Chemistry

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medium importance~1 Q in Tier 113 formulas⚡ 11 shortcuts5 subtopics

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Acids, bases, pH, common compounds and reactions

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⏱ 8 min read🧩 6 question types🎯 22 practice Q

The idea in one minute

Acids, 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.

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.

Question types you will see

Each type: how to recognise it, the method step by step, and one question to try.

Type 1very common4 practice Q

Common name ↔ chemical name/formula

How to spot it:

'Baking soda is…', 'chemical name of washing soda / bleaching powder / plaster of Paris', 'which is CaO', formula asked for a common name.

Method

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 it works:

these names are fixed by tradition, so the only route is the pair-table.

Try this

The chemical formula of baking soda is —

Show solution

NaHCO₃ (sodium bicarbonate). Washing soda is the stronger base Na₂CO₃·10H₂O.

Type 2very common3 practice Q

Natural source → acid

How to spot it:

'Tamarind contains…', 'the acid in an ant sting is…', 'which acid is in curd/tomato/apple', match-the-column of foods and acids.

Method

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).

Try this

The acid present in tamarind is —

Show solution

Tartaric acid (grapes also contain it). Lemon gives citric acid, curd lactic acid.

Type 3very common4 practice Q

Indicators and pH values

How to spot it:

'Phenolphthalein turns pink in…', 'what colour does turmeric give with a base', pH of blood/tooth decay threshold, 'solution of pH 3 is…'.

Method

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.

Try this

A solution has pH = 3; another has pH = 5. The first is —

Show solution

100 times more acidic — each unit below 7 multiplies H⁺ ten-fold (3 is two units below 5).

Type 4common3 practice Q

Salt chemistry and standard reactions

How to spot it:

'CO₂ turns lime water…', 'plaster of Paris is made from…', chlor-alkali products, what happens when water is added to quicklime.

Method

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.

Try this

Lime water turns milky when CO₂ is passed through it because —

Show solution

Insoluble calcium carbonate (CaCO₃) forms — the standard test for carbon dioxide.

Type 5common2 practice Q

Uses of baking soda, washing soda, bleaching powder

How to spot it:

'Which chemical is used as an antacid / to remove hardness of water / to disinfect water', why cakes rise, composition of a soda-acid extinguisher.

Method

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.

Try this

Permanent hardness of water is removed by adding —

Show solution

Washing soda (Na₂CO₃·10H₂O), which precipitates the calcium and magnesium ions.

Type 6occasional2 practice Q

Vitriols and water of crystallisation

How to spot it:

'Oil of vitriol is…', 'blue vitriol is…', why blue copper sulphate turns white on heating, formula with dot-water.

Method

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.

Try this

On strong heating, blue vitriol (CuSO₄·5H₂O) turns —

Show solution

White, as it loses its five waters of crystallisation; adding water turns it blue again.

Formula sheet

\text{pH} = -\log_{10}[\text{H}^{+}]

pH + pOH = 14 at 25 °C

Neutralisation

\text{Acid} + \text{Base} \rightarrow \text{Salt} + \text{H}_2\text{O}

Setting of Plaster of Paris

\text{CaSO}_4\cdot\tfrac{1}{2}\text{H}_2\text{O} + 1\tfrac{1}{2}\,\text{H}_2\text{O} \rightarrow \text{CaSO}_4\cdot 2\text{H}_2\text{O}

PoP → gypsum

Shortcuts that save time

⚡ 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?

Show solution

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.

Mistakes to avoid

Where most students lose marks on this subtopic.

Mistake 01

Swapping baking soda (NaHCO₃) and washing soda (Na₂CO₃·10H₂O).

Mistake 02

Writing aqua regia as 3 HNO₃ : 1 HCl — it is 1 HNO₃ : 3 HCl.

Mistake 03

Thinking rust forms in dry air — it needs both oxygen and moisture.

Quick revision

Read this the night before the exam.

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.

Practice: 22 questions

Sets of 10, mixed across the question types above. Every answer has a step-by-step explanation.

Topic test · 10 questions

Suggested time 3 min · wrong answers go to your mistake notebook automatically.

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