NDA Chemistry · Teaching notes
Acids, Bases and Salts — NDA Chemistry
Acids, Bases and Salts is a pure recall chapter in NDA Chemistry — 33 PYQs across 2017–2026, almost all EASY or MODERATE. The bank rarely asks you to calculate; it asks you to KNOW: which acid is in a bee sting, which oxide is neutral, the pH of milk of magnesia, the formula of washing soda, the water-of-crystallization count of ferrous sulphate. Two questions in three are 'which one of the following' or 'which is NOT correct', so the win comes from memorising the tables cold. The chapter teaches in five movements, building from the underlying theory up to the everyday compounds: (1) Acid-base theory, oxides and electrolytes — the three definitions (Arrhenius, Brønsted-Lowry, Lewis), how oxides are classified acidic/basic/neutral/amphoteric, and why some solutions conduct and others do not; (2) Common acids — names, formulas, sources and uses, from the natural acids in food to the mineral acids in the lab; (3) The pH scale and common substances — what pH measures, the 0–14 range, and the pH values of everyday solutions; (4) Salts and common compounds — the household name↔formula table, what is made from common salt, and bleaching powder; (5) Water of crystallization — the fixed water counts locked into hydrated salt crystals. Most concepts are reference tables: memorise the table, win the marks.
Subtopic notes
Acid-Base Theory, Oxides and Electrolytes
7 PYQsThe three definitions of acids and bases (Arrhenius, Bronsted-Lowry, Lewis), how oxides are sorted into acidic, basic, neutral and amphoteric, and why some solutions conduct electricity while others do not.
Open note
Common Acids — Names, Formulas, Sources and Uses
8 PYQsThe everyday acids the NDA tests by source (the acid in a bee sting, in tomatoes, in vinegar), by use (etching glass, cleaning gold), and by formula (naming the oxy-acids of a halogen).
Open note
The pH Scale and Common Substances
8 PYQsWhat pH measures (hydrogen-ion concentration), the 0–14 range, and the pH values of everyday solutions — from gastric juice to milk of magnesia.
Open note
Salts and Common Compounds
7 PYQsThe household name↔formula table for the common salts (washing soda, baking soda, bleaching powder, gypsum, brine), what is manufactured from common salt, and the properties of bleaching powder.
Open note
Water of Crystallization
3 PYQsThe fixed number of water molecules locked into a salt crystal's structure, written after a dot in the formula — and the salts that have none.
Open note
PYQ weightage by concept
18 concepts · 33 PYQs — where the marks actually sit, so you know what to drill first
PYQ weightage by concept
18 concepts · 33 PYQs — where the marks actually sit, so you know what to drill first
| Concept | PYQs | Share |
|---|---|---|
| Electrolytes and electrical conductivity of solutions | 2 | 6% |
| The three acid-base concepts: Arrhenius, Bronsted-Lowry and Lewis | 1 | 3% |
| Identifying Lewis acids | 1 | 3% |
| Basicity of acids (monobasic, dibasic, tribasic) | 1 | 3% |
| Classification of oxides: acidic, basic, neutral and amphoteric | 1 | 3% |
| The first mineral acid discovered | 1 | 3% |
| Concept | PYQs | Share |
|---|---|---|
| Natural acids and their sources | 3 | 9% |
| Mineral acids and their uses | 2 | 6% |
| Acid reactions: nitric acid with metals, and carbonates with HCl | 2 | 6% |
| Naming oxy-acids: hypo-, -ous, -ic, per- | 1 | 3% |
| Concept | PYQs | Share |
|---|---|---|
| pH values of common substances | 4 | 12% |
| Indicators and the acid-base nature of household items | 3 | 9% |
| What the pH scale measures | 1 | 3% |
| Concept | PYQs | Share |
|---|---|---|
| Common names and formulas of salts | 4 | 12% |
| Bleaching powder: formula, uses and properties | 2 | 6% |
| Compounds manufactured from common salt | 1 | 3% |
| Concept | PYQs | Share |
|---|---|---|
| Water of crystallization of common salts | 2 | 6% |
| Water of crystallization and colour | 1 | 3% |
Formula & revision sheet
2 formulas · 13 reference tables · 21 gotchas across all subtopics — the exam-eve cheat-sheet
Formula & revision sheet
2 formulas · 13 reference tables · 21 gotchas across all subtopics — the exam-eve cheat-sheet
Reference tables (3)
