Acids, Bases and Salts

1. What acids and bases are

You already know from earlier classes that the sour taste of food (lemon, vinegar, tamarind) comes from acids, while the bitter taste and soapy feel (baking soda, soap) come from bases. We never taste lab chemicals — instead we test them with indicators.

• Acids turn blue litmus red; they are sour.
• Bases turn red litmus blue; they are bitter and soapy to touch.

Common lab acids: hydrochloric acid (HCl), sulphuric acid (H₂SO₄), nitric acid (HNO₃), acetic acid (CH₃COOH). Common bases: sodium hydroxide (NaOH), potassium hydroxide (KOH), calcium hydroxide [Ca(OH)₂], magnesium hydroxide [Mg(OH)₂], ammonium hydroxide (NH₄OH).

2. Indicators — how we detect acids and bases

An acid–base indicator is a dye (or mixture of dyes) that shows a different colour in acid and in base. There are three families:

• Natural indicators: litmus (a purple dye from lichen, division Thallophyta — purple when neutral), turmeric (yellow → reddish-brown in base, which is why a soap stain on turmeric-stained cloth turns red), red cabbage, petals of Hydrangea, Petunia, Geranium, beetroot.
• Synthetic indicators: phenolphthalein and methyl orange.
• Olfactory indicators: substances whose smell changes with acid/base — onion and vanilla essence lose their smell in a base (NaOH) but keep it in acid; clove oil behaves similarly. Useful for visually impaired students.

3. Reaction of acids and bases with metals

When an acid reacts with a metal, the metal displaces hydrogen from the acid: hydrogen gas is released and a salt forms.

Test for hydrogen: pass the gas through soap solution and bring a burning candle near the gas-filled bubble — it burns with a pop sound (Activity 2.3, Fig. 2.1).

Bases with metals are less common, but some active metals (like zinc) react with a strong base to give hydrogen and a salt:

Such reactions are not possible with all metals — only with a few (Activity 2.4).

4. Acids with metal carbonates and hydrogencarbonates

All metal carbonates and hydrogencarbonates react with acids to give a salt, carbon dioxide and water (Activity 2.5, Fig. 2.2):

Test for CO₂: pass the gas through lime water [Ca(OH)₂] — it turns milky due to a white precipitate of calcium carbonate:

On passing excess CO₂ the milkiness disappears because soluble calcium hydrogencarbonate forms:

Limestone, chalk and marble are all different forms of CaCO₃.

5. Neutralisation — acids and bases react with each other

When an acid and a base react, they cancel each other's effect, giving a salt and water. This is a neutralisation reaction (Activity 2.6).

In Activity 2.6, NaOH + phenolphthalein is pink; adding HCl drop by drop makes the pink disappear (acid neutralises the base); adding more NaOH brings the pink back.

6. Reaction with metal oxides and non-metal oxides

Metal oxide + acid → salt + water. Copper oxide (black) + dilute HCl gives a blue-green solution of copper(II) chloride (Activity 2.7):

Because metal oxides react with acids just as bases do, metal oxides are called basic oxides.

Non-metal oxide + base → salt + water. CO₂ (a non-metal oxide) reacts with Ca(OH)₂ (a base), just like an acid would — so non-metal oxides are acidic oxides.

7. What all acids and all bases have in common

All acids release hydrogen gas with metals, so hydrogen is common to all acids. But not every compound containing hydrogen is acidic (glucose, alcohol contain H but are not acids). The real test is conduction of electricity (Activity 2.8, Fig. 2.3): dilute HCl and H₂SO₄ make the bulb glow, glucose and alcohol do not.

Conduction means the solution contains ions. Acids give H⁺ as the cation (Cl⁻ in HCl, NO₃⁻ in HNO₃, SO₄²⁻ in H₂SO₄, CH₃COO⁻ in CH₃COOH). So:

8. What happens in water — H₃O⁺ and OH⁻

Acids release H⁺ ions only in the presence of water (Activity 2.9, Fig. 2.4): dry HCl gas does not change dry litmus, but moist litmus turns red. The separation of H⁺ from HCl needs water:

A bare H⁺ ion cannot exist alone; it joins a water molecule to form the hydronium ion H₃O⁺:

Bases in water give hydroxide (OH⁻) ions. A base that dissolves in water is an alkali (e.g. NaOH, KOH; Mg(OH)₂, Ca(OH)₂ are only slightly soluble).

