Chemistry
🔒 Log in to trackAcids, bases, pH, common compounds and reactions
🔒 Log in to trackAcids, bases and indicators
| Indicator | In acid | In base |
|---|---|---|
| Litmus | Blue → red | Red → blue |
| Phenolphthalein | Colourless | Pink |
| Methyl orange | Red | Yellow |
| Turmeric | Yellow (no change) | Reddish-brown |
- pH scale (Sørensen): 0–14 at 25 °C. < 7 acidic, 7 neutral, > 7 basic. pH = −log[H⁺]; each unit is a 10-fold change in H⁺.
- Typical values: gastric juice about 1.2–2 (HCl), lemon juice about 2–2.5, pure water 7, human blood about 7.4 (slightly basic), milk of magnesia about 10.
- Tooth decay starts when mouth pH falls below about 5.5. Antacids (milk of magnesia — Mg(OH)₂, baking soda) neutralise excess stomach acid.
- Ant and bee stings inject formic (methanoic) acid — rub baking soda. Wasp stings are alkaline — use vinegar.
- Acid rain has pH below about 5.6.
Natural sources of acids
| Source | Acid |
|---|---|
| Vinegar | Acetic |
| Lemon, orange (citrus) | Citric |
| Tamarind, grapes | Tartaric |
| Tomato, spinach | Oxalic |
| Curd, sour milk | Lactic |
| Apple | Malic |
| Ant sting, nettle | Formic |
| Stomach | Hydrochloric |
Common names of compounds
| Common name | Chemical name / formula |
|---|---|
| Baking soda | Sodium hydrogen carbonate, NaHCO₃ |
| Washing soda | Sodium carbonate decahydrate, Na₂CO₃·10H₂O |
| Bleaching powder | Calcium oxychloride, CaOCl₂ |
| Plaster of Paris | Calcium sulphate hemihydrate, CaSO₄·½H₂O |
| Gypsum | CaSO₄·2H₂O |
| Common salt | NaCl |
| Quicklime / slaked lime | CaO / Ca(OH)₂ |
| Limestone, marble, chalk | CaCO₃ |
| Caustic soda / caustic potash | NaOH / KOH |
| Blue vitriol | Copper sulphate, CuSO₄·5H₂O |
| Green vitriol | Ferrous sulphate, FeSO₄·7H₂O |
| White vitriol | Zinc sulphate, ZnSO₄·7H₂O |
| Oil of vitriol | Sulphuric acid, H₂SO₄ |
| Muriatic acid | Hydrochloric acid, HCl |
| Epsom salt | MgSO₄·7H₂O |
| Potash alum (fitkari) | K₂SO₄·Al₂(SO₄)₃·24H₂O — water purification |
| Hypo | Sodium thiosulphate, Na₂S₂O₃ — photography |
| Laughing gas | Nitrous oxide, N₂O |
| Dry ice | Solid CO₂ |
| Heavy water | D₂O — moderator in reactors |
| Marsh gas | Methane, CH₄ |
| Lunar caustic | Silver nitrate, AgNO₃ |
| Sal ammoniac | Ammonium chloride, NH₄Cl |
| Aqua regia | Conc. HNO₃ + conc. HCl in 1 : 3 — dissolves gold and platinum |
Types of reactions and everyday chemistry
- Oxidation = gain of oxygen / loss of electrons; reduction = the opposite. Redox reactions do both.
- Rusting of iron needs both oxygen and water (moisture); rust is hydrated iron(III) oxide, Fe₂O₃·xH₂O. Prevention: painting, oiling, galvanisation (zinc coating), alloying (stainless steel).
- Rancidity — oxidation of fats/oils; chips packets are flushed with nitrogen to prevent it.
- Neutralisation: acid + base → salt + water. Displacement: a more reactive metal displaces a less reactive one (iron nail turns brown in copper sulphate solution).
- Hard water: temporary hardness (bicarbonates of Ca and Mg) is removed by boiling; permanent hardness (chlorides and sulphates) by washing soda or ion exchange.
- Plaster of Paris sets into gypsum when mixed with water — used for plasters on fractured bones and statues.
Detailed notes
Acids, bases and how we test them
An acid releases H⁺ ions in water (sour taste); a base releases OH⁻ ions (slippery feel) — a soluble base is an alkali. Indicators tell them apart:
| Indicator | In acid | In base |
|---|---|---|
| Litmus | Blue → red | Red → blue |
| Phenolphthalein | Colourless | Pink |
| Methyl orange | Red | Yellow |
| Turmeric | Yellow (no change) | Reddish-brown |
| The pH scale (Sørensen) runs 0–14: below 7 acidic, 7 neutral, above 7 basic; each step is a ten-fold change. Key values: gastric juice ≈ 1–2 (HCl), lemon ≈ 2, pure water 7, blood ≈ 7.4 (slightly basic), milk of magnesia ≈ 10. Tooth decay begins below pH 5.5; acid rain is below about 5.6. Antacids (milk of magnesia Mg(OH)₂) neutralise excess stomach acid; an ant sting injects formic acid — soothe it with baking soda. |
Where natural acids come from
Vinegar → acetic; lemon/orange → citric; tamarind and grapes → tartaric; tomato and spinach → oxalic; curd/sour milk → lactic; apple → malic; ant sting/nettle → formic; stomach → hydrochloric. This table is asked in nearly every paper.
