Acids, Bases and Salts

Why this chapter matters for UPSC: This is Chapter 2 of NCERT's Class X Science (Reprint 2026-27). It explains why acids behave alike (they all give H⁺ ions in water), what the pH number really measures, and how one raw material, common salt, becomes sodium hydroxide, bleaching powder, baking soda and washing soda. Prelims general science draws on all of it (antacids, tooth decay, bee stings, Plaster of Paris, acid rain), and the same ideas sit under GS3 questions on soil, drinking water and the ocean.

Contemporary hook: On 2 June 2026, NOAA said the fourth global coral bleaching event "likely concluded in mid-2025", after "bleaching-level heat stress impacted 84% of the world's coral reef area" between early 2023 and mid-2025. Bleaching is a heat effect. The slower change in seawater chemistry as it takes up carbon dioxide, a non-metallic oxide, is this chapter's chemistry, and keeping the two apart is a common answer-writing test.

🧠 First Principles — Read This First

One ion explains all acids. Hydrochloric, sulphuric, nitric and acetic acids all react with metals to give hydrogen, and all turn blue litmus red. The common cause is that in water they "produce hydrogen ions, H+(aq), in solution, which are responsible for their acidic properties". Bases, in the same way, generate hydroxide (OH⁻) ions in water.

No water, no acid behaviour. "The separation of H+ ion from HCl molecules cannot occur in the absence of water", and the H⁺ ion never exists alone: it joins a water molecule as the hydronium ion, H₃O⁺. That is why dry HCl gas does not change the colour of dry litmus paper.

Neutralisation is ions meeting. Whatever the acid and base, the core reaction is H⁺(aq) + OH⁻(aq) → H₂O(l). The rest of the acid and base becomes a salt.

pH is a concentration number. The pH scale measures hydrogen ion concentration: the more H₃O⁺, the lower the pH. Strength is a separate idea: at the same concentration, a strong acid gives more H⁺ ions than a weak one.

Salts carry their parents' character. A salt of a strong acid and a strong base is neutral; strong acid with weak base gives an acidic salt; strong base with weak acid gives a basic one. Water locked inside a crystal (water of crystallisation) explains blue copper sulphate, washing soda and Plaster of Paris.

PART 1 — Quick Reference

Table 1: How acids and bases react

Source: NCERT, Science Class X, ch. 2, Reprint 2026-27, sections 2.1.2-2.1.6 and Activities 2.3-2.7 (pp. 19-22); the CuO equation is NCERT's "write and balance" task, balanced on this page.

Table 2: Indicators

Source: NCERT, Science Class X, ch. 2, Reprint 2026-27, pp. 17-19, 21 and 25.

Table 3: pH values NCERT gives

Source: NCERT, Science Class X, ch. 2, Reprint 2026-27, pp. 25-27 and Exercise 11 (p. 35).

Table 4: Chemicals from common salt, and Plaster of Paris

Source: NCERT, Science Class X, ch. 2, Reprint 2026-27, sections 2.4.3 and 2.4.4 (pp. 29-33).

Table 5: Naturally occurring acids

Source: NCERT, Science Class X, ch. 2, Reprint 2026-27, Table 2.3 (p. 28).

Table 6: NCERT lines to read with care

Source: NCERT, Science Class X, Reprint 2026-27, ch. 2 pp. 17, 25-26, 33-35; NCERT, Biology Class XI, ch. 2 "Biological Classification", section 2.6 (Reprint 2025-26).

PART 2 — Concepts & Narrative

Acids and bases in the laboratory (2.1.1)

"Acids are sour in taste and change the colour of blue litmus to red, whereas, bases are bitter and change the colour of the red litmus to blue." NCERT never asks you to taste: indicators show the change by colour, and olfactory indicators by odour. In Activity 2.2, onion-soaked cloth strips are tested with dilute HCl and dilute NaOH, and vanilla essence and clove oil are tried the same way. The in-text puzzle (three test tubes, only red litmus) is solved by using the colour change twice: the base turns red litmus blue, and that blue paper then identifies the acid.

Acids and bases with metals (2.1.2)

Zinc granules in dilute sulphuric acid give a gas. Passed through soap solution, it fills bubbles, and a burning candle brought near a bubble shows the gas burns: it is hydrogen (NCERT's Fig. 2.1, "testing hydrogen gas by burning"). The activity is repeated with HCl, HNO₃ and CH₃COOH. "The metal in the above reactions displaces hydrogen atoms from the acids as hydrogen gas and forms a compound called a salt."

