CDS Chemistry · Teaching notes
Acids, Bases and Salts — CDS Chemistry
Acids, Bases and Salts is the largest chemistry chapter in CDS General Knowledge: 40 past-year questions from 2016 (II) to 2026 (II), in 18 of the 20 sittings. Almost all of it is recall of names, formulas and pairs of source and acid, and only two questions are HARD. The one idea worth building rather than memorising is the oxide rule: metal oxides are basic, non-metal oxides acidic, and zinc and aluminium oxides both. Each page stands alone, so start with the one you know least.
Every subtopic, worked example, formula and trap in one printable document — answers shown, ready to share.
Subtopic notes
Acids, Bases and Oxides
12 PYQsThe 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.
The pH Scale and Indicators
7 PYQsWhat pH measures, the 0 to 14 scale, the pH of everyday substances in order, and the indicators that show it.
Acids in Food and the Body
11 PYQsWhich acid is in which fruit, food, sting or body fluid, and what common acids and antacids are used for.
Common Salts and Carbonates
10 PYQsThe trade names and formulas of everyday salts, how many water molecules their crystals carry, and the carbonates behind marble, pearl and the fire extinguisher.
Formula & revision sheet
1 formulas · 10 reference tables · 17 gotchas across all subtopics — the exam-eve cheat-sheet
Formula & revision sheet
1 formulas · 10 reference tables · 17 gotchas across all subtopics — the exam-eve cheat-sheet
Acids, Bases and Oxides
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
The pH Scale and Indicators
Formulas (1)
Reference tables (2)
pH of everyday substances7 rows
| Substance | Approx. pH | Nature |
|---|---|---|
| Gastric juice | 1.5–2 | Strongly acidic |
| Lemon juice | 2.2 | Acidic |
| Coffee | 5 | Weakly acidic |
| Pure water | 7 | Neutral |
| Blood | 7.4 | Slightly basic |
| Milk of magnesia | 10 | Mild base (antacid)Q CDS 2022 (II): milk of magnesia is a mild base. |
| Sodium hydroxide solution | 13–14 | Strong base |
CDS 2023 (I) order of increasing pH: lemon juice < coffee < blood < milk of magnesia.
Acid-base indicators5 rows
| Indicator | In acid | In base |
|---|---|---|
| Litmus (from lichen) | Red | BlueQ CDS 2022 (I): litmus comes from a lichen. |
| Phenolphthalein | Colourless | Pink |
| Methyl orange | Red | Yellow |
| Turmeric | Yellow | Reddish-brown |
| Universal indicator | Red to yellow as pH rises | Green at 7, then blue to violetQ CDS 2021 (II): it is a mixture and shows many colours, but it is not used for titrations. |
Watch out for (3)
- pH 0 is the most acidic, not neutral→ The pH scale
- Liming soil RAISES its pH→ pH of everyday substances
- Universal indicator is not a titration indicator→ Acid-base indicators
Acids in Food and the Body
Reference tables (2)
Acids in fruits, foods and household products7 rows
| Source or use | Acid |
|---|---|
| Lemon, orange | Citric acid |
| Vitamin C (amla, citrus) | Ascorbic acidQ CDS 2019 (II): vitamin C is ascorbic acid. |
| Tomato | Oxalic acidQ Asked in CDS 2021 (I) and again in a 2026 (II) match-the-list. |
| Tamarind, grapes | Tartaric acidQ CDS 2023 (II): 'oxalic acid is found in tamarind paste' was the incorrect statement. |
| Curd, sour milk | Lactic acid |
| Vinegar (5–8% in water), pickles | Acetic acidQ |
| Mild antiseptic | Boric acidQ CDS 2020 (I) matched boric acid to antiseptic, citric acid to preservative, Mg(OH)₂ to antacid, acetic acid to pickle. |
Acids in stings and in the stomach4 rows
| Source | Acid | Kind |
|---|---|---|
| Ant or bee sting | Methanoic (formic) acid | OrganicQ |
| Nettle leaf hairs | Methanoic (formic) acid | OrganicQ Asked twice: CDS 2021 (II) and 2022 (II). |
| Stomach (gastric juice) | Hydrochloric acid | MineralQ Asked twice: CDS 2020 (I) and 2022 (I). HCl is the only mineral acid among the usual options. |
| Tired muscles | Lactic acid | Organic |
Watch out for (4)
- Tomato is oxalic, tamarind is tartaric→ Acids in fruits, foods and household products
- Milk of magnesia is a base, not an acid→ Acids in fruits, foods and household products
- The stomach acid is hydrochloric, not sulphuric→ Acids in stings and in the stomach
- Nettle and ant share one acid→ Acids in stings and in the stomach
Common Salts and Carbonates
Reference tables (3)
Trade names and formulas of common salts8 rows
| Trade name | Chemical name | Formula |
|---|---|---|
| Baking soda (cooking soda) | Sodium hydrogen carbonate | NaHCO₃Q CDS 2026 (II): cooking soda is NaHCO₃, not Na₂CO₃. |
| Washing soda | Sodium carbonate decahydrate | Na₂CO₃·10H₂O |
| Soda ash | Sodium carbonate (anhydrous) | Na₂CO₃ |
| Bleaching powder | Calcium oxychloride | CaOCl₂ |
| Plaster of Paris | Calcium sulphate hemihydrate | CaSO₄·½H₂O (2CaSO₄·H₂O)Q CDS 2017 (II): its paste holds a fractured bone in place. |
| Gypsum | Calcium sulphate dihydrate | CaSO₄·2H₂O |
| Slaked lime | Calcium hydroxide | Ca(OH)₂ |
| Quicklime | Calcium oxide | CaO |
Water of crystallisation4 rows
| Salt | Formula | Waters per formula unit |
|---|---|---|
| Copper sulphate (blue vitriol) | CuSO₄·5H₂O | 5Q CDS 2020 (I): copper sulphate crystals carry five water molecules. |
| Washing soda | Na₂CO₃·10H₂O | 10 |
| Gypsum | CaSO₄·2H₂O | 2 |
| Plaster of Paris | CaSO₄·½H₂O | ½ |
Calcium carbonate and hydrogen carbonates4 rows
| Material or use | Compound |
|---|---|
| Marble, chalk, limestone | Calcium carbonate, CaCO₃Q CDS 2026 (II): all three are forms of calcium carbonate. |
| Pearl | Calcium carbonate (nacre)Q |
| Soda-acid fire extinguisher | Sodium hydrogen carbonate with sulphuric acidQ |
| No solid hydrogen carbonate | LithiumQ CDS 2016 (II), HARD: LiHCO₃ exists only in solution. |
Watch out for (4)
- Baking soda and washing soda are different salts→ Trade names and formulas of common salts
- Plaster of Paris has half a water, not two→ Trade names and formulas of common salts
- Blue vitriol has five waters→ Water of crystallisation
- The extinguisher uses the hydrogen carbonate→ Calcium carbonate and hydrogen carbonates
PYQ weightage by concept
11 concepts · 40 PYQs — where the marks actually sit, so you know what to drill first
PYQ weightage by concept
11 concepts · 40 PYQs — where the marks actually sit, so you know what to drill first
Acids, Bases and Oxides12 PYQs · 30%
| Concept | PYQs | Share |
|---|---|---|
| Acidic, basic, amphoteric and neutral oxides | 5 | 13% |
| Bases, alkalis and acids in water | 4 | 10% |
| Three definitions of an acid and a base | 3 | 8% |
The pH Scale and Indicators7 PYQs · 18%
| Concept | PYQs | Share |
|---|---|---|
| pH of everyday substances | 3 | 8% |
| The pH scale | 2 | 5% |
| Acid-base indicators | 2 | 5% |
Acids in Food and the Body11 PYQs · 28%
| Concept | PYQs | Share |
|---|---|---|
| Acids in fruits, foods and household products | 6 | 15% |
| Acids in stings and in the stomach | 5 | 13% |
Common Salts and Carbonates10 PYQs · 25%
| Concept | PYQs | Share |
|---|---|---|
| Trade names and formulas of common salts | 5 | 13% |
| Calcium carbonate and hydrogen carbonates | 4 | 10% |
| Water of crystallisation | 1 | 3% |
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.