So neutralisation, at the ionic level, is simply:

Dilution & safety: dissolving an acid or base in water is highly exothermic (Activity 2.10). Always add acid to water, slowly with stirring — never water to concentrated acid, or the heat may splash the acid out and crack the glass. Dilution lowers the concentration of H₃O⁺/OH⁻ ions per unit volume.

9. The pH scale — how strong an acid or base is

"p" in pH is from German potenz = power. The pH scale runs 0 to 14 and measures the H⁺ ion concentration. A universal indicator (mixture of dyes) shows a different colour at each pH.

• Strong acid = gives more H⁺ (e.g. HCl, H₂SO₄); weak acid = gives fewer H⁺ (e.g. CH₃COOH). At the same concentration, the strong acid has the lower pH.
• Likewise a strong base gives more OH⁻ than a weak base.

Some pH values (Fig. 2.7): gastric juice ~1.2, lemon juice ~2.2, pure water/blood ~7.4, milk of magnesia ~10, NaOH solution ~14.

10. Importance of pH in everyday life

• Living organisms: the body works in a narrow pH range (~7.0–7.8). When rain water has pH below 5.6 it is acid rain; flowing into rivers it lowers their pH and threatens aquatic life.
• Soil & plants: plants need a specific pH range. Acidic soil is treated with bases — quick lime (CaO), slaked lime [Ca(OH)₂] or chalk (CaCO₃) (Activity 2.12).
• Digestion: the stomach makes HCl for digestion. Excess acid (indigestion) is neutralised by antacids — mild bases like milk of magnesia [Mg(OH)₂] or baking soda.
• Tooth decay: starts when mouth pH falls below 5.5 — bacteria turn sugar into acid that corrodes enamel (calcium hydroxyapatite, the hardest body substance). Brushing with basic toothpaste neutralises the acid.
• Self-defence in nature: bee/ant stings inject methanoic (formic) acid ⇒ relief from a mild base like baking soda; nettle leaves also sting with methanoic acid (remedy: rub dock leaf, which is basic).

11. Family and pH of salts

A salt is the ionic compound formed when an acid neutralises a base; the H⁺ of the acid and OH⁻ of the base leave as water. Salts sharing the same positive radical (NaCl, Na₂SO₄) are the sodium family; sharing the same negative radical (NaCl, KCl) are the chloride family (Activity 2.13).

pH of salts depends on the parent acid and base (Activity 2.14):

12. Common salt — the raw material for chemicals

Sodium chloride (NaCl) = the salt formed from HCl + NaOH; it is the table salt we eat, a neutral salt. Seawater and rock-salt deposits (formed from dried-up seas, mined like coal) are its sources. Common salt is the raw material for NaOH, baking soda, washing soda and bleaching powder.

Sodium hydroxide — the chlor-alkali process. Passing electricity through brine (concentrated aqueous NaCl):

"Chlor-alkali" = chlorine (at anode) + alkali NaOH (at cathode); H₂ at cathode. All three products are useful: Cl₂ → water treatment, PVC, CFCs; H₂ → fuels, margarine, ammonia; NaOH → soaps, detergents, paper.

13. Bleaching powder, baking soda, washing soda

Bleaching powder, Ca(OCl)Cl (calcium oxychloride; written CaOCl₂). Made by passing chlorine over dry slaked lime:

Uses: bleaching cotton, linen and wood-pulp; oxidising agent in industry; making drinking water germ-free (disinfectant).

Baking soda — sodium hydrogencarbonate, NaHCO₃ (a mild, non-corrosive basic salt). Made from common salt:

On heating during cooking it gives CO₂ that makes food fluffy:

Uses of baking soda: (i) in baking powder (baking soda + a mild edible acid like tartaric acid) — on heating it releases CO₂ [NaHCO₃ + H⁺ → CO₂ + H₂O + sodium salt of acid] making cakes/bread soft and spongy (the tartaric acid removes the bitter Na₂CO₃ taste); (ii) in antacids, neutralising stomach acid; (iii) in soda-acid fire extinguishers.

Washing soda — sodium carbonate, Na₂CO₃·10H₂O. Recrystallising sodium carbonate (got by heating baking soda) gives washing soda — a basic salt:

Uses of washing soda: glass, soap and paper industries; making borax; domestic cleaning agent; removing permanent hardness of water.

