Chemistry Basics — TNPSC Group II/IIA Science
Overview
Chemistry questions in TNPSC Group 2/2A cover the periodic table, acids, bases, salts, types of chemical reactions, and real-world applications. Focus on the properties and uses of common acids and bases, and how chemistry applies to everyday life in Tamil Nadu (salt pans, cooking, medicine).
The Periodic Table
Structure
- Periods: Horizontal rows (7 periods); period number = number of electron shells
- Groups: Vertical columns (18 groups); group number indicates valence electrons (for main group elements)
- Metals: Left side; shiny, malleable, good conductors
- Non-metals: Right side; brittle, poor conductors
- Metalloids (Semi-metals): B, Si, Ge, As — properties of both
First 20 Elements (Must memorise)
| No. | Symbol | Name | No. | Symbol | Name |
|---|---|---|---|---|---|
| 1 | H | Hydrogen | 11 | Na | Sodium |
| 2 | He | Helium | 12 | Mg | Magnesium |
| 3 | Li | Lithium | 13 | Al | Aluminium |
| 4 | Be | Beryllium | 14 | Si | Silicon |
| 5 | B | Boron | 15 | P | Phosphorus |
| 6 | C | Carbon | 16 | S | Sulphur |
| 7 | N | Nitrogen | 17 | Cl | Chlorine |
| 8 | O | Oxygen | 18 | Ar | Argon |
| 9 | F | Fluorine | 19 | K | Potassium |
| 10 | Ne | Neon | 20 | Ca | Calcium |
Memory aid: "Hi He Likes Beer But Could Not Obtain Five Neat Nurses Might Allow Silly People Somewhat Clean Arse Kindly Call" (H He Li Be B C N O F Ne Na Mg Al Si P S Cl Ar K Ca)
Important Groups
| Group | Name | Examples | Properties |
|---|---|---|---|
| Group 1 | Alkali Metals | Li, Na, K, Rb | Highly reactive; soft; react with water |
| Group 2 | Alkaline Earth Metals | Mg, Ca, Ba | Reactive; form oxides |
| Group 17 | Halogens | F, Cl, Br, I | Reactive non-metals; salt-formers |
| Group 18 | Noble Gases | He, Ne, Ar, Kr | Unreactive; zero valency |
Acids
Properties of Acids
- Taste sour
- Turn blue litmus red
- pH < 7
- Release H⁺ ions in solution (Arrhenius definition)
- React with metals to release hydrogen
Common Acids
| Acid | Formula | Found In / Uses |
|---|---|---|
| Hydrochloric acid | HCl | Stomach acid (gastric acid); cleaning metals |
| Sulphuric acid | H₂SO₄ | Car batteries; fertilizers; "oil of vitriol" |
| Nitric acid | HNO₃ | Fertilizers; explosives |
| Acetic acid (Ethanoic acid) | CH₃COOH | Vinegar (5% solution); food preservative |
| Citric acid | C₆H₈O₇ | Lemons, limes, oranges |
| Lactic acid | C₃H₆O₃ | Sour milk, yoghurt, muscles after exercise |
| Carbonic acid | H₂CO₃ | Fizzy drinks (CO₂ dissolved in water) |
| Tannic acid | — | Tea (tannic taste) |
Strong vs Weak Acids
- Strong acids: Completely ionise in water — HCl, H₂SO₄, HNO₃, HClO₄
- Weak acids: Partially ionise — Acetic acid, Citric acid, Carbonic acid
Bases and Alkalis
Properties of Bases
- Taste bitter
- Feel soapy
- Turn red litmus blue
- pH > 7
- Release OH⁻ ions in solution
Alkali: A base that dissolves in water
Common Bases
| Base/Alkali | Formula | Uses |
|---|---|---|
| Sodium hydroxide (Caustic soda) | NaOH | Soap making, paper, textiles |
| Potassium hydroxide | KOH | Soft soap |
| Calcium hydroxide (Slaked lime) | Ca(OH)₂ | Whitewash, water treatment |
| Calcium oxide (Quick lime) | CaO | Cement, water treatment; reacts violently with water |
| Magnesium hydroxide | Mg(OH)₂ | Antacid (Milk of Magnesia) |
| Ammonia | NH₃ | Fertilizers, cleaning agents |
| Sodium bicarbonate (Baking soda) | NaHCO₃ | Baking, antacid, fire extinguisher |
Salts
Formation
- Acid + Base → Salt + Water (Neutralisation)
- Acid + Metal → Salt + Hydrogen
- Acid + Metal Carbonate → Salt + Water + CO₂
Common Salts
| Salt | Chemical Name | Formula | Uses |
|---|---|---|---|
| Common salt | Sodium chloride | NaCl | Food, preservation |
| Baking soda | Sodium bicarbonate | NaHCO₃ | Baking, antacid |
| Washing soda | Sodium carbonate | Na₂CO₃ | Washing, glass making |