Common names and formulae — the core table
| Common name | Chemical name / formula |
|---|---|
| Baking soda | Sodium bicarbonate, NaHCO₃ |
| Washing soda | Sodium carbonate, Na₂CO₃·10H₂O |
| Common salt | NaCl |
| Caustic soda | Sodium hydroxide, NaOH |
| Caustic potash | KOH |
| Bleaching powder | Ca(OCl)Cl (calcium oxychloride) |
| Plaster of Paris | Calcium sulphate hemihydrate, CaSO₄·½H₂O |
| Gypsum | CaSO₄·2H₂O |
| Quicklime | CaO |
| Limestone, marble, chalk | CaCO₃ |
| Lime water | Ca(OH)₂ |
| Milk of magnesia | Mg(OH)₂ |
| Laughing gas | N₂O |
| Blue vitriol | CuSO₄·5H₂O |
| Green vitriol | FeSO₄·7H₂O |
| White vitriol | ZnSO₄·7H₂O |
| Oil of vitriol | H₂SO₄ (sulphuric acid) |
| Alum (fitkari) | KAl(SO₄)₂·12H₂O |
| Heavy water | D₂O |
| Dry ice | Solid CO₂ |
| Quartz, sand | SiO₂ |
Reactions and everyday chemistry
- Neutralisation: acid + base → salt + water. Acid + metal → salt + H₂; acid + carbonate → salt + water + CO₂ (the gas turns lime water milky — the standard test for CO₂).
- Chlor-alkali process: electrolysis of brine (NaCl solution) gives NaOH + Cl₂ + H₂.
- Baking soda (NaHCO₃): mild, non-poisonous antacid; raises cakes and idli batter with CO₂; in soda acid fire extinguishers. Washing soda (Na₂CO₃·10H₂O): removes permanent hardness of water, cleans glass, makes borax and glass. Bleaching powder (Ca(OCl)Cl): disinfects drinking water, bleaches cotton.
- Plaster of Paris: made by heating gypsum to 100 °C; on adding water it sets back into hard gypsum — casts for fractures, statues, chalk.
- Water of crystallisation: CuSO₄·5H₂O is blue; on heating it loses water and turns white, regaining colour on moistening.
Quick revision
- pH: <7 acid, 7 neutral, >7 base; blood 7.4; tooth decay below 5.5; acid rain below 5.6.
- Phenolphthalein: colourless in acid, pink in base. Ant sting = formic acid; tamarind = tartaric; tomato = oxalic.
- Baking soda NaHCO₃; washing soda Na₂CO₃·10H₂O; caustic soda NaOH; bleaching powder Ca(OCl)Cl.
- Plaster of Paris CaSO₄·½H₂O from gypsum CaSO₄·2H₂O; lime water milky = CO₂ test.
- Vitriols: oil–H₂SO₄, blue–CuSO₄·5H₂O, green–FeSO₄·7H₂O, white–ZnSO₄.
Types of questions asked
Every way this subtopic shows up in exams — how to recognise it, the formula or logic to use, and a solved example.
Type 1: Common name ↔ chemical name/formulavery common4 practice Q
'Baking soda is…', 'chemical name of washing soda / bleaching powder / plaster of Paris', 'which is CaO', formula asked for a common name.
- Learn the core table by pairing: baking soda NaHCO₃ ↔ washing soda Na₂CO₃·10H₂O; quicklime CaO ↔ lime water Ca(OH)₂ ↔ limestone CaCO₃.
- Gypsum CaSO₄·2H₂O loses water → plaster of Paris CaSO₄·½H₂O.
- Vitriol family: oil of vitriol H₂SO₄; blue CuSO₄·5H₂O; green FeSO₄·7H₂O; white ZnSO₄. Why: these names are fixed by tradition, so the only route is the pair-table.
Example: The chemical formula of baking soda is —
NaHCO₃ (sodium bicarbonate). Washing soda is the stronger base Na₂CO₃·10H₂O.
Type 2: Natural source → acidvery common3 practice Q
'Tamarind contains…', 'the acid in an ant sting is…', 'which acid is in curd/tomato/apple', match-the-column of foods and acids.