Some metals also react with bases. Zinc warmed with sodium hydroxide gives hydrogen and sodium zincate: 2NaOH(aq) + Zn(s) → Na₂ZnO₂(s) + H₂(g). NCERT adds the limit that such reactions are not possible with all metals. The salt formed "has a negative ion composed of the metal and oxygen".

Carbonates and hydrogencarbonates with acids (2.1.3)

Sodium carbonate and sodium hydrogencarbonate both fizz with dilute HCl, and the gas turns lime water milky:

• Na₂CO₃(s) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g)
• NaHCO₃(s) + HCl(aq) → NaCl(aq) + H₂O(l) + CO₂(g)
• Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l): the white precipitate.

Pass excess carbon dioxide and the milkiness clears, because CaCO₃(s) + H₂O(l) + CO₂(g) → Ca(HCO₃)₂(aq), which is soluble in water. "Limestone, chalk and marble are different forms of calcium carbonate", and all metal carbonates and hydrogencarbonates react with acids to give a salt, carbon dioxide and water. That is the logic of Exercise 2: a solution that fizzes with crushed egg-shells and turns lime water milky contains an acid (HCl).

Neutralisation, and the oxides (2.1.4-2.1.6)

In Activity 2.6, NaOH solution with phenolphthalein is pink; dilute HCl added drop by drop removes the colour; a little more NaOH brings the pink back. "The reaction between an acid and a base to give a salt and water is known as a neutralisation reaction": Base + Acid → Salt + Water, as in NaOH + HCl → NaCl + H₂O.

Copper(II) oxide dissolves in dilute HCl and the solution turns blue-green, the colour of copper(II) chloride: CuO + 2HCl → CuCl₂ + H₂O. Because metal oxides react with acids to give salt and water, just as bases do, they are called basic oxides. The reverse also holds. Carbon dioxide, a non-metal oxide, reacts with the base calcium hydroxide to give a salt (CaCO₃) and water, so "non-metallic oxides are acidic in nature".

In-text questions draw on this. Curd and other sour substances should not be kept in brass and copper vessels, because their acids react with the metal. A metal compound that fizzes with dilute HCl, gives a gas that puts out a candle and forms calcium chloride is calcium carbonate: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂.

What all acids and all bases have in common (2.2)

In Activity 2.8, two nails on a cork are wired to a 6-volt battery through a bulb. The bulb glows in dilute HCl and dilute H₂SO₄, but not in glucose or alcohol solution, though both contain hydrogen. "The electric current is carried through the acidic solution by ions." Acids contain H⁺ as the cation, with anions such as Cl⁻, NO₃⁻, SO₄²⁻ and CH₃COO⁻. Glucose and alcohol do not release H⁺, so they are not acids (Exercise 6).

In Activity 2.9, concentrated sulphuric acid on dry common salt gives HCl gas (in humid weather it is passed through a guard tube of calcium chloride to dry it). The gas changes wet blue litmus but not dry litmus, so hydrogen ions form only in the presence of water:

• HCl + H₂O → H₃O⁺ + Cl⁻
• H⁺ + H₂O → H₃O⁺

Hydrogen ions are therefore always shown as H⁺(aq) or as the hydronium ion, H₃O⁺. Bases dissolve to give OH⁻: NaOH(s) → Na⁺(aq) + OH⁻(aq); KOH(s) → K⁺(aq) + OH⁻(aq); Mg(OH)₂(s) → Mg²⁺(aq) + 2OH⁻(aq).

Distilled water does not conduct electricity because it has almost no ions; rain water does, because it carries dissolved substances that form ions (Exercise 7).

Dilution, and why acid goes into water (2.2.1)

A few drops of concentrated sulphuric acid swirled into 10 mL of water warm the beaker, and sodium hydroxide pellets do the same. "The process of dissolving an acid or a base in water is a highly exothermic one."

How strong are acid or base solutions? (2.3)

A universal indicator, "a mixture of several indicators", shows different colours at different concentrations of hydrogen ions. On that basis a scale for measuring hydrogen ion concentration, the pH scale, has been developed. "The p in pH stands for 'potenz' in German", meaning power. Paper impregnated with universal indicator is generally used to measure pH.

Strength is a different question. "The strength of acids and bases depends on the number of H+ ions and OH– ions produced, respectively." Take hydrochloric acid and acetic acid "of the same concentration, say one molar": they give different amounts of H⁺. "Acids that give rise to more H+ ions are said to be strong acids, and acids that give less H+ ions are said to be weak acids." This is Exercise 10: with the same amount and concentration of HCl and acetic acid on equal lengths of magnesium ribbon, fizzing is more vigorous with HCl because it gives more H⁺.

