Acids, Bases & Salts — 50 Q&A

An acid is a substance that produces H₊ ions in aqueous solution. They taste sour and turn blue litmus red.

A base is a substance that produces OH^- ions in aqueous solution. Soluble bases are called alkalis; they taste bitter and turn red litmus blue.

A salt is an ionic compound formed when the H⁺ of an acid is replaced by a metal or ammonium ion. Example: NaCl from HCl + NaOH.

Neutralisation is a reaction between an acid and a base to form salt and water. Example: HCl + NaOH → NaCl + H₂O.

Indicators are substances that change colour to show whether a solution is acidic or basic. Examples: litmus, phenolphthalein, methyl orange.

An acid reacts with many metals to produce a salt and hydrogen gas. Example: Zn + 2HCl → ZnCl₂ + H₂↑.

Acids react with carbonates/hydrogen carbonates to give salt, CO₂ gas and water. Example: CaCO₃ + 2HCl → CaCl₂ + CO₂↑ + H₂O.

Examples: Antacid tablets neutralising stomach acid; treating acidic soil with lime (CaO) to neutralise acidity.

pH is a measure of acidity/basicity. It ranges from 0-14: pH <7 acidic, pH =7 neutral, pH >7 basic. Lower pH means higher acidity.

A universal indicator is a mixture of indicators that shows a range of colours for different pH values, allowing approximate pH determination.

Strong acids (e.g., HCl, HNO₃, H₂SO₄) ionise completely in water. Weak acids (e.g., CH₃COOH) ionise partially.

Strong bases (e.g., NaOH, KOH) dissociate completely in water; weak bases (e.g., NH₃) accept protons partially or produce fewer OH^- ions.

Concentrated acids have a large amount of acid per unit volume (less water); dilute acids have more water and less acid per unit volume. Concentration is different from strength.

Na₂CO₃ + 2HCl → 2NaCl + CO₂↑ + H₂O.

Mg + 2HCl → MgCl₂ + H₂↑ (magnesium reacts vigorously producing hydrogen gas).

Acid + metal oxide → salt + water. Example: HCl + CuO → CuCl₂ + H₂O.

Acid + base → salt + water. Example: HCl + NaOH → NaCl + H₂O (neutralisation).

Phenolphthalein is colourless in acidic and neutral solutions and turns pink in basic solutions (pH > ~8.2).

Methyl orange turns red in acidic solutions (pH < ~3.1) and yellow in basic solutions (pH > ~4.4); useful in titrations.

Red litmus turns blue in a basic solution; blue litmus turns red in an acidic solution. Neutral solutions do not change litmus colour.

Baking soda is sodium hydrogen carbonate (NaHCO₃). Uses: in baking as a leavening agent, and to neutralise acids (e.g., antacid).

Washing soda is sodium carbonate decahydrate (Na₂CO₃·10H₂O). Uses: in laundry detergents to soften water and remove grease.

Bleaching powder is calcium oxychloride, CaOCl₂. Uses: disinfection of drinking water and bleaching of clothes.

POP is calcium sulfate hemihydrate (CaSO₄·1/2H₂O). Uses: making casts for broken bones and decorative mouldings.

Carbon dioxide is passed through a solution of sodium carbonate: Na₂CO₃ + CO₂ + H₂O → 2NaHCO₃ (laboratory method).

Brine (NaCl solution) is treated in the Solvay process or electrolysed to get Na₂CO₃ by converting to sodium bicarbonate and then heating to form sodium carbonate.

Common salt is obtained by evaporation of seawater or mining rock salt. Uses: food seasoning, chemical industry (production of NaOH, Cl₂), preservation.

Human blood pH is about 7.35–7.45. It must be maintained within a narrow range; deviation can be harmful to enzyme function and metabolism.

Acid rain is rainwater with lowered pH caused by dissolved oxides of sulfur and nitrogen (SO₂, NO_x) produced by burning fossil fuels; it damages vegetation, structures, and aquatic life.

Bubble the gas through lime water (Ca(OH)₂). It turns milky due to CaCO₃ formation; excess CO₂ clears the solution forming Ca(HCO₃)₂.

Sea water is collected in shallow ponds and allowed to evaporate under the sun. Salt crystallises and is collected (solar evaporation method).

NaOH is used in paper manufacturing, soap and detergent production, and as a strong chemical base in many processes.

POP (CaSO₄·½H₂O) reacts with water to form gypsum (CaSO₄·2H₂O), setting into a hard mass. This is an exothermic hydration reaction.

Acidic foods/drinks can erode tooth enamel leading to cavities and sensitivity. Saliva helps neutralise acids; good oral hygiene prevents damage.

Stomach acid (HCl) helps denature proteins, activate digestive enzymes (pepsin), and kill many microbes present in food.

A neutral salt is formed from strong acid and strong base; its solution is neutral (pH ≈7). Example: NaCl from HCl + NaOH.

An acidic salt is formed when a strong acid and weak base react (or partial neutralisation). Example: NH₄Cl (from NH₃ + HCl) — solution is slightly acidic.

A basic salt is formed from strong base and weak acid. Example: Na₂CO₃ (from NaOH + H₂CO₃) — solution is basic.

Titration is an analytical technique to determine the concentration of an acid or base by reacting it with a solution of known concentration (standard solution) using an indicator to find the end point.

The equivalence point is where stoichiometrically equal amounts of acid and base have reacted. The end point is observed (indicator colour change) and should be close to the equivalence point.

Antiseptics are used on living tissues to prevent infection (e.g., mild acids/bases sometimes), while disinfectants are used on inanimate objects (e.g., bleach — a basic/oxidising chemical).

Wear gloves and goggles, add acid to water (never water to acid), use fume hood for strong acids, neutralise spills appropriately, and keep neutralising agents (sodium bicarbonate) handy.

Acids react with metals producing salts and hydrogen; acid rain (containing H₂SO₄, HNO₃) accelerates corrosion of metallic structures.

Amphoteric substances can react as acid or base. Example: Aluminium hydroxide Al(OH)₃ reacts with acids and with strong bases.

NH₃ + HCl → NH₄Cl (white smoke of ammonium chloride forms on mixing gases).

Soil pH affects nutrient availability. Acidic soils (low pH) may limit plant growth; adding lime (a base) can neutralise acidity and improve fertility.

Choose an indicator whose colour change range covers the pH at equivalence point. E.g., phenolphthalein for strong acid vs strong base titration; methyl orange for strong acid vs weak base.

React dilute HCl with marble chips (CaCO₃): CaCO₃ + 2HCl → CaCl₂ + CO₂↑ + H₂O.

Common mistakes: confusing strength vs concentration, adding water to acid (dangerous), wrong equations, ignoring states, incorrect indicator choice. Tips: practise equations, remember litmus/indicator rules, always add acid to water, and revise reaction types.

1. Know definitions: acid, base, salt, neutralisation, pH.
2. Memorise litmus, phenolphthalein, methyl orange behaviour.
3. Practice reaction types: acid+metal, acid+carbonate, acid+base, acid+metal oxide.
4. Understand strength vs concentration and pH scale.
5. Remember important salts & their uses: NaCl, NaHCO₃, Na₂CO₃, CaOCl₂, POP.