9.3 Acids, Bases & Salts
Key Takeaways
- Acids are substances that donate hydrogen ions (H⁺) in water and turn blue litmus red; bases produce hydroxide ions (OH⁻) and turn red litmus blue.
- The pH scale runs from 0 to 14: below 7 is acidic, 7 is neutral and above 7 is basic/alkaline, with each unit representing a ten-fold change in H⁺ concentration.
- Neutralisation is the reaction of an acid with a base to form a salt and water: HCl + NaOH → NaCl + H₂O.
- Salts are classified as normal (NaCl), acidic (NH₄Cl), basic (Na₂CO₃) or double (potash alum); common household chemicals — baking soda (NaHCO₃), washing soda (Na₂CO₃·10H₂O), bleaching powder (CaOCl₂) — are salts.
- Indicators reveal acidity or basicity: litmus (acid → red, base → blue), phenolphthalein (colourless in acid, pink in base) and methyl orange (red in acid, yellow in base).
Acids, bases and salts touch daily life — lemon juice (citric acid), tamarind (tartaric acid), milk of magnesia (Mg(OH)₂), common salt (NaCl), baking soda. RRB repeatedly tests the definitions, the pH scale, indicators, neutralisation and the formulas and uses of common salts.
What Are Acids and Bases?
- Acids are sour substances that release H⁺ ions (hydronium ions, H₃O⁺) when dissolved in water. Examples: HCl, H₂SO₄, HNO₃, CH₃COOH (acetic acid, vinegar), citric acid (lemon), lactic acid (curd), tartaric acid (tamarind, baking powder).
- Bases are bitter, soapy substances that release OH⁻ ions in water. Examples: NaOH (caustic soda), KOH (caustic potash), Ca(OH)₂ (slaked lime), NH₄OH, Mg(OH)₂ (milk of magnesia, antacid).
Strong acids (HCl, H₂SO₄, HNO₃) ionise completely; weak acids (CH₃COOH, H₂CO₃) ionise partially. Strong bases (NaOH, KOH) ionise completely; weak bases (NH₄OH) partially.
Indicators
Indicators change colour depending on whether a solution is acidic or basic.
| Indicator | Acidic | Neutral | Basic |
|---|---|---|---|
| Litmus (blue) | Turns red | No change | Stays blue |
| Litmus (red) | Stays red | No change | Turns blue |
| Phenolphthalein | Colourless | Colourless | Pink |
| Methyl orange | Red | Orange | Yellow |
| China rose | Dark pink | — | Green |
Natural indicators include litmus (extracted from lichens), turmeric (turns red with bases — why a turmeric stain on cloth turns red when washed with soap) and red cabbage leaves. Synthetic indicators include phenolphthalein and methyl orange. Olfactory indicators (onion, vanilla, clove oil) change smell in acid/base — useful for visually impaired students.
The pH Scale
pH = −log₁₀[H⁺]; it measures hydrogen-ion concentration. The scale runs 0–14 at 25 °C.
| pH range | Nature | Examples |
|---|---|---|
| 0–2 | Strongly acidic | HCl (~1), gastric juice (~1.2), lemon juice (~2) |
| 3–6 | Weakly acidic | Vinegar (~3), coffee (~5), milk (~6.5), rainwater (~5.6, slightly acidic due to CO₂) |
| 7 | Neutral | Pure water, NaCl solution |
| just above 7 | Very slightly basic | Blood (~7.4) |
| 8–10 | Weakly basic | Baking soda solution (~8.2), seawater (~8), soap solution (~10) |
| 11–14 | Strongly basic | Milk of magnesia (~10.5), bleach (~12), NaOH (~14) |
A change of 1 pH unit means a 10-fold change in H⁺ concentration. Acid rain (pH < 5.6) damages plants, aquatic life and buildings (marble, limestone). Tooth decay starts when mouth pH < 5.5; tooth enamel (calcium phosphate) dissolves. Self-defence by plants: nettles inject formic acid (pH ~3) causing stinging.
Neutralisation
An acid reacts with a base to give a salt and water; heat is released (exothermic).
HCl + NaOH → NaCl + H₂O
The resulting pH depends on which reactant is in excess. If exactly stoichiometric, the salt solution's pH depends on whether the salt is normal, acidic or basic.