Classification of oxides: acidic, basic, neutral and amphoteric4 rows
| Type | Reacts with | Examples |
|---|---|---|
| Acidic oxide | Bases (forms a salt) | CO2, SO2, NO2 |
| Basic oxide | Acids (forms a salt) | Na2O, MgO, CaO |
| Neutral oxide | Neither acids nor bases | CO, N2O, NO, H2O CO (carbon monoxide) is the neutral oxide the bank loves — CO2 in contrast is acidic. |
| Amphoteric oxide | Both acids and bases | Al2O3, ZnO |
Electrolytes and electrical conductivity of solutions6 rows
| Substance in water | Conducts? | Reason |
|---|---|---|
| NaOH (sodium hydroxide) | Yes (basic solution) | Strong electrolyte — gives Na+ and OH- |
| NaCl, CuSO4 (salts) | Yes | Ionise fully into free ions |
| HCl (strong acid) | Yes | Ionises fully into H+ and Cl- |
| CH3COOH (acetic acid) | Weakly | Weak electrolyte — partial ionisation |
| Sugar | No | Non-electrolyte — dissolves as whole molecules, no ions Sugar is the classic non-conducting solution — it is covalent and produces no ions. |
| CH3OH (methanol) | No | Non-electrolyte — covalent, no ions |
The first mineral acid discovered4 rows
| Mineral acid | Formula | Note |
|---|---|---|
| Nitric acid | HNO3 | The first mineral acid discovered Nitric acid is the bank's answer for 'first mineral acid discovered'. |
| Hydrochloric acid | HCl | Mineral acid — discovered later |
| Sulphuric acid | H2SO4 | 'King of chemicals', dibasic |
| Phosphoric acid | H3PO4 | Tribasic mineral acid |
Watch out for (5)
- Match the definition to the right concept→ The three acid-base concepts: Arrhenius, Bronsted-Lowry and Lewis
- NH3 is a base, not an acid→ Identifying Lewis acids
- Tribasic = phosphoric acid→ Basicity of acids (monobasic, dibasic, tribasic)
- CO is neutral, CO2 is acidic→ Classification of oxides: acidic, basic, neutral and amphoteric
- Sugar dissolves but does not ionise→ Electrolytes and electrical conductivity of solutions
Formulas (1)
Reference tables (3)
Natural acids and their sources6 rows
| Source | Acid present |
|---|---|
| Bee sting / Nettle sting / Ant | Methanoic acid (formic acid) Bee, nettle and ant stings all inject methanoic (formic) acid — the cause of the burning pain. |
| Vinegar | Ethanoic acid (acetic acid) |
| Curd / Sour milk | Lactic acid |
| Lemon, orange (citrus) | Citric acid |
| Tamarind, grapes | Tartaric acid |
| Tomato, spinach | Oxalic acid For 'acid in tomatoes', the NCERT-listed answer is oxalic acid (not citric, which is not offered in the bank's options). |
Mineral acids and their uses4 rows
| Acid | Principal use |
|---|---|
| Hydrofluoric acid (HF) | Etching glass (attacks SiO2) HF is stored in plastic, not glass, because it dissolves glass — hence its use in etching. |
| Dilute nitric acid (HNO3) | Cleaning gold and silver articles (goldsmith) Goldsmiths use dilute HNO3 — it removes base-metal impurities but leaves the noble metal. |
| Sulphuric acid (H2SO4) | Fertilisers, batteries, industry ('king of chemicals') |
| Hydrochloric acid (HCl) | Cleaning metal surfaces (pickling), lab reagent |
Naming oxy-acids: hypo-, -ous, -ic, per-4 rows
| Name | Formula | Oxygen count |
|---|---|---|
| Hypobromous acid | HOBr (HBrO) | Lowest (hypo...ous) The 'hypo-' prefix marks the lowest oxidation state — hypobromous acid is HOBr, not HBr. |
| Bromous acid | HBrO2 | Low (...ous) |
| Bromic acid | HBrO3 | High (...ic) |
| Perbromic acid | HBrO4 | Highest (per...ic) |
Watch out for (6)
- Bee and nettle stings = methanoic acid, not acetic→ Natural acids and their sources
- Tomato = oxalic acid (in the bank)→ Natural acids and their sources
- Glass etching = hydrofluoric acid only→ Mineral acids and their uses
- Hypobromous acid is HOBr, not HBr→ Naming oxy-acids: hypo-, -ous, -ic, per-
- Quick lime has no carbonate, so no CO2→ Acid reactions: nitric acid with metals, and carbonates with HCl
- Dilute HNO3 gives NO, concentrated gives NO2→ Acid reactions: nitric acid with metals, and carbonates with HCl
Formulas (1)
Reference tables (2)
pH values of common substances7 rows
| Substance | Approx. pH | Nature |
|---|---|---|
| Gastric juice | 1.5–2 | Strongly acidic — highest H+ Gastric juice has the lowest pH and therefore the highest H+ concentration of the common options. |
| Lemon juice | 2–3 | Acidic |
