
Concept 2 of 6
Identifying Lewis acids
Intuition
A Lewis acid is anything hungry for an electron pair. The two tell-tale signs: an incomplete octet (the central atom has fewer than 8 electrons) or empty d-orbitals on a metal ion. Spot those and you have a Lewis acid.
Definition
A Lewis acid accepts an electron pair. Common Lewis acids and why:
- BF3 and AlCl3 — the central atom (B, Al) has an incomplete octet, so it accepts a pair.
- FeCl3 — Fe(III) has empty d-orbitals to accept a pair.
- NH3 is a Lewis base, not an acid — its nitrogen has a lone pair to donate.
Worked example
From the list CO2, BCl3, H2O and NH3, identify the Lewis acid.
Practice this concept4 quick reps
From the bank · past-year question
Consider the following:
I.
II.
III.
IV.
How many of the above are Lewis acids?
[Q87 · Apr · 2026]
NH3 is a base, not an acid
In a 'how many are Lewis acids?' list, NH3 is the base — its nitrogen lone pair is donated. AlCl3, BF3 and FeCl3 are the Lewis acids (all electron-pair acceptors).
Concept 3 of 6
Basicity of acids (monobasic, dibasic, tribasic)
Intuition
The 'basicity' of an acid is just how many replaceable (ionisable) hydrogen ions it can give per molecule. One H+ is monobasic, two is dibasic, three is tribasic. Count the acidic hydrogens.
Definition
Basicity = number of replaceable H+ ions an acid releases per molecule:
- Monobasic (1 H+): HCl, HNO3.
- Dibasic (2 H+): H2SO4, H2CO3.
- Tribasic (3 H+): H3PO4 (phosphoric acid).
Worked example
What is the basicity of sulphuric acid (H2SO4), and what is the term for it?
Practice this concept4 quick reps
From the bank · past-year question
Which one of the following is a tribasic acid?
[Q80 · Sep · 2018]
Tribasic = phosphoric acid
Among HCl, HNO3, H2SO4 and H3PO4, the tribasic one is phosphoric acid (H3PO4) — three replaceable H+. HCl and HNO3 are monobasic; H2SO4 is dibasic.
Concept 4 of 6
Classification of oxides: acidic, basic, neutral and amphoteric
Intuition
An oxide is an element bonded to oxygen, and how it behaves with acid or base sorts it into four buckets. Non-metal oxides are usually acidic; metal oxides are usually basic; a few non-metal oxides react with neither (neutral); and a few react with both (amphoteric).
Definition
The four oxide buckets:
- Acidic oxides — react with bases to form salts; mostly non-metal oxides: CO2, SO2, NO2.
- Basic oxides — react with acids to form salts; mostly metal oxides: Na2O, MgO, CaO.
- Neutral oxides — react with neither acids nor bases: CO, N2O, NO, H2O.
- Amphoteric oxides — react with both acids and bases: Al2O3, ZnO.
| Type | Reacts with | Examples |
|---|---|---|
| Acidic oxide | Bases (forms a salt) | CO2, SO2, NO2 |
| Basic oxide | Acids (forms a salt) | Na2O, MgO, CaO |
| Neutral oxide | Neither acids nor bases | CO, N2O, NO, H2O CO (carbon monoxide) is the neutral oxide the bank loves — CO2 in contrast is acidic. |
| Amphoteric oxide | Both acids and bases | Al2O3, ZnO |
Non-metal oxides tend to be acidic; metal oxides tend to be basic; CO/N2O/NO are neutral; Al2O3/ZnO are amphoteric.
Practice this conceptself-check · 4 quick reps
From the bank · past-year question
Which one of the following oxides is a neutral oxide?
[Q76 · Apr · 2025]
CO is neutral, CO2 is acidic
Carbon monoxide (CO) is a neutral oxide — it forms no salt with acids or bases. Do not confuse it with CO2, which is acidic (it forms carbonic acid / carbonates).
Concept 5 of 6
Electrolytes and electrical conductivity of solutions
Intuition
A solution conducts electricity only if it contains free ions. Acids, bases and salts dissolve into ions (electrolytes), so their solutions conduct. Covalent compounds like sugar and alcohol dissolve as whole molecules — no ions, no conduction.
Definition
What conducts and what does not:
- Strong electrolytes (conduct well) — strong acids (HCl), strong bases (NaOH), and salts (NaCl, CuSO4). They ionise fully in water.
- Weak electrolytes (conduct poorly) — weak acids like CH3COOH (acetic acid). They ionise only partly.
- Non-electrolytes (do NOT conduct) — sugar, CH3OH (methanol), and other covalent molecules that dissolve without forming ions.
- A base dissolved in water that conducts and turns the solution basic is NaOH (gives Na+ and OH- ions).
| Substance in water | Conducts? | Reason |
|---|---|---|
| NaOH (sodium hydroxide) | Yes (basic solution) | Strong electrolyte — gives Na+ and OH- |
| NaCl, CuSO4 (salts) | Yes | Ionise fully into free ions |
| HCl (strong acid) | Yes | Ionises fully into H+ and Cl- |
| CH3COOH (acetic acid) | Weakly | Weak electrolyte — partial ionisation |
| Sugar | No | Non-electrolyte — dissolves as whole molecules, no ions Sugar is the classic non-conducting solution — it is covalent and produces no ions. |
| CH3OH (methanol) | No | Non-electrolyte — covalent, no ions |
Practice this concept4 quick reps
From the bank · past-year question
Which one of the following solutions is not capable of conducting electricity?
[Q70 · Sep · 2022]
Sugar dissolves but does not ionise
Sugar dissolves freely in water, but it dissolves as neutral molecules, not ions — so its solution does not conduct electricity. Salts, strong acids and strong bases all produce ions and DO conduct.
Concept 6 of 6
The first mineral acid discovered
Intuition
A mineral acid is an inorganic acid derived from minerals — HCl, HNO3, H2SO4, H3PO4. The bank tests one historical fact here: which of these was discovered first.
Definition
The mineral acids and the historical fact:
- The common mineral (inorganic) acids are HCl, HNO3, H2SO4 and H3PO4.
- Nitric acid (HNO3) was the first mineral acid to be discovered — credited to the Arab alchemist Jabir ibn Hayyan (around the 8th century).
| Mineral acid | Formula | Note |
|---|---|---|
| Nitric acid | HNO3 | The first mineral acid discovered Nitric acid is the bank's answer for 'first mineral acid discovered'. |
| Hydrochloric acid | HCl | Mineral acid — discovered later |
| Sulphuric acid | H2SO4 | 'King of chemicals', dibasic |
| Phosphoric acid | H3PO4 | Tribasic mineral acid |
Practice this concept3 quick reps
From the bank · past-year question
Which one of the following was the first mineral acid discovered?
[Q148 · Apr · 2020]
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (3)
Classification of oxides: acidic, basic, neutral and amphoteric4 rows
| Type | Reacts with | Examples |
|---|---|---|
| Acidic oxide | Bases (forms a salt) | CO2, SO2, NO2 |
| Basic oxide | Acids (forms a salt) | Na2O, MgO, CaO |
| Neutral oxide | Neither acids nor bases | CO, N2O, NO, H2O CO (carbon monoxide) is the neutral oxide the bank loves — CO2 in contrast is acidic. |
| Amphoteric oxide | Both acids and bases | Al2O3, ZnO |
Non-metal oxides tend to be acidic; metal oxides tend to be basic; CO/N2O/NO are neutral; Al2O3/ZnO are amphoteric.
Electrolytes and electrical conductivity of solutions6 rows
| Substance in water | Conducts? | Reason |
|---|---|---|
| NaOH (sodium hydroxide) | Yes (basic solution) | Strong electrolyte — gives Na+ and OH- |
| NaCl, CuSO4 (salts) | Yes | Ionise fully into free ions |
| HCl (strong acid) | Yes | Ionises fully into H+ and Cl- |
| CH3COOH (acetic acid) | Weakly | Weak electrolyte — partial ionisation |
| Sugar | No | Non-electrolyte — dissolves as whole molecules, no ions Sugar is the classic non-conducting solution — it is covalent and produces no ions. |
| CH3OH (methanol) | No | Non-electrolyte — covalent, no ions |
The first mineral acid discovered4 rows
| Mineral acid | Formula | Note |
|---|---|---|
| Nitric acid | HNO3 | The first mineral acid discovered Nitric acid is the bank's answer for 'first mineral acid discovered'. |
| Hydrochloric acid | HCl | Mineral acid — discovered later |
| Sulphuric acid | H2SO4 | 'King of chemicals', dibasic |
| Phosphoric acid | H3PO4 | Tribasic mineral acid |
Watch out for (5)
- Match the definition to the right concept→ The three acid-base concepts: Arrhenius, Bronsted-Lowry and Lewis
- NH3 is a base, not an acid→ Identifying Lewis acids
- Tribasic = phosphoric acid→ Basicity of acids (monobasic, dibasic, tribasic)
- CO is neutral, CO2 is acidic→ Classification of oxides: acidic, basic, neutral and amphoteric
- Sugar dissolves but does not ionise→ Electrolytes and electrical conductivity of solutions
Drill every past-year question on this subtopic
7 questions from the bank — paginated, with cart and Word-export support.