14. Water of crystallisation & Plaster of Paris

Water of crystallisation = the fixed number of water molecules present in one formula unit of a salt (Activity 2.15, Fig. 2.9). Blue copper sulphate crystals CuSO₄·5H₂O turn white on heating (water driven off) and blue again on adding water. The "10H₂O" in washing soda is also water of crystallisation — so the crystals are not wet.

Gypsum, CaSO₄·2H₂O has two water molecules of crystallisation. Heating gypsum to 373 K drives off most of this water to give Plaster of Paris (POP):

The "½H₂O" means two formula units of CaSO₄ share one water molecule. POP + water → gypsum, a hard solid mass:

Uses of POP: doctors set fractured bones; making toys, decorative material, smooth surfaces and chalk. It must be stored in a moisture-proof container, else it absorbs water and sets into hard gypsum.

15. NCERT in-text QUESTIONS — answered

Page 18 — Q1. Three test tubes (distilled water, acid, base), only red litmus. Dip red litmus in each: it turns blue only in the base. Take that now-blue litmus and dip it in the other two: it turns red in the acid and stays blue in distilled water (no change). So all three are identified.

Page 22 — Q1. Curd and sour foods contain acids that react with brass/copper, forming toxic compounds and spoiling the food — so avoid such vessels.

Q2. Hydrogen gas is liberated when an acid reacts with a metal: e.g. 2HCl + Zn → ZnCl₂ + H₂. Test: bring a burning candle near — it burns with a pop sound.

Q3. Gas extinguishing a candle → CO₂ → compound A is a carbonate; product is calcium chloride → A is calcium carbonate: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂.

Page 25 — Q1. HCl, HNO₃ ionise in water to give H⁺(aq) (acidic); alcohol and glucose do not release H⁺, so they are not acidic.

Q2. An acid solution conducts because it has free ions (H⁺ and the anion) that carry current.

Q3. Dry HCl gas has no water, so it cannot form H⁺ ions — no ions, no acidic behaviour, dry litmus stays unchanged.

Q4. Dilution is highly exothermic; add acid to water so heat spreads through the water. Adding water to acid releases heat in a small spot, splashing acid and cracking glass.

Q5. Diluting an acid decreases the H₃O⁺ concentration per unit volume.

Q6. Adding more base to NaOH solution increases the OH⁻ concentration.

Page 28 — Q1. pH 6 (solution A) < pH 8 (solution B), so A has more H⁺; A is acidic, B is basic.

Q2. The greater the H⁺(aq) concentration, the more acidic the solution (lower pH).

Q3. Yes, basic solutions also contain H⁺(aq) ions, but their OH⁻ concentration is greater than the H⁺ concentration — that excess OH⁻ makes them basic.

Q4. When the soil is too acidic, the farmer treats it with bases — quick lime (CaO), slaked lime [Ca(OH)₂] or chalk (CaCO₃).

Page 33 — Q1. Common name of Ca(OCl)Cl (CaOCl₂) is bleaching powder.

Q2. Slaked lime, Ca(OH)₂, on treatment with chlorine yields bleaching powder.

Q3. Sodium carbonate / washing soda (Na₂CO₃·10H₂O) softens (removes permanent hardness of) hard water.

Q4. Heating sodium hydrogencarbonate: 2NaHCO₃ + heat → Na₂CO₃ + H₂O + CO₂.

Q5. POP + water: CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O (gypsum).

16. NCERT EXERCISES — fully solved

Q1. A solution turns red litmus blue → it is basic → pH likely (d) 10.

Q2. Reacts with egg-shells (CaCO₃) giving a gas that turns lime-water milky (CO₂) → an acid → (b) HCl.

Q3. 10 mL NaOH needs 8 mL HCl, so 20 mL NaOH needs (d) 16 mL HCl (double the volume).

Q4. Medicine for indigestion (excess acid) → (c) Antacid.

