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
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
Practice this concept4 quick reps
The same idea in a real exam question:
Example 1 · Acids, Bases and Salts · Acid-Base Theory: Concepts, Oxides and Electrolytes
Match the definition to the right concept
Concept 2 of 6: Identifying Lewis acids
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
Practice this concept4 quick reps
The same idea in a real exam question:
Example 2 · Acids, Bases and Salts · Acid-Base Theory: Concepts, Oxides and Electrolytes
NH3 is a base, not an acid
Concept 3 of 6: Basicity of acids (monobasic, dibasic, tribasic)
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
Practice this concept4 quick reps
The same idea in a real exam question:
Example 3 · Acids, Bases and Salts · Acid-Base Theory: Concepts, Oxides and Electrolytes
Tribasic = phosphoric acid
Concept 4 of 6: Classification of oxides: acidic, basic, neutral and 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 |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
Example 4 · Acids, Bases and Salts · Acid-Base Theory: Concepts, Oxides and Electrolytes
CO is neutral, CO2 is acidic
Concept 5 of 6: Electrolytes and electrical conductivity of solutions
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
The same idea in a real exam question:
Example 5 · Acids, Bases and Salts · Acid-Base Theory: Concepts, Oxides and Electrolytes
Sugar dissolves but does not ionise
Concept 6 of 6: The first mineral acid discovered
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
The same idea in a real exam question:
Example 6 · Acids, Bases and Salts · Acid-Base Theory: Concepts, Oxides and Electrolytes
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 |
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
Test yourself on Acids, Bases and Salts
15 past NDA questions from this chapter, timed at 15 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.