| Bleaching powder | Calcium hypochlorite | CaOCl₂ | Bleaching, disinfection |
| Plaster of Paris | Calcium sulphate hemihydrate | CaSO₄·½H₂O | Plastering, casts |
| Potash alum | Potassium aluminium sulphate | KAl(SO₄)₂·12H₂O | Water purification, pickles |
| Mohr's salt | Ammonium iron(II) sulphate | (NH₄)₂Fe(SO₄)₂·6H₂O | Standard solution |
pH Scale and Indicators
pH Values of Common Substances
| Substance | pH | Nature |
|---|---|---|
| Stomach acid (HCl) | 1–2 | Strongly acidic |
| Lemon juice | 2–3 | Acidic |
| Vinegar | 3 | Acidic |
| Coffee | 5 | Weakly acidic |
| Milk | 6.5 | Slightly acidic |
| Pure water | 7 | Neutral |
| Blood | 7.4 | Slightly basic |
| Baking soda solution | 8–9 | Weakly basic |
| Milk of Magnesia | 10–11 | Basic |
| Household ammonia | 11 | Basic |
| Bleach | 12 | Strongly basic |
| Caustic soda | 13–14 | Strongly basic |
Types of Chemical Reactions
| Type | Description | Example |
|---|---|---|
| Combination | A + B → AB | 2Mg + O₂ → 2MgO |
| Decomposition | AB → A + B | 2H₂O₂ → 2H₂O + O₂ |
| Displacement | A + BC → AC + B | Fe + CuSO₄ → FeSO₄ + Cu |
| Double Displacement | AB + CD → AD + CB | NaCl + AgNO₃ → AgCl↓ + NaNO₃ |
| Oxidation-Reduction (Redox) | Simultaneous oxidation and reduction | Burning of fuels |
| Neutralisation | Acid + Base → Salt + Water | HCl + NaOH → NaCl + H₂O |
| Combustion | Substance + O₂ → Oxide + heat | CH₄ + 2O₂ → CO₂ + 2H₂O |
Everyday Chemistry
In Food and Kitchen
- Baking powder: NaHCO₃ + Tartaric acid → CO₂ (makes bread rise)
- Curd: Lactic acid bacteria convert milk sugars to lactic acid
- Vinegar: Acetic acid (5%); preservative
- Turmeric: Contains curcumin; turns red in alkaline solution
In Cleaning and Hygiene
- Soap: Made from fats + NaOH (saponification)
- Detergent: Synthetic surfactants; work in hard water unlike soap
- Bleaching powder: Kills bacteria; whitens clothes
In Environment
- Acid rain: SO₂ + NO₂ from factories + rain → H₂SO₄ + HNO₃ (pH 4–5)
- Greenhouse gases: CO₂, CH₄, N₂O, H₂O vapour — trap heat
- Ozone depletion: CFCs (chlorofluorocarbons) from refrigerants destroy O₃ layer
Tamil Nadu Salt Industry
- Thoothukudi (Tuticorin): Major salt-producing area
- Salt obtained by solar evaporation of seawater
- India is 3rd largest salt producer in world
- Tamil Nadu produces over 30% of India's salt
Key Facts for TNPSC Group II/IIA
- Hydrogen (H): Lightest element; most abundant element in universe
- Carbon (C): Basis of all organic chemistry; element 6
- Sodium (Na): Soft metal; from Latin "Natrium"; highly reactive with water
- pH < 7: Acidic; pH 7: Neutral; pH > 7: Basic
- Litmus: Red in acid; Blue in base — most commonly tested indicator
- Neutralisation: Acid + Base → Salt + Water; basis of antacids
- NaCl (Common salt): Used in food, preservation; mined at Thoothukudi, TN
- NaHCO₃ (Baking soda): Used in baking, antacid, fire extinguisher
- Ca(OH)₂ (Slaked lime): Whitewash; also used in water treatment
- Acid rain: pH 4–5; caused by SO₂ and NO₂ from industrial emissions
Previous Year MCQ Patterns
Q: Which of the following turns blue litmus paper red? a) Lemon juice b) Sodium hydroxide c) Pure water d) Baking soda
Q: The chemical formula of baking soda is: a) Na₂CO₃ b) NaHCO₃ c) NaCl d) NaOH
Q: Neutralisation reaction produces: a) Acid and Base b) Salt and Water c) Oxide and Hydrogen d) Carbon dioxide and Water
Frequently Asked Questions
1-H (Hydrogen), 2-He (Helium), 3-Li (Lithium), 4-Be (Beryllium), 5-B (Boron), 6-C (Carbon), 7-N (Nitrogen), 8-O (Oxygen), 9-F (Fluorine), 10-Ne (Neon), 11-Na (Sodium), 12-Mg (Magnesium), 13-Al (Aluminium), 14-Si (Silicon), 15-P (Phosphorus), 16-S (Sulphur), 17-Cl (Chlorine), 18-Ar (Argon), 19-K (Potassium), 20-Ca (Calcium). Memory aid: H He Li Be B C N O F Ne Na Mg Al Si P S Cl Ar K Ca