Salts and Their Classification
A salt is the ionic compound formed when the replaceable hydrogen of an acid is partly or fully replaced by a metal (or ammonium).
Two different classifications share these words, and mixing them is the commonest error on this topic. Keep them apart.
(a) By how much of the acid's hydrogen has been replaced — a statement about the formula:
- Normal salt: every replaceable H⁺ has been replaced. NaCl (from HCl + NaOH), Na₂SO₄, KNO₃.
- Acid salt: only part of the H⁺ has been replaced, so the formula still contains hydrogen. NaHCO₃, NaHSO₄, KHSO₄.
- Basic salt: the salt still contains a hydroxide or oxide group, e.g., basic copper carbonate Cu(OH)₂·CuCO₃.
- Double salt: two salts crystallised together in a fixed ratio, e.g., potash alum K₂SO₄·Al₂(SO₄)₃·24H₂O (used in water purification), Mohr's salt FeSO₄·(NH₄)₂SO₄·6H₂O.
(b) By the strength of the parent acid and base — a statement about the pH of the solution:
| Parent acid + base | Solution is | Example |
|---|---|---|
| Strong acid + strong base | Neutral (pH ≈ 7) | NaCl (HCl + NaOH) |
| Strong acid + weak base | Acidic (pH < 7) | NH₄Cl (HCl + NH₄OH) |
| Weak acid + strong base | Basic (pH > 7) | Na₂CO₃, CH₃COONa (H₂CO₃ or CH₃COOH + NaOH) |
The trap RRB actually sets: NaHCO₃ is an acid salt by formula, yet its solution is basic (pH about 8.2), because it comes from a weak acid (carbonic) and a strong base (sodium hydroxide). "Acid salt" describes the leftover hydrogen in the formula, not the pH of the solution.
Common Salts RRB Names
| Name | Formula | Use |
|---|---|---|
| Common salt | NaCl | Food, brine, raw material for NaOH, Cl₂, washing soda |
| Baking soda | NaHCO₃ (sodium hydrogen carbonate) | Antacid, baking (releases CO₂ → makes batter fluffy), fire extinguishers |
| Washing soda | Na₂CO₃·10H₂O | Cleaning, water softening, glass making |
| Bleaching powder | CaOCl₂ (calcium oxychloride) | Bleaching cotton/linen, disinfecting drinking water |
| Plaster of Paris | CaSO₄·½H₂O | Casts for fractured bones, statues |
| Quicklime | CaO | Manufacture of cement |
| Slaked lime | Ca(OH)₂ | Whitewash, neutralising acidic soil |
| Chalk / marble | CaCO₃ | Building material, antacid |
Chlor-alkali process: electrolysis of brine (NaCl solution) gives NaOH (caustic soda, near cathode), Cl₂ (at anode) and H₂ (at cathode): 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂. Cl₂ is used for PVC, disinfectants; NaOH for soaps, paper; H₂ for fuels, margarine.
RRB Exam Scenarios and Traps
- Trap: thinking 'all acids are strong'. Curd contains lactic acid (weak); vinegar is acetic acid (weak).
- Trap: confusing baking soda (NaHCO₃) with washing soda (Na₂CO₃·10H₂O) — the latter cannot be used as an antacid.
- Trap: pH 7 is neutral only at 25 °C; pH of pure water changes slightly with temperature.
- Expect: 'Which indicator is colourless in acid and pink in base?' Phenolphthalein.
- Expect: 'A soil pH is 4.5; which compound corrects it?' Quicklime (CaO) or slaked lime (Ca(OH)₂) — neutralisation of acid soil.
- Expect: 'Gas released when dilute HCl reacts with Na₂CO₃?' CO₂ (turns limewater milky).
- Expect: 'Bleaching powder formula?' CaOCl₂ — written as Ca(OCl)Cl to show the two different chlorine atoms.
Remember the formula–use pairs by drilling them. RRB likes to give a use and ask the formula, or vice versa.
Which salt is used as an antacid and also in baking powder to make batter fluffy?
A solution turns phenolphthalein pink and red litmus blue. Its pH is most likely:
Which gas is released when dilute hydrochloric acid reacts with sodium carbonate, and what is its test?