Acids, Bases and Salts

This is the second chapter of NCERT's Class 10 Science textbook (pages 17–36 of the 2026-27 reprint). It has four parts — chemical properties of acids and bases (2.1), what all acids and all bases have in common (2.2), how strong acid or base solutions are — the pH scale and its importance in daily life (2.3) — and more about salts (2.4: families, pH of salts, chemicals from common salt, water of crystallisation) — with 15 activities, Figures 2.1–2.10, Tables 2.1–2.4, three ‘Do You Know?’ boxes, a box on acids on other planets, four in-text question boxes, a summary, 15 exercise questions and two group activities. As in the previous chapter, every equation on this page is checked in the source — here for electric charge as well as atoms.

• Neighbouring chapters: the previous chapter, Chemical Reactions and Equations, supplies the reaction types used here (displacement of hydrogen, double displacement in neutralisation, decomposition of baking soda, lime water). The next, Metals and Non-metals, explains why some metals react with acids and others do not, and why metal oxides are basic and non-metal oxides acidic. Live geography chapters give the setting: Resources and Development (Class 10 — soils, and treating acidic and alkaline soils) and Minerals and Energy Resources (Class 10 — limestone, gypsum and salt as minerals).
• Prelims — science: UPSC has asked whether washing soda solution is basic (1984), which acid ants inject (1999), what vinegar is (1993), why acid is added to water (1999), the common names of salts (1998), baking soda in fire extinguishers and gypsum to plaster of Paris (2004, 2005), the pH of blood (2008), why copper sulphate solution is acidic (2001), which acids occur in milk, vinegar and lemon (1988), alkalis for cleaning (1986) and chlorine in municipal water (1980).
• GS3 — science, agriculture and environment: acid rain; soil pH and liming of acidic soils; water disinfection with chlorine and bleaching powder; the chlor-alkali industry; oral health and diet; ocean acidification (Beyond NCERT).

• Indicators show acid or base: litmus (red in acid, blue in base), turmeric (reddish-brown in base), phenolphthalein (pink in base), methyl orange; olfactory ones change smell (onion, vanilla, clove).
• Acids + metal → salt + H₂; + carbonate/hydrogencarbonate → salt + CO₂ + water; + metal oxide → salt + water. Bases + zinc → salt + H₂; + non-metal oxide → salt + water. Acid + base → salt + water (neutralisation).
• What they share: acids give H⁺(aq)/H₃O⁺ — only in water (dry HCl gas does not change dry litmus); bases give OH⁻(aq); soluble bases are alkalis. Dissolving them is highly exothermic — add acid to water, never water to acid.
• pH 0–14: 7 neutral, below 7 acidic, above 7 basic; more H⁺ = lower pH. Body 7.0–7.8; acid rain below 5.6; tooth enamel corrodes below 5.5; antacids such as milk of magnesia neutralise excess stomach acid.
• Salts belong to families (sodium salts, chloride salts…). Strong acid + strong base → neutral salt (pH 7); strong acid + weak base → acidic; strong base + weak acid → basic.
• From common salt: NaOH, Cl₂ and H₂ (chlor-alkali), bleaching powder Ca(ClO)₂, baking soda NaHCO₃, washing soda Na₂CO₃·10H₂O. Water of crystallisation: CuSO₄·5H₂O (blue), CaSO₄·2H₂O (gypsum) → plaster of Paris CaSO₄·½H₂O at 373 K.

NCERT's ‘Do You Know?’ box says litmus is a purple dye extracted from lichen; when neither acidic nor basic, litmus solution is purple. Other natural indicators include red cabbage leaves, turmeric and coloured petals of Hydrangea, Petunia and Geranium. In Activity 2.1 students test hydrochloric, sulphuric, nitric and acetic acids and sodium, calcium, potassium, magnesium and ammonium hydroxides with red litmus, blue litmus, phenolphthalein and methyl orange, and tabulate the colours in Table 2.1. Some substances change their smell in acid or base: these are olfactory indicators (Activity 2.2 — onion-scented cloth strips, vanilla essence, clove oil).

With metals (Activity 2.3): about 5 mL of dilute sulphuric acid on zinc granules gives bubbles; passed through soap solution, the gas fills soap bubbles that burn with a pop near a candle — hydrogen: Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g). The metal displaces hydrogen from the acid and forms a salt: acid + metal → salt + hydrogen gas. Some bases react with metals too (Activity 2.4): warm zinc with sodium hydroxide solution — 2NaOH(aq) + Zn(s) → Na₂ZnO₂(s) + H₂(g) (sodium zincate) — but such reactions are not possible with all metals.