Exercise 9 puts the scale to work. Solutions A to E show pH 4, 1, 11, 7 and 9. D is neutral, C strongly alkaline, B strongly acidic, A weakly acidic and E weakly alkaline. In increasing order of hydrogen-ion concentration: C, E, D, A, B.

pH in everyday life (2.3.1)

The body and rain. "Our body works within the pH range of 7.0 to 7.8", and living organisms survive only within a narrow range. Rain with pH below 5.6 is acid rain; flowing into rivers, it lowers the pH of river water and makes the survival of aquatic life difficult. NCERT adds that "the atmosphere of venus is made up of thick white and yellowish clouds of sulphuric acid".

Soil. Plants need a particular pH range. Activity 2.12 tests soil pH: about 2 g of soil in 5 mL of water, shaken and filtered, the filtrate checked with universal indicator paper. A farmer whose soil is too acidic treats it with quick lime (calcium oxide), slaked lime (calcium hydroxide) or chalk (calcium carbonate), all of which neutralise acid (in-text question, p. 28).

Digestion. The stomach produces hydrochloric acid, which "helps in the digestion of food without harming the stomach". In indigestion it produces too much, causing pain and irritation; antacids, mild bases such as magnesium hydroxide (milk of magnesia), neutralise the excess. This answers both the chapter's opening question (baking soda solution, not lemon juice or vinegar) and Exercise 4 (antacid).

Teeth. "Tooth decay starts when the pH of the mouth is lower than 5.5." Enamel, made of calcium hydroxyapatite (a crystalline form of calcium phosphate), is the hardest substance in the body and does not dissolve in water, but it corrodes below pH 5.5. Bacteria in the mouth "produce acids by degradation of sugar and food particles remaining in the mouth after eating". Cleaning the mouth after meals, with toothpastes that are generally basic, neutralises the excess acid.

Stings. "Bee-sting leaves an acid" that causes pain and irritation; a mild base such as baking soda on the stung area gives relief. Nettle leaves have stinging hair that cause painful stings because of "the methanoic acid secreted by them". The traditional remedy is rubbing the area with the leaf of the dock plant, which often grows beside the nettle, and NCERT asks the reader to work out the dock plant's nature.

Milk and curd. Fresh milk has pH 6 (Exercise 11); as it turns to curd, acid forms and the pH falls. A milkman who adds a little baking soda shifts the milk to slightly alkaline so that it does not turn sour quickly; such milk takes longer to set as curd because the acid formed must first neutralise the base (Exercise 12).

More about salts (2.4)

Families. Salts with the same positive or negative radical belong to a family: NaCl and Na₂SO₄ are sodium salts; NaCl and KCl are chloride salts.

pH of salts. Activity 2.14 tests sodium chloride, potassium nitrate, aluminium chloride, zinc sulphate, copper sulphate, sodium acetate, sodium carbonate and sodium hydrogencarbonate. The rule: "Salts of a strong acid and a strong base are neutral with pH value of 7"; salts of a strong acid and weak base are acidic (pH below 7); those of a strong base and weak acid are basic (pH above 7). Common salt, from hydrochloric acid and sodium hydroxide, is neutral.

Common salt. Seawater holds many dissolved salts, and sodium chloride is separated from them. Large crystals of solid salt, often brown from impurities, are rock salt; beds of it formed "when seas of bygone ages dried up", and rock salt "is mined like coal". NCERT recalls Gandhi's Dandi March: salt was an important symbol in the freedom struggle.

Chemicals from common salt (2.4.3)

Sodium hydroxide. Passing electricity through brine (aqueous sodium chloride) gives 2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g). This is the chlor-alkali process, "chlor for chlorine and alkali for sodium hydroxide". "Chlorine gas is given off at the anode, and hydrogen gas at the cathode. Sodium hydroxide solution is formed near the cathode." All three products are useful.

Bleaching powder. The chlorine from this process is used to make bleaching powder, by the action of chlorine on dry slaked lime: 2Ca(OH)₂ + 2Cl₂ → Ca(ClO)₂ + CaCl₂ + 2H₂O. NCERT now represents bleaching powder as Ca(ClO)₂, "though the actual composition is quite complex" (the 2020-21 edition wrote CaOCl₂). It bleaches cotton and linen in the textile industry, wood pulp in paper factories and washed clothes in laundries; it is an oxidising agent in many chemical industries; and it makes drinking water free from germs. In-text questions 1 and 2 (p. 33): Ca(ClO)₂ is bleaching powder, and slaked lime is the substance that yields it with chlorine.

Baking soda. Sodium hydrogencarbonate is "produced using sodium chloride as one of the raw materials": NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃. It is a "mild non-corrosive basic salt". Heated during cooking, 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂ (in-text question 4).

Washing soda. Heating baking soda gives sodium carbonate, and "recrystallisation of sodium carbonate gives washing soda": Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O. "It is also a basic salt." It is used in the glass, soap and paper industries, to make sodium compounds such as borax, as a cleaning agent for domestic purposes, and "for removing permanent hardness of water" (in-text question 3: the sodium compound for softening hard water).

Are the crystals of salts really dry? (2.4.4)

Heating copper sulphate crystals in a dry boiling tube leaves water droplets in the tube, and "the salt turns white"; two or three drops of water bring the blue colour back (Activity 2.15). "Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt." Hydrated copper sulphate is CuSO₄·5H₂O, and washing soda's 10H₂O is the same kind of water, so the crystals are not wet.

Gypsum, CaSO₄·2H₂O, has two molecules of water of crystallisation. "On heating gypsum at 373 K, it loses water molecules and becomes calcium sulphate hemihydrate", CaSO₄·½H₂O, called Plaster of Paris. It is a white powder that on mixing with water changes to gypsum once again, "giving a hard solid mass": CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O. Doctors use it to support fractured bones in the right position. It must be kept in a moisture-proof container, because moisture would set it into hard gypsum (Exercise 13).

Did the 2020-21 edition differ?

Yes, in three places. Bleaching powder: the 2020-21 book represented it as CaOCl₂, made by Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O; the current book writes Ca(ClO)₂ with the equation 2Ca(OH)₂ + 2Cl₂ → Ca(ClO)₂ + CaCl₂ + 2H₂O and asks for the common name of Ca(ClO)₂. Older guides and answer keys still use CaOCl₂. Baking powder: "a mild edible acid such as tartaric acid" became "tartaric acid which is a weak edible acid". Exercise 8: "Why do acids not show acidic behaviour in the absence of water?" became a question about dry HCl gas. The rest of the chapter, its activities and exercises are unchanged.

Source: NCERT, Science Class X, 2020-21 edition (whole-book zip, Wayback Machine capture of 9 October 2021), compared with Reprint 2026-27.

PART 3 — UPSC Integration

Frames for Mains Answers

1. Soil acidity and liming. Plants need a specific pH range (NCERT Activity 2.12). Acidic soil is corrected with bases: quick lime, slaked lime or chalk neutralise the excess acid. Use this as the chemistry under any answer on soil health or fertiliser balance.

2. Acid rain. Rain below pH 5.6 is acid rain; it lowers river pH and makes survival of aquatic life difficult (NCERT). Acids react with carbonates (limestone, chalk and marble are all CaCO₃), which is why carbonate stone and shells are vulnerable.

3. Coral reefs under two stresses. Separate heat (bleaching: 84% of reef area under bleaching-level heat stress, early 2023 to mid-2025, NOAA) from chemistry (more dissolved CO₂, an acidic oxide, makes seawater less basic). Then link both to the source, carbon dioxide emissions.

Exam Strategy

Prelims fact-traps:

• pH measures hydrogen ion concentration; strength compares acids at the same concentration. A lower pH does not by itself mean a stronger acid.
• Dry HCl gas does not turn dry blue litmus red: H⁺ forms only in water, as H₃O⁺.
• Every alkali is a base; not every base is an alkali (only water-soluble bases are).
• Add acid to water, never water to acid.
• Baking soda NaHCO₃; washing soda Na₂CO₃·10H₂O; baking powder = baking soda + tartaric acid.
• Bleaching powder: chlorine on dry slaked lime; current NCERT formula Ca(ClO)₂ (older books CaOCl₂).
• Plaster of Paris: gypsum heated at 373 K; CaSO₄·½H₂O; sets back to gypsum with water.
• Tooth decay below pH 5.5; acid rain below 5.6; body 7.0 to 7.8.
• Ant and nettle stings: methanoic acid. Tamarind: tartaric. Curd: lactic. Vinegar: acetic.
• In the chlor-alkali process chlorine forms at the anode and hydrogen at the cathode, with NaOH near the cathode.

Mains: NCERT's chemistry is the explanation layer (liming acidic soil, carbonates and acid rain, CO₂ as an acidic oxide); date any data you add.