- Kitchen map: vinegar–acetic, lemon–citric, tamarind+grapes–tartaric, tomato+spinach–oxalic, curd–lactic, apple–malic, ant sting+nettle–formic, stomach–HCl.
- Trap pairs: tamarind is TARTARIC (not citric); tomato is OXALIC; apple is MALIC.
- Base for an ant sting: baking soda (neutralises the acid).
Example: The acid present in tamarind is —
Tartaric acid (grapes also contain it). Lemon gives citric acid, curd lactic acid.
Type 3: Indicators and pH valuesvery common4 practice Q
'Phenolphthalein turns pink in…', 'what colour does turmeric give with a base', pH of blood/tooth decay threshold, 'solution of pH 3 is…'.
- Indicator colours: litmus blue→red in acid; phenolphthalein colourless in acid, PINK in base; methyl orange red in acid, yellow in base; turmeric reddish-brown in base.
- pH: <7 acid, 7 neutral (pure water), >7 base; blood ≈ 7.4 (slightly basic); tooth enamel dissolves below pH 5.5.
- Lower pH = stronger acid; each pH unit is a ten-fold change.
Example: A solution has pH = 3; another has pH = 5. The first is —
100 times more acidic — each unit below 7 multiplies H⁺ ten-fold (3 is two units below 5).
Type 4: Salt chemistry and standard reactionscommon3 practice Q
'CO₂ turns lime water…', 'plaster of Paris is made from…', chlor-alkali products, what happens when water is added to quicklime.
- Acid + carbonate → CO₂; CO₂ + Ca(OH)₂ (lime water) → white CaCO₃ = milkiness (the CO₂ test).
- Gypsum heated → plaster of Paris; water added → sets to gypsum again (fracture casts).
- Chlor-alkali: electrolysis of brine → NaOH + Cl₂ + H₂.
- Quicklime + water → slaked lime, releasing much heat.
Example: Lime water turns milky when CO₂ is passed through it because —
Insoluble calcium carbonate (CaCO₃) forms — the standard test for carbon dioxide.
Type 5: Uses of baking soda, washing soda, bleaching powdercommon2 practice Q
'Which chemical is used as an antacid / to remove hardness of water / to disinfect water', why cakes rise, composition of a soda-acid extinguisher.
- Baking soda NaHCO₃: antacid, baking powder (CO₂ makes dough rise), fire extinguishers, mild alkali for stings.
- Washing soda Na₂CO₃·10H₂O: removes permanent hardness, glass and borax manufacture.
- Bleaching powder Ca(OCl)Cl: sterilises drinking water, bleaches cotton, makes chloroform.
- Match the job to the chemical; 'permanent hardness' always points to washing soda.
Example: Permanent hardness of water is removed by adding —
Washing soda (Na₂CO₃·10H₂O), which precipitates the calcium and magnesium ions.
Type 6: Vitriols and water of crystallisationoccasional2 practice Q
'Oil of vitriol is…', 'blue vitriol is…', why blue copper sulphate turns white on heating, formula with dot-water.
- Vitriol set: oil of vitriol = H₂SO₄; blue vitriol = CuSO₄·5H₂O; green vitriol = FeSO₄·7H₂O; white vitriol = ZnSO₄·7H₂O.
- The dot-water is water of crystallisation: heating drives it off (blue → white); moistening restores it.
- Same idea: washing soda's 10H₂O; gypsum's 2H₂O.
Example: On strong heating, blue vitriol (CuSO₄·5H₂O) turns —
White, as it loses its five waters of crystallisation; adding water turns it blue again.
Formulas
pH + pOH = 14 at 25 °C
PoP → gypsum
Shortcut tricks
⚡ Kitchen acid map: 'Vinegar-Acetic, Lemon-Citric, Tamarind-Tartaric, Tomato-Oxalic, Curd-Lactic, Apple-Malic, Ant-Formic'
Pairs that start alike help: Tamarind–Tartaric, Apple–mAlic (think 'mAlus', the apple genus), Formica (Latin for ant)–Formic.
Example: Which acid is present in tomato?
Oxalic acid.
⚡ Phenolphthalein goes 'Pink in Base'
'Phenol → Pink' only in a base; in acid it stays colourless.
⚡ Vitriol colours by metal
Blue — copper (CuSO₄ crystals are blue), Green — iron (ferrous), White — zinc; Oil of vitriol — H₂SO₄.
Where students lose marks
Swapping baking soda (NaHCO₃) and washing soda (Na₂CO₃·10H₂O).
Writing aqua regia as 3 HNO₃ : 1 HCl — it is 1 HNO₃ : 3 HCl.
Thinking rust forms in dry air — it needs both oxygen and moisture.
Practice sets — 22 questions
Sets of 10, mixed across the question types above. Each answer comes with a step-by-step explanation.
Topic test · 10 questions
Suggested time 3 min · wrong answers go to your mistake notebook automatically.