Elements, Compounds and Acids-Bases
Overview
Elements, Compounds, and Acids-Bases is a high-yield chapter for SSC CGL and CHSL. It covers pure substances, mixtures, chemical bonding basics, pH scale, properties of acids and bases, and important salts. Questions frequently test pH values of common substances, names of elements by symbol, acid-base indicators, and reactions like neutralisation. Mastering this chapter ensures 2-4 marks in the General Science section.
Typed Notes
Elements and Their Symbols
An element cannot be broken down into simpler substances by chemical means. The periodic table has 118 elements. SSC frequently asks Latin-origin symbols: Gold (Au-Aurum), Silver (Ag-Argentum), Iron (Fe-Ferrum), Copper (Cu-Cuprum), Mercury (Hg-Hydrargyrum), Lead (Pb-Plumbum), Sodium (Na-Natrium), Potassium (K-Kalium), Tin (Sn-Stannum), Tungsten (W-Wolfram). Non-metals like Carbon (C), Nitrogen (N), Oxygen (O) use English-based symbols. Hydrogen is the lightest element; Osmium is the densest element.
Compounds vs Mixtures
A compound is a pure substance formed by chemical combination of elements in a definite mass ratio; it has a fixed melting/boiling point. A mixture has no fixed composition and can be separated by physical methods. Key compounds for SSC: NaCl (common salt), H₂SO₄ (sulphuric acid — king of chemicals), HNO₃ (nitric acid — queen of chemicals), NaOH (caustic soda), Ca(OH)₂ (slaked lime), CaCO₃ (limestone/marble), NaHCO₃ (baking soda), Na₂CO₃ (washing soda), CuSO₄ (blue vitriol), FeSO₄ (green vitriol), ZnSO₄ (white vitriol).
Acids — Types and Properties
Acids donate H⁺ ions in solution. Strong acids fully ionise: H₂SO₄, HCl (hydrochloric acid), HNO₃. Weak acids partially ionise: CH₃COOH (acetic acid in vinegar), H₂CO₃ (carbonic acid in soda water), citric acid (in lemon), lactic acid (in curd/sour milk), formic acid (in ant sting), oxalic acid (in tomato). Aqua Regia = 3 parts HCl + 1 part HNO₃; it dissolves gold and platinum. Acid rain has pH below 5.6, caused by SO₂ and NO₂ emissions.
Bases, Salts and pH Applications
Bases accept H⁺ ions or donate OH⁻ ions. Strong bases: NaOH, KOH, Ca(OH)₂. Weak base: NH₄OH (ammonium hydroxide). Antacids like Mg(OH)₂ (milk of magnesia) neutralise excess stomach acid (HCl, pH ≈ 1.5–2). Baking soda (NaHCO₃) is a mild base used in cooking and as antacid. Bleaching powder = Ca(OCl)Cl; used to disinfect water. Plaster of Paris = CaSO₄·½H₂O. Tooth enamel erodes below pH 5.5. Soil acidity is corrected by adding lime (CaO or Ca(OH)₂).
Key Concepts
Revision ke liye ek nazar — exam se pehle yahi padhna hai.
- 01An Element is a pure substance made of only one kind of atom; 118 elements are known, 94 occur naturally. Symbol of Gold = Au (Aurum), Silver = Ag (Argentum), Iron = Fe (Ferrum), Sodium = Na (Natrium), Potassium = K (Kalium).
- 02A Compound is formed when two or more elements chemically combine in a fixed ratio; it has different properties from its constituent elements. Examples: Water (H₂O), Common Salt (NaCl), Baking Soda (NaHCO₃), Washing Soda (Na₂CO₃·10H₂O).
- 03pH Scale ranges from 0 to 14. pH < 7 = Acidic, pH = 7 = Neutral (pure water), pH > 7 = Basic/Alkaline. Human blood pH = 7.35–7.45 (slightly alkaline). Lemon juice pH ≈ 2, Milk pH ≈ 6.8.
- 04Acids taste sour, turn blue litmus RED, have pH < 7. Bases taste bitter, feel soapy, turn red litmus BLUE, have pH > 7. Neutralisation: Acid + Base → Salt + Water.
- 05Important Indicators: Litmus (natural, from lichen), Phenolphthalein (colourless in acid, pink in base), Methyl Orange (red in acid, yellow in base). Universal Indicator shows a range of colours for different pH values.
Questions & Solutions
Pehle khud try karo — phir solution kholo. Yahi tariqa topper banata hai.
Why is H₂SO₄ called the 'King of Chemicals'? What is the 'Queen of Chemicals'?
What is Aqua Regia and why is it significant?
Differentiate between Baking Soda and Washing Soda with their chemical formulas.
Name the natural acids found in common food items tested in SSC exams.
What happens when an acid reacts with a metal carbonate? Give an example.
Elements are pure single-atom substances; compounds are chemically bonded combinations with fixed ratios. The pH scale (0–14) measures acidity and alkalinity — acids below 7, bases above 7, neutral at 7. Acids turn blue litmus red; bases turn red litmus blue. Key reactions include neutralisation (acid + base → salt + water). Memorise Latin symbols, common acid/base names, and pH values of everyday substances for guaranteed SSC marks.
Chapter-end quiz
4 MCQsOption tap karo — turant pata chalega sahi ya galat, explanation ke saath.
Q1.The chemical symbol 'Hg' for Mercury is derived from which Latin word?
Q2.Which of the following has the LOWEST pH value?
Q3.Phenolphthalein indicator is colourless in acid and turns which colour in a basic solution?
Q4.Plaster of Paris is chemically known as:
Mark karo — dashboard par progress dikhega.