Practice Questions

Questions 1-4 are the NCERT exercise MCQs. Practice (UPSC-pattern, not past papers): questions 5-9.

1. A solution turns red litmus blue, its pH is likely to be
(a) 1
(b) 4
(c) 5
(d) 10

Answer: (d). Turning red litmus blue means a base, so pH above 7.

2. A solution reacts with crushed egg-shells to give a gas that turns lime-water milky. The solution contains
(a) NaCl
(b) HCl
(c) LiCl
(d) KCl

Answer: (b). An acid reacts with calcium carbonate to give CO₂.

3. 10 mL of a solution of NaOH is found to be completely neutralised by 8 mL of a given solution of HCl. If we take 20 mL of the same solution of NaOH, the amount HCl solution (the same solution as before) required to neutralise it will be
(a) 4 mL
(b) 8 mL
(c) 12 mL
(d) 16 mL

Answer: (d). Twice the base needs twice the acid.

4. Which one of the following types of medicines is used for treating indigestion?
(a) Antibiotic
(b) Analgesic
(c) Antacid
(d) Antiseptic

Answer: (c). Antacids are mild bases that neutralise excess stomach acid.

5. Solutions P, Q and R have pH 2, 7 and 12. Consider the following statements:
1. P has the highest hydrogen ion concentration of the three.
2. Q is neutral.
3. R would turn blue litmus red.
Which of the statements given above are correct?
(a) 1 only
(b) 2 and 3 only
(c) 1 and 2 only
(d) 1, 2 and 3

Answer: (c). R is basic: it turns red litmus blue.

6. Consider the following statements:
1. Dry HCl gas does not change the colour of dry blue litmus paper.
2. A concentrated acid should be added slowly to water, not water to the acid.
3. Bleaching powder is made by the action of chlorine on dry quick lime.
Which of the statements given above are correct?
(a) 1 and 2 only
(b) 2 and 3 only
(c) 1 and 3 only
(d) 1, 2 and 3

Answer: (a). Bleaching powder is made from dry slaked lime, Ca(OH)₂.

7. Consider the following pairs (natural source : acid):
1. Tamarind : Tartaric acid
2. Nettle sting : Methanoic acid
3. Sour milk : Citric acid
How many of the pairs given above are correctly matched?
(a) Only one
(b) Only two
(c) All three
(d) None

Answer: (b). Sour milk (curd) contains lactic acid.

8. With reference to drinking water and its treatment, consider the following statements:
1. Under IS 10500:2012, the acceptable limit for fluoride is 1.0 mg/l, and 1.5 mg/l is permitted only in the absence of an alternate source.
2. IS 10500:2012 expresses total hardness as calcium carbonate.
3. Washing soda is used to remove the permanent hardness of water.
Which of the statements given above are correct?
(a) 1 and 2 only
(b) 2 and 3 only
(c) 1 and 3 only
(d) 1, 2 and 3

Answer: (d). All three are correct.

9. Ocean acidification and coral bleaching are often treated as one problem. Distinguish between them, and explain why carbon dioxide emissions link the two. (150 words)

NCERT exercises (where the answers are on this page): 5 Table 1 (Zn + H₂SO₄ → ZnSO₄ + H₂; Mg + 2HCl → MgCl₂ + H₂; 2Al + 3H₂SO₄ → Al₂(SO₄)₃ + 3H₂; Fe + 2HCl → FeCl₂ + H₂); 6 and 7 "What all acids and all bases have in common"; 8 the H₃O⁺ explanation; 9 and 10 "How strong are acid or base solutions?"; 11 and 12 "Milk and curd"; 13 Plaster of Paris; 14 neutralisation (NaOH + HCl; CuO + 2HCl); 15 Table 4 (its baking soda and washing soda rows).

📦 Revision Capsule

Sources

• NCERT, Science, Textbook for Class X, ch. 2 "Acids, Bases and Salts", Reprint 2026-27 — ncert.nic.in PDF.
• NCERT, Biology, Textbook for Class XI, ch. 2 "Biological Classification", Reprint 2025-26 (lichens, section 2.6) — ncert.nic.in PDF.
• NCERT, Science, Class X, 2020-21 edition (whole-book zip), as archived on 9 October 2021 — Wayback Machine.
• Bureau of Indian Standards, IS 10500:2012 Drinking Water — Specification (Second Revision), with Amendment No. 2 (September 2018) — PDF hosted by the National Consumer Helpline.
• NOAA NESDIS, "World's Fourth Mass Coral Bleaching Event Likely Ended in 2025", 2 June 2026 — nesdis.noaa.gov.