Q5. Word + balanced equations:

• (a) dilute H₂SO₄ + zinc: zinc + sulphuric acid → zinc sulphate + hydrogen. Zn + H₂SO₄ → ZnSO₄ + H₂
• (b) dilute HCl + magnesium: magnesium + hydrochloric acid → magnesium chloride + hydrogen. Mg + 2HCl → MgCl₂ + H₂
• (c) dilute H₂SO₄ + aluminium: aluminium + sulphuric acid → aluminium sulphate + hydrogen. 2Al + 3H₂SO₄ → Al₂(SO₄)₃ + 3H₂
• (d) dilute HCl + iron: iron + hydrochloric acid → iron(II) chloride + hydrogen. Fe + 2HCl → FeCl₂ + H₂

Q6. To show alcohol/glucose are not acids: set up the conductivity test of Activity 2.8 (two nails on a cork connected to a 6 V battery, bulb and switch). With dilute HCl the bulb glows; with glucose or alcohol it does not — they release no H⁺ ions, so they are not acidic.

Q7. Distilled water has no ions, so it cannot conduct. Rain water dissolves CO₂ and other gases, forming ions (carbonic acid → H⁺), so it conducts.

Q8. Dry HCl gas has no water to ionise into H⁺, so it shows no acidic behaviour; acidity needs H⁺(aq) ions, which form only in water.

Q9. Five solutions with pH A=4, B=1, C=11, D=7, E=9. (a) neutral → D (pH 7); (b) strongly alkaline → C (pH 11); (c) strongly acidic → B (pH 1); (d) weakly acidic → A (pH 4); (e) weakly alkaline → E (pH 9). Increasing H⁺ concentration = decreasing pH: C < E < D < A < B (i.e. pH 11, 9, 7, 4, 1).

Q10. Test tube A (HCl, a strong acid) gives more H⁺ than B (CH₃COOH, a weak acid) at the same concentration, so fizzing is more vigorous in A.

Q11. Fresh milk has pH 6 (slightly acidic). As it turns to curd, lactic acid forms, so the pH falls below 6 (more acidic).

Q12. (a) Baking soda is basic, so it shifts milk's pH from 6 to slightly alkaline; (b) the milk takes longer to set into curd because curdling needs acidic conditions, and the alkaline milk must first be brought back to acidic by the bacteria.

Q13. POP (CaSO₄·½H₂O) absorbs moisture and sets into hard, useless gypsum (CaSO₄·2H₂O); a moisture-proof container keeps it usable.

Q14. A neutralisation reaction is the reaction of an acid with a base to give a salt and water. Examples: NaOH + HCl → NaCl + H₂O; Ca(OH)₂ + H₂SO₄ → CaSO₄ + 2H₂O.

Q15. Washing soda: (i) removes permanent hardness of water; (ii) used in glass, soap and paper industries. Baking soda: (i) ingredient of baking powder for soft, spongy cakes; (ii) used in antacids to neutralise stomach acid.

17. Common mistakes to avoid

• Confusing colour changes — acid turns blue litmus red, base turns red litmus blue. Phenolphthalein is pink in base, colourless in acid.
• Forgetting acids give H⁺ only in water — dry HCl is not acidic.
• Reversing the pH rule: more H⁺ means lower pH (more acidic).
• Mixing up baking soda NaHCO₃ with washing soda Na₂CO₃·10H₂O.
• Writing POP as CaSO₄·2H₂O (that's gypsum) — POP is CaSO₄·½H₂O.
• Saying "add water to acid" — it must be acid to water, slowly.
• Forgetting the milkiness with lime water disappears on passing excess CO₂.

18. Quick revision checklist

• Acid → H⁺(aq)/H₃O⁺; base → OH⁻(aq); soluble base = alkali.
• Acid + metal → salt + H₂ (pop test); acid + carbonate → salt + CO₂ + H₂O (lime-water test).
• Acid + base → salt + water = neutralisation; H⁺ + OH⁻ → H₂O.
• Metal oxides basic, non-metal oxides acidic.
• pH: <7 acid, 7 neutral, >7 base; controls digestion, tooth decay (<5.5), soil, acid rain (<5.6), stings.
• Salt pH: strong+strong = neutral, strong acid+weak base = acidic, weak acid+strong base = basic.
• From NaCl: NaOH (chlor-alkali), bleaching powder CaOCl₂, baking soda NaHCO₃, washing soda Na₂CO₃·10H₂O.
• Water of crystallisation: CuSO₄·5H₂O; gypsum CaSO₄·2H₂O → POP CaSO₄·½H₂O at 373 K.