Basic Chemistry — Study Notes for UPSSSC PET

Overview

Basic Chemistry forms a core component of the General Science section in UPSSSC PET. Questions typically test fundamental concepts rather than advanced calculations: atomic structure basics, types of chemical bonds, properties of acids and bases, and identification of common compounds used in daily life. Most questions are factual recall—knowing valencies, chemical formulas, pH values, and practical applications of substances. Students must balance memorization of key facts (like "baking soda = NaHCO₃") with conceptual clarity (why ionic compounds conduct electricity when dissolved). This topic intersects with everyday life—medicines, fertilizers, fuels, cleaning agents—making it both scoring and practical. Expect 4–6 questions directly from this domain, often phrased as "Which compound is used in X?" or "What happens when acid reacts with metal?"

Your preparation strategy should prioritize common compounds and their uses, acid-base indicators, types of bonds, and the periodic table's basic organization. Avoid deep dive into quantum mechanics or complex organic chemistry; PET tests breadth, not depth. A solid grasp of NCERT Class 9–10 chemistry suffices for 90% of questions.

Key Concepts

• **Atom and subatomic particles**: An atom consists of a nucleus (protons + neutrons) surrounded by electrons in shells. Protons are positive, electrons negative, neutrons neutral. Atomic number = number of protons; mass number = protons + neutrons.

• **Electronic configuration**: Electrons occupy shells (K, L, M, N) with maximum capacities 2, 8, 18, 32. Valence electrons (outermost shell) determine chemical behