CDS Chemistry · Acids, Bases and Salts
Acids, Bases and Oxides
The three definitions of an acid and a base, what makes a base an alkali, how acids behave in water, and which oxides are acidic, basic or amphoteric.
Why this matters
Twelve CDS questions, almost one a year since 2017. Most are statement checks: is water an acid or a base here, is every base an alkali, which oxide reacts with both acids and bases. Two harder ones need the Brønsted and Lewis definitions, which go beyond the Class 10 book.
Concept 1 of 3: Three definitions of an acid and a base
Each definition widens the one before it. Arrhenius talks about ions in water. Brønsted–Lowry talks about who gives a proton (H⁺) to whom. Lewis talks about who gives a pair of electrons. A substance that is not an acid in one sense can be one in the next.
Definition
The three definitions:
- Arrhenius: an acid gives H⁺ ions in water; a base gives OH⁻ ions in water.
- Brønsted–Lowry: an acid is a proton donor; a base is a proton acceptor. Water can be either, depending on its partner.
- Lewis: an acid is an electron-pair acceptor; a base is an electron-pair donor.
- A proton-loving (protophilic) substance is a base. Hydrophilic and hydrophobic describe water, not protons.
| Definition | Acid | Base | Example |
|---|---|---|---|
| Arrhenius | Gives H⁺ in water | Gives OH⁻ in water | HCl → H⁺ + Cl⁻; NaOH → Na⁺ + OH⁻ |
| Brønsted–Lowry | Proton donor | Proton acceptor | HSO₄⁻ + H₂O → H₃O⁺ + SO₄²⁻: water takes the proton, so water is the baseQ CDS 2017 (II): in this reaction water acts as a base. |
| Lewis | Electron-pair acceptor | Electron-pair donor | Acids: BF₃, AlCl₃, Co³⁺, Mg²⁺. Bases: NH₃, H₂O, OH⁻, F⁻Q CDS 2025 (I): AlCl₃, Co³⁺ and BF₃ are all Lewis acids. Anything with a lone pair to give, such as NH₃ or OH⁻, is a Lewis base. |
Electron-poor molecules (BF₃, AlCl₃) and small, highly charged metal ions are the usual Lewis acids.
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2017 · CDS (II) 2017 — General Knowledge · Q17Moderate
Example 1 · Acids, Bases and Salts · Acids, Bases and Oxides
In the reaction between hydrogen sulphate ion and water
the water acts as
Water is not always neutral in a reaction
Water is amphiprotic: it is a base when it takes a proton from HSO₄⁻ or HCl, and an acid when it gives a proton to NH₃. Read which way the proton moves.
A Lewis acid ACCEPTS electrons
Lewis acids are electron-pair acceptors (BF₃, AlCl₃, metal ions). A set containing NH₃, H₂O, OH⁻ or F⁻ is not 'all Lewis acids', because each of those has a lone pair to donate.
Concept 2 of 3: Bases, alkalis and acids in water
A base is anything that neutralises an acid. An alkali is a base that also dissolves in water. So every alkali is a base, but many bases (copper oxide, iron hydroxide) are not alkalis. Acids and alkalis in water are full of ions, which is why they conduct and why mixing them with water gives out heat.
Definition
What the bank checks:
- Alkali = a water-soluble base: NaOH, KOH, Ca(OH)₂, NH₄OH. Insoluble bases such as CuO and Cu(OH)₂ are bases but not alkalis.
- Alkalis are bitter, soapy to touch and corrosive, and turn red litmus blue.
- Dissolving an alkali such as NaOH in water is exothermic (gives out heat).
- An acid solution conducts electricity because the acid ionises into H⁺ (H₃O⁺) ions.
- Diluting an acid spreads the same ions through more water, so the H₃O⁺ concentration falls.
- Always add acid to water, slowly, while stirring. Water added to concentrated acid can boil and splash.
| Statement | True or false | Why |
|---|---|---|
| All bases are alkalis | False | Only the bases that dissolve in water are alkalisQ |
| All alkalis dissolve in water | True | Dissolving is what makes a base an alkali |
| Alkalis are soapy, bitter and corrosive | True | The standard properties of an alkali |
| Adding water to an alkali gives out heat | True | Dissolving NaOH or KOH is exothermicQ |
| An acid solution conducts because it holds H⁺ ions | True | The ions carry the chargeQ |
| Diluting an acid raises its H₃O⁺ concentration | False | The ions spread through more volume, so it fallsQ |
| To dilute, add water to the concentrated acid | False | Add the acid to water, slowly |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2026 · CDS (I) 2026 — General Knowledge · Q29Moderate
Example 2 · Acids, Bases and Salts · Acids, Bases and Oxides
Consider the following statements regarding bases and alkalis :
1. All bases dissolve in water.
2. Bases which are soluble in water are called alkalis.
3. An alkali is soapy to touch, bitter and corrosive.
4. Addition of water to an alkali generates heat.
Which of the statements given above are correct ?
Every alkali is a base, not the reverse
'All bases are alkalis' is false. Copper oxide, copper hydroxide and iron hydroxide are bases that do not dissolve in water, so they are not alkalis.
Dilution lowers concentration
Adding water does not create H₃O⁺ ions; it spreads them out. The number of ions per unit volume decreases on dilution.
Concept 3 of 3: Acidic, basic, amphoteric and neutral oxides
Most metal oxides are basic and most non-metal oxides are acidic. A few oxides, zinc oxide and aluminium oxide above all, sit in the middle and react with both acids and bases. A handful, like carbon monoxide, react with neither.
Definition
The four kinds of oxide:
- Basic oxides (metals): Na₂O, MgO, CaO, CuO. They react with acids to give a salt and water.
- Acidic oxides (non-metals): CO₂, SO₂, P₄O₁₀. In water they form acids, e.g. CO₂ + H₂O → H₂CO₃ (carbonic acid).
- Amphoteric oxides react with both acids and bases: ZnO, Al₂O₃ (also PbO, SnO).
- Neutral oxides react with neither: CO, NO, N₂O.
- Acid + basic oxide → salt + water. CuO + 2HCl → CuCl₂ (copper(II) chloride, blue-green) + H₂O.
| Kind of oxide | Examples | Reacts with |
|---|---|---|
| Basic | Na₂O, MgO, CaO, CuO | Acids, giving a salt and waterQ CDS 2026 (II): CuO in hydrochloric acid turns blue-green because copper(II) chloride forms. |
| Acidic | CO₂, SO₂, P₄O₁₀ | Bases; in water it gives an acidQ CDS 2018 (I): CO₂ is the gas that dissolves in water to give an acidic solution. |
| Amphoteric | ZnO, Al₂O₃ | Both acids and basesQ Both have been asked: ZnO (CDS 2020 II) and Al₂O₃ (CDS 2024 I). |
| Neutral | CO, NO, N₂O | Neither acids nor bases |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2024 · CDS (I) 2024 — General Knowledge · Q33Moderate
Example 3 · Acids, Bases and Salts · Acids, Bases and Oxides
Which one of the following is an amphoteric oxide?
Non-metal oxides are acidic
Non-metals give acidic oxides (CO₂, SO₂). Metals give basic oxides. A statement that non-metals dissolved in water 'produce basic oxides' or 'provide hydroxides' has it backwards.
Amphoteric is not the same as neutral
ZnO and Al₂O₃ react with acids and with bases. CO and NO react with neither. MgO, CaO and Na₂O are purely basic, and P₄O₁₀ is purely acidic.
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)
Three definitions of an acid and a base3 rows
| Definition | Acid | Base | Example |
|---|---|---|---|
| Arrhenius | Gives H⁺ in water | Gives OH⁻ in water | HCl → H⁺ + Cl⁻; NaOH → Na⁺ + OH⁻ |
| Brønsted–Lowry | Proton donor | Proton acceptor | HSO₄⁻ + H₂O → H₃O⁺ + SO₄²⁻: water takes the proton, so water is the baseQ CDS 2017 (II): in this reaction water acts as a base. |
| Lewis | Electron-pair acceptor | Electron-pair donor | Acids: BF₃, AlCl₃, Co³⁺, Mg²⁺. Bases: NH₃, H₂O, OH⁻, F⁻Q CDS 2025 (I): AlCl₃, Co³⁺ and BF₃ are all Lewis acids. Anything with a lone pair to give, such as NH₃ or OH⁻, is a Lewis base. |
Electron-poor molecules (BF₃, AlCl₃) and small, highly charged metal ions are the usual Lewis acids.
Bases, alkalis and acids in water7 rows
| Statement | True or false | Why |
|---|---|---|
| All bases are alkalis | False | Only the bases that dissolve in water are alkalisQ |
| All alkalis dissolve in water | True | Dissolving is what makes a base an alkali |
| Alkalis are soapy, bitter and corrosive | True | The standard properties of an alkali |
| Adding water to an alkali gives out heat | True | Dissolving NaOH or KOH is exothermicQ |
| An acid solution conducts because it holds H⁺ ions | True | The ions carry the chargeQ |
| Diluting an acid raises its H₃O⁺ concentration | False | The ions spread through more volume, so it fallsQ |
| To dilute, add water to the concentrated acid | False | Add the acid to water, slowly |
Acidic, basic, amphoteric and neutral oxides4 rows
| Kind of oxide | Examples | Reacts with |
|---|---|---|
| Basic | Na₂O, MgO, CaO, CuO | Acids, giving a salt and waterQ CDS 2026 (II): CuO in hydrochloric acid turns blue-green because copper(II) chloride forms. |
| Acidic | CO₂, SO₂, P₄O₁₀ | Bases; in water it gives an acidQ CDS 2018 (I): CO₂ is the gas that dissolves in water to give an acidic solution. |
| Amphoteric | ZnO, Al₂O₃ | Both acids and basesQ Both have been asked: ZnO (CDS 2020 II) and Al₂O₃ (CDS 2024 I). |
| Neutral | CO, NO, N₂O | Neither acids nor bases |
Watch out for (6)
- Water is not always neutral in a reaction→ Three definitions of an acid and a base
- A Lewis acid ACCEPTS electrons→ Three definitions of an acid and a base
- Every alkali is a base, not the reverse→ Bases, alkalis and acids in water
- Dilution lowers concentration→ Bases, alkalis and acids in water
- Non-metal oxides are acidic→ Acidic, basic, amphoteric and neutral oxides
- Amphoteric is not the same as neutral→ Acidic, basic, amphoteric and neutral oxides
Test yourself on Acids, Bases and Salts
15 past CDS 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.