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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.

All Acids, Bases and Salts notesNDA Chemistry strategy

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.
  1. BF3 has an incomplete octet on boron, so it can accept an electron pair.
  2. NH3 has a lone pair on nitrogen, so it can donate that electron pair.
  3. Lewis acid = electron-pair acceptor; Lewis base = electron-pair donor.
Answer:BF3 is the Lewis acid (accepts the pair); NH3 is the Lewis base (donates the pair).
Practice this concept4 quick reps

Practice — Level 1 (4 reps)

Quick reps to lock in the method. Try each, then check.

  1. 1.
    Under the Arrhenius concept, an acid produces which ion in water?
  2. 2.
    Under the Bronsted-Lowry concept, an acid is defined as a what?
  3. 3.
    Under the Lewis concept, a base is defined as a what?
  4. 4.
    Which acid-base concept defines acids and bases only in aqueous solution?

From the bank · past-year question

Example 1Acids, Bases and SaltsEASY
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.
  1. BCl3 has an incomplete octet on boron — it accepts an electron pair, so it is a Lewis acid.
  2. H2O and NH3 each have lone pairs to donate — they are Lewis bases.
  3. CO2 can act as a weak Lewis acid at carbon, but BCl3 is the clear electron-pair acceptor here.
Answer:BCl3 is the Lewis acid (incomplete octet on boron accepts an electron pair).
Practice this concept4 quick reps

Practice — Level 1 (4 reps)

Quick reps to lock in the method. Try each, then check.

  1. 1.
    Is BF3 a Lewis acid or a Lewis base?
  2. 2.
    Is NH3 a Lewis acid or a Lewis base?
  3. 3.
    Why is AlCl3 a Lewis acid?
  4. 4.
    Why is FeCl3 a Lewis acid?

From the bank · past-year question

Example 2Acids, Bases and SaltsMODERATE
Consider the following: I. AlCl3\text{AlCl}_3 II. NH3\text{NH}_3 III. BF3\text{BF}_3 IV. FeCl3\text{FeCl}_3 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?
  1. H2SO4 has two replaceable hydrogen ions per molecule.
  2. An acid releasing two H+ ions is called dibasic.
Answer:Basicity 2 — sulphuric acid is dibasic.
Practice this concept4 quick reps

Practice — Level 1 (4 reps)

Quick reps to lock in the method. Try each, then check.

  1. 1.
    Which acid is tribasic: HCl, HNO3, H2SO4 or H3PO4?
  2. 2.
    Basicity of hydrochloric acid (HCl)?
  3. 3.
    Basicity of sulphuric acid (H2SO4)?
  4. 4.
    Basicity of phosphoric acid (H3PO4)?

From the bank · past-year question

Example 3Acids, Bases and SaltsEASY
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.
TypeReacts withExamples
Acidic oxideBases (forms a salt)CO2, SO2, NO2
Basic oxideAcids (forms a salt)Na2O, MgO, CaO
Neutral oxideNeither acids nor basesCO, N2O, NO, H2O
CO (carbon monoxide) is the neutral oxide the bank loves — CO2 in contrast is acidic.
Amphoteric oxideBoth acids and basesAl2O3, 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

Try it yourself

Classify each oxide as acidic, basic or neutral: CO2, MgO, NO.

Practice — Level 1 (4 reps)

Quick reps to lock in the method. Try each, then check.

  1. 1.
    Which is a neutral oxide: CO, CO2, Na2O or MgO?
  2. 2.
    Is CO2 an acidic, basic or neutral oxide?
  3. 3.
    Is MgO an acidic, basic or neutral oxide?
  4. 4.
    Name an amphoteric oxide.

From the bank · past-year question

Example 4Acids, Bases and SaltsMODERATE
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 waterConducts?Reason
NaOH (sodium hydroxide)Yes (basic solution)Strong electrolyte — gives Na+ and OH-
NaCl, CuSO4 (salts)YesIonise fully into free ions
HCl (strong acid)YesIonises fully into H+ and Cl-
CH3COOH (acetic acid)WeaklyWeak electrolyte — partial ionisation
SugarNoNon-electrolyte — dissolves as whole molecules, no ions
Sugar is the classic non-conducting solution — it is covalent and produces no ions.
CH3OH (methanol)NoNon-electrolyte — covalent, no ions
Practice this concept4 quick reps

Practice — Level 1 (4 reps)

Quick reps to lock in the method. Try each, then check.

  1. 1.
    Which does NOT conduct electricity in solution: copper sulphate, sodium chloride, sugar or sodium hydroxide?
  2. 2.
    Which compound conducts electricity and forms a basic solution: HCl, CH3COOH, CH3OH or NaOH?
  3. 3.
    Why does a sugar solution not conduct electricity?
  4. 4.
    Acetic acid (CH3COOH) is a strong or weak electrolyte?

From the bank · past-year question

Example 5Acids, Bases and SaltsEASY
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.
Drill 1 more on electrolytes and electrical conductivity of solutions

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 acidFormulaNote
Nitric acidHNO3The first mineral acid discovered
Nitric acid is the bank's answer for 'first mineral acid discovered'.
Hydrochloric acidHClMineral acid — discovered later
Sulphuric acidH2SO4'King of chemicals', dibasic
Phosphoric acidH3PO4Tribasic mineral acid
Practice this concept3 quick reps

Practice — Level 1 (3 reps)

Quick reps to lock in the method. Try each, then check.

  1. 1.
    Which was the first mineral acid to be discovered?
  2. 2.
    Name three mineral acids other than nitric acid.
  3. 3.
    Is acetic acid a mineral acid?

From the bank · past-year question

Example 6Acids, Bases and SaltsHARD
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
TypeReacts withExamples
Acidic oxideBases (forms a salt)CO2, SO2, NO2
Basic oxideAcids (forms a salt)Na2O, MgO, CaO
Neutral oxideNeither acids nor basesCO, N2O, NO, H2O
CO (carbon monoxide) is the neutral oxide the bank loves — CO2 in contrast is acidic.
Amphoteric oxideBoth acids and basesAl2O3, 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 waterConducts?Reason
NaOH (sodium hydroxide)Yes (basic solution)Strong electrolyte — gives Na+ and OH-
NaCl, CuSO4 (salts)YesIonise fully into free ions
HCl (strong acid)YesIonises fully into H+ and Cl-
CH3COOH (acetic acid)WeaklyWeak electrolyte — partial ionisation
SugarNoNon-electrolyte — dissolves as whole molecules, no ions
Sugar is the classic non-conducting solution — it is covalent and produces no ions.
CH3OH (methanol)NoNon-electrolyte — covalent, no ions
The first mineral acid discovered4 rows
Mineral acidFormulaNote
Nitric acidHNO3The first mineral acid discovered
Nitric acid is the bank's answer for 'first mineral acid discovered'.
Hydrochloric acidHClMineral acid — discovered later
Sulphuric acidH2SO4'King of chemicals', dibasic
Phosphoric acidH3PO4Tribasic mineral acid

Watch out for (5)

Mastery check — 1 interleaved questions

Try each one before clicking. Questions are interleaved across the concepts above, not grouped — interleaving sharpens transfer.

Example 1Acids, Bases and SaltsEASY
Which compound, when dissolved in water, conducts electricity and forms a basic solution?

[Q81 · Sep · 2017]

Drill every past-year question on this subtopic

7 questions from the bank — paginated, with cart and Word-export support.

Drill the 7 questions