| Pure water | 7 | Neutral |
| Human body / blood | 7.0–7.8 | Slightly basic (narrow range) The human body operates in the pH range 7.0–7.8 — the bank's answer. |
| Milk of magnesia | 10 | Basic (antacid) Milk of magnesia (magnesium hydroxide) has pH about 10. |
| Sodium hydroxide solution | 13–14 | Strongly basic |
| Acid rain | below 5.6 | Acidic — rain turns acidic below pH 5.6 For rain to be called 'acid rain', its pH must fall below 5.6. |
Indicators and the acid-base nature of household items5 rows
| Item / indicator | Behaviour |
|---|---|
| Turmeric stain + soap then water | Yellow → reddish-brown → yellow Turmeric goes reddish-brown in alkaline soap and back to yellow when the soap is washed away. |
| Toothpaste | Basic (neutralises mouth acid) |
| FeCl3 solution | pH < 7 (acidic, by hydrolysis) FeCl3 = strong-acid + weak-base salt, so it hydrolyses to an acidic solution (pH < 7). |
| NaCl, KCl solution | pH ≈ 7 (neutral) |
| NaOH solution | Basic (pH > 7) |
Watch out for (3)
- Higher H+ means LOWER pH→ What the pH scale measures
- Acid rain threshold is pH 5.6, not 7→ pH values of common substances
- FeCl3 is acidic, not neutral→ Indicators and the acid-base nature of household items
Reference tables (3)
Common names and formulas of salts7 rows
| Common name | Chemical name / formula |
|---|---|
| Washing soda | Sodium carbonate, Na2CO3·10H2OQ |
| Baking soda | Sodium bicarbonate, NaHCO3 |
| Bleaching powder | Calcium oxychloride, CaOCl2 |
| Gypsum | Calcium sulphate dihydrate, CaSO4·2H2O |
| Brine | Aqueous solution of NaCl (common salt) Brine is NaCl in water — not NaOH, NaHCO3 or Na2CO3. |
| Milk of magnesia | Magnesium hydroxide, Mg(OH)2 |
| Lime water | Calcium hydroxide solution, Ca(OH)2 Lime water is Ca(OH)2 — it does NOT represent calcium carbonate. Limestone, chalk and marble are the CaCO3 ones. |
Compounds manufactured from common salt4 rows
| Compound | Made from common salt? | Actual source |
|---|---|---|
| Washing soda (Na2CO3) | Yes | NaCl, via Solvay process |
| Baking soda (NaHCO3) | Yes | NaCl, via Solvay process |
| Bleaching powder | Yes | Chlorine (from NaCl) + slaked lime |
| Plaster of Paris | No | Gypsum (CaSO4·2H2O) Plaster of Paris is the trap — it is made from gypsum, NOT from common salt. |
Bleaching powder: formula, uses and properties5 rows
| Property / use | Detail |
|---|---|
| Chemical nature | Oxidising agent (NOT reducing) The bank's wrong statement is 'bleaching powder is a reducing agent' — it is an oxidising agent. |
| Use 1 | Bleaching wood pulp in paper factories |
| Use 2 | Bleaching linen and cotton in textiles |
| Use 3 | Disinfecting drinking water |
| Shared with DDT | Both contain chlorine Bleaching powder and DDT both contain chlorine — bleaching powder is inorganic, DDT is organic. |
Watch out for (5)
- Lime water is Ca(OH)2, not CaCO3→ Common names and formulas of salts
- Milk of magnesia = Mg(OH)2, not a carbonate→ Common names and formulas of salts
- Plaster of Paris comes from gypsum, not salt→ Compounds manufactured from common salt
- Bleaching powder oxidises, it does not reduce→ Bleaching powder: formula, uses and properties
- Bleaching powder and DDT share chlorine, not calcium→ Bleaching powder: formula, uses and properties
Reference tables (2)
Water of crystallization of common salts6 rows
| Salt | Formula | Water molecules |
|---|---|---|
| Blue vitriol (copper sulphate) | CuSO4·5H2O | 5 |
| Green vitriol (ferrous sulphate) | FeSO4·7H2O | 7 Ferrous sulphate crystal carries 7 water molecules. |
| Washing soda | Na2CO3·10H2O | 10 |
| Gypsum | CaSO4·2H2O | 2 |
| Mohr's salt | FeSO4·(NH4)2SO4·6H2O | 6 |
| Potassium permanganate | KMnO4 | 0 (none) KMnO4 has NO water of crystallization — the bank's answer for 'which salt has none'. |
Water of crystallization and colour2 rows
| Form | Colour | Cause |
|---|---|---|
| CuSO4·5H2O (hydrated) | Blue | Water of crystallization The blue colour of copper sulphate crystals is due to water — heat it out and it turns white. |
| CuSO4 (anhydrous, after heating) | White | Water removed |
Watch out for (2)
- KMnO4 has no water of crystallization→ Water of crystallization of common salts
- Blue copper sulphate owes its colour to water→ Water of crystallization and colour