CDS Chemistry · Formula sheet
Acids, Bases and Salts formulas
1 formula, 10 reference tables and 17 common traps for CDS Chemistry Acids, Bases and Salts, grouped by subtopic.
Acids, Bases and Oxides
Learn this subtopic in the notesThree definitions of an acid and a base
| 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 water
| 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 oxides
| 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 |
Common traps
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.
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.
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.
The pH Scale and Indicators
Learn this subtopic in the notesThe pH scale
Definition of pH
- hydrogen-ion concentration in mol/L
pH of everyday substances
| 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 indicators
| 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. |
Common traps
pH 0 is the most acidic, not neutral
pH 0 means [H⁺] = 1 mol/L, the strong-acid end of the scale. Neutral is 7, not 0.
Liming soil RAISES its pH
Lime and dolomite are bases. Spreading them neutralises soil acidity, so the pH goes up. They do not add nitrogen or phosphorus.
Universal indicator is not a titration indicator
Universal indicator changes colour gradually across the whole scale, so it cannot mark a sharp end point. Titrations use one indicator such as phenolphthalein or methyl orange.
Acids in Food and the Body
Learn this subtopic in the notesAcids in fruits, foods and household products
| 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 stomach
| 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 |
Common traps
Tomato is oxalic, tamarind is tartaric
The two are swapped often in the options. Oxalic acid is in tomato and spinach; tartaric acid is in tamarind and grapes.
Milk of magnesia is a base, not an acid
In a list of 'compound and use', magnesium hydroxide is the antacid: it is a mild base that neutralises excess stomach acid.
The stomach acid is hydrochloric, not sulphuric
Gastric juice contains hydrochloric acid. Sulphuric, nitric and formic acids are distractors; lactic and uric acids are organic, not mineral.
Nettle and ant share one acid
Both inject methanoic (formic) acid. Citric, tartaric and acetic acids are food acids and do not sting.
Common Salts and Carbonates
Learn this subtopic in the notesTrade names and formulas of common salts
| 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 crystallisation
| 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 carbonates
| 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. |
Common traps
Baking soda and washing soda are different salts
Baking soda is NaHCO₃ (a hydrogen carbonate). Washing soda is Na₂CO₃·10H₂O (a carbonate with ten waters). Most wrong options swap these two.
Plaster of Paris has half a water, not two
Plaster of Paris is CaSO₄·½H₂O. Gypsum, which it turns into when it sets, is CaSO₄·2H₂O.
Blue vitriol has five waters
Copper sulphate crystals are CuSO₄·5H₂O. Options of 2, 4 or 6 are distractors.
The extinguisher uses the hydrogen carbonate
A soda-acid extinguisher uses sodium hydrogen carbonate (NaHCO₃), not sodium carbonate or sodium chloride.