With carbonates and hydrogencarbonates (Activity 2.5): about 0.5 g each of sodium carbonate (tube A) and sodium hydrogencarbonate (tube B) with about 2 mL dilute HCl: Na₂CO₃(s) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g); NaHCO₃(s) + HCl(aq) → NaCl(aq) + H₂O(l) + CO₂(g). The gas turns lime water milky: Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l) (white precipitate); with excess CO₂ the milkiness clears: CaCO₃(s) + H₂O(l) + CO₂(g) → Ca(HCO₃)₂(aq) (soluble). Limestone, chalk and marble are forms of calcium carbonate. So metal carbonate/hydrogencarbonate + acid → salt + carbon dioxide + water.

With each other (Activity 2.6): about 2 mL dilute NaOH with two drops of phenolphthalein is pink; adding dilute HCl drop by drop makes it colourless; a few more drops of NaOH bring the pink back. The base's effect is nullified by the acid and vice versa: NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l). Base + acid → salt + water is a neutralisation reaction. With metal oxides (Activity 2.7): black copper oxide dissolves slowly in dilute HCl and the solution turns blue-green (copper(II) chloride): CuO(s) + 2HCl(aq) → CuCl₂(aq) + H₂O(l). Because metal oxide + acid → salt + water, like a base, metal oxides are called basic oxides. Non-metal oxides with bases: lime water (a base) with CO₂ gives a salt and water, so non-metal oxides are acidic.

In Activity 2.8, two nails fixed on a cork in a 100 mL beaker are wired through a bulb and a switch to a 6 volt battery. With dilute HCl or dilute sulphuric acid the bulb glows; with glucose or alcohol solution it does not. Current is carried by ions. Acids contain H⁺ as cation with anions such as Cl⁻ in HCl, NO₃⁻ in HNO₃, SO₄²⁻ in H₂SO₄ and CH₃COO⁻ in CH₃COOH; so acids produce H⁺(aq) in solution, which gives them their acidic properties. Repeated with alkalis (NaOH, Ca(OH)₂), the bulb also glows.

Only in water. In Activity 2.9, concentrated sulphuric acid is added to about 1 g of solid NaCl in a dry test tube; the gas (HCl) is tested with dry and then wet blue litmus — only the wet paper turns red. H⁺ ions separate from HCl molecules only in the presence of water: HCl + H₂O → H₃O⁺ + Cl⁻. A hydrogen ion cannot exist alone; it joins a water molecule, H⁺ + H₂O → H₃O⁺, so it is always written H⁺(aq) or as the hydronium ion H₃O⁺. Bases in water give hydroxide ions: NaOH(s) —[water]→ Na⁺(aq) + OH⁻(aq); KOH(s) —[water]→ K⁺(aq) + OH⁻(aq); Mg(OH)₂(s) —[water]→ Mg²⁺(aq) + 2OH⁻(aq). Bases soluble in water are alkalis. Neutralisation is then simply H⁺(aq) + OH⁻(aq) → H₂O(l) — in general, HX + MOH → MX + HOH.

In Activity 2.10, a few drops of concentrated H₂SO₄ added to 10 mL of water make the beaker warm; sodium hydroxide pellets do the same. Dissolving an acid or a base in water is highly exothermic. With concentrated nitric or sulphuric acid, the acid must always be added slowly to water with constant stirring; if water is added to concentrated acid, the heat may make the mixture splash out and cause burns, and the glass may break from local heating. Mixing an acid or base with water lowers the concentration of ions (H₃O⁺/OH⁻) per unit volume — dilution.

A universal indicator, a mixture of several indicators, shows different colours at different hydrogen-ion concentrations. The pH scale measures hydrogen-ion concentration; the p stands for German potenz, ‘power’. pH generally runs from 0 (very acidic) to 14 (very alkaline). Higher the hydronium ion concentration, lower the pH. A neutral solution has pH 7; values below 7 are acidic; from 7 to 14 the OH⁻ concentration — the strength of the alkali — increases. pH is usually measured with paper soaked in universal indicator.

Strong and weak. The strength of acids and bases depends on how many H⁺ or OH⁻ ions they produce. Hydrochloric acid and acetic acid at the same concentration (say one molar) give different amounts of H⁺: acids giving more H⁺ are strong (HCl), those giving fewer are weak (acetic acid). Likewise for bases: NaOH and KOH are strong; NH₄OH is weak (Beyond NCERT examples).

Plants and animals are pH-sensitive. Our body works within pH 7.0 to 7.8; living organisms survive only in a narrow range. Rain with pH below 5.6 is acid rain; when it flows into rivers, it lowers their pH and makes life hard for aquatic organisms. A box notes that the atmosphere of Venus has thick white and yellowish clouds of sulphuric acid. Soil: plants need a specific pH range; Activity 2.12 tests a soil filtrate (about 2 g soil shaken with 5 mL water) with universal indicator. Digestion: the stomach makes hydrochloric acid, which helps digestion without harming the stomach; in indigestion, too much acid causes pain and irritation, relieved by bases called antacids such as magnesium hydroxide (milk of magnesia) — Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O (equation Beyond NCERT).

Tooth decay starts when the pH of the mouth falls below 5.5. Enamel — calcium hydroxyapatite, a crystalline calcium phosphate and the hardest substance in the body — does not dissolve in water but is corroded below pH 5.5. Bacteria in the mouth make acids by degrading sugar and food particles left after eating; cleaning the mouth after eating, and toothpastes, which are generally basic, neutralise the excess acid. Self-defence by chemical warfare: a honey-bee sting leaves an acid — a mild base like baking soda gives relief; the stinging hairs of nettle inject methanoic acid, and the traditional remedy is rubbing with a dock leaf, which often grows nearby.

In Activity 2.13, students write formulae for nine salts and identify the acid and base each may come from. Salts with the same positive or negative radical belong to a family: NaCl and Na₂SO₄ are sodium salts; NaCl and KCl are chloride salts. In Activity 2.14, solutions of eight salts are tested with litmus and pH paper. NCERT's rule: salts of a strong acid and strong base are neutral (pH 7); of a strong acid and weak base are acidic (pH below 7); of a strong base and weak acid are basic (pH above 7).

Families in the list: sodium salts (NaCl, Na₂SO₄, NaNO₃, Na₂CO₃ — 4); sulphate salts (K₂SO₄, Na₂SO₄, CaSO₄, MgSO₄, CuSO₄ — 5); chloride salts (NaCl, NH₄Cl — 2); and single members of potassium, calcium, magnesium, copper, ammonium, nitrate and carbonate families.

Sodium chloride — the salt of HCl and NaOH, and a neutral salt — is separated from the many salts in seawater; solid deposits, often brown large crystals called rock salt, formed when ancient seas dried up and are mined like coal. NCERT recalls Mahatma Gandhi's Dandi March: salt was an important symbol in the freedom struggle. Sodium hydroxide: electricity through brine (aqueous NaCl) gives 2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g) — the chlor-alkali process (chlor for chlorine, alkali for sodium hydroxide). Chlorine is given off at the anode, hydrogen at the cathode, and sodium hydroxide solution forms near the cathode.

Bleaching powder is made by the action of chlorine on dry slaked lime: 2Ca(OH)₂ + 2Cl₂ → Ca(ClO)₂ + CaCl₂ + 2H₂O. It is represented as Ca(ClO)₂, though its actual composition is quite complex. Uses: bleaching cotton and linen in the textile industry, wood pulp in paper factories and washed clothes in laundry; an oxidising agent in chemical industries; and making drinking water free from germs. Baking soda, sodium hydrogencarbonate, is made using sodium chloride as a raw material: NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃. It is a mild, non-corrosive basic salt; on heating during cooking, 2NaHCO₃ —[heat]→ Na₂CO₃ + H₂O + CO₂. Baking powder is baking soda plus tartaric acid, a weak edible acid; heated or wetted, NaHCO₃ + H⁺ → Na⁺ + H₂O + CO₂ (NCERT: ‘+ sodium salt of acid’) — the CO₂ makes bread and cake rise, soft and spongy. Baking soda is also in antacids and in soda-acid fire extinguishers. Washing soda: sodium carbonate from heating baking soda is recrystallised: Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O. It is a basic salt, used in the glass, soap and paper industries, to make sodium compounds such as borax, for domestic cleaning, and to remove permanent hardness of water.

In Activity 2.15, a few blue copper sulphate crystals heated in a dry boiling tube turn white, and water droplets appear in the tube; 2–3 drops of water on the white powder bring the blue colour back. The crystals that seemed dry contained water of crystallisation — the fixed number of water molecules in one formula unit of a salt: CuSO₄·5H₂O(s) —[heat]→ CuSO₄(s) + 5H₂O(g); CuSO₄(s) + 5H₂O(l) → CuSO₄·5H₂O(s) (equations Beyond NCERT; NCERT gives the formula CuSO₄·5H₂O). So Na₂CO₃·10H₂O is not ‘wet’ — its ten waters are part of the crystal. Gypsum, CaSO₄·2H₂O, has two.

Plaster of Paris. Heating gypsum at 373 K drives off water and gives calcium sulphate hemihydrate, CaSO₄·½H₂O — plaster of Paris: CaSO₄·2H₂O —[373 K]→ CaSO₄·½H₂O + 1½ H₂O (equation Beyond NCERT). It is a white powder that, mixed with water, becomes gypsum again as a hard solid mass: CaSO₄·½H₂O + 1½ H₂O → CaSO₄·2H₂O. The ½ means two formula units of CaSO₄ share one water molecule. Doctors use it to support fractured bones; it is also used for toys, decorative materials and making surfaces smooth.

Short outlines in page order, for questions in the running text, the activities and the four ‘Questions’ boxes.

Numbers

One-liners

• Acid + metal → salt + hydrogen; the gas burns with a pop.
• Acid + carbonate or hydrogencarbonate → salt + CO₂ + water; CO₂ turns lime water milky.
• Metal oxides are basic oxides; non-metal oxides are acidic oxides.
• Acids give H⁺(aq) — really H₃O⁺ — only in water; bases give OH⁻(aq).
• An alkali is a base that dissolves in water.
• Add acid to water slowly with stirring — never water to acid.
• Higher H⁺ concentration = lower pH.
• Strong acid + strong base → neutral salt; strong acid + weak base → acidic salt; strong base + weak acid → basic salt.
• Chlor-alkali: Cl₂ at the anode, H₂ at the cathode, NaOH near the cathode.
• Bleaching powder = chlorine + dry slaked lime; represented as Ca(ClO)₂.
• Baking soda NaHCO₃; baking powder = baking soda + tartaric acid; washing soda Na₂CO₃·10H₂O.
• Gypsum CaSO₄·2H₂O at 373 K → plaster of Paris CaSO₄·½H₂O; with water it sets back to gypsum.
• Ant and nettle stings: methanoic acid; vinegar: acetic; curd: lactic; tamarind: tartaric; lemon and orange: citric.

Traps to avoid

• Every alkali is a base, but not every base is an alkali.
• Dry HCl gas does not turn dry blue litmus red — ions need water.
• Strong ≠ concentrated: dilution lowers concentration, not strength.
• Acid rain is below pH 5.6; tooth decay begins below 5.5.
• Washing soda and baking soda solutions are basic, though both are salts.
• Baking soda is NaHCO₃; anhydrous Na₂CO₃ is soda ash, not baking soda.
• Lime water turns milky with CO₂ and clears again with excess CO₂.
• Phenolphthalein is colourless in acids AND in neutral water; only bases turn it pink.
• Plaster of Paris is the hemihydrate (½H₂O), gypsum the dihydrate (2H₂O) — heat gypsum to get POP, add water to POP to get gypsum.
• NCERT slips: litmus is from lichens (not ‘a plant’); Figure 2.7 lumps pure water (7) with blood (7.4); Na₂ZnO₂ is shown as (s).

Prelims — what UPSC asks

• Names, formulae and uses of salts are the most-asked part: blue vitriol, Epsom salt, baking soda and caustic soda (1998); baking soda in fire extinguishers, quicklime in glass, gypsum to plaster of Paris (2004); ‘anhydrous sodium carbonate is baking soda’ (false), Hasenclever plant (2005); washing soda solution is basic (1984); copper sulphate solution is acidic by hydrolysis (2001).
• Natural acids: ants inject formic (methanoic) acid (1999); vinegar is acetic acid (1993); lactic–milk, acetic–vinegar, citric–lemon, butyric–rancid butter (1988). Learn Table 2.3 by heart and add butyric acid.
• Safety, ions and pH: acid into water, with the specific-heat reason keyed as not the explanation (1999); blood pH 7.35–7.45 (2008); alkalis for washing utensils (1986); chlorine for municipal water (1980).
• Likely next: pH thresholds (5.6, 5.5), antacids, soil liming, the chlor-alkali products and their uses, water of crystallisation, olfactory indicators. Acid-rain causes (SO₂ and NO_x; asked in 1988, 1993, 2010, 2013, 2022) need the environment chapters as well as this one.
• Answer-key note (1986, alkalis for washing utensils): the stored key says the reason ‘alkalis are readily soluble in water’ correctly explains the assertion. Solubility is what defines an alkali; alkalis clean because they act on grease and oil (turning fats into soap). Treat this key as disputed — if it appears again, ‘both true, R not the correct explanation’ is the safer reading.

Mains — where it appears and how to answer

• No GS Mains question in the bank so far is built on this chapter's core content. It supplies the science for GS3 answers on acid rain (sources, effects on lakes, soils and monuments), soil acidity and liming or gypsum for alkaline soils, drinking-water disinfection, the chlor-alkali and soda-ash industries, fluoride and oral health, and ocean acidification (Beyond NCERT).
• Answer frame: define with the pH number or a balanced equation → cause → effects (ecosystem, health, economy) → remedies (liming, emission control, chlorination) → a line on monitoring or policy, attributed. A correct threshold (‘rain below pH 5.6’) is a quick credibility marker.
• A small sketch — the pH bar with thresholds, or the salt family tree — fits in a 150-word answer.

Tap an option. NCERT's own exercise questions come first; the rest test the figures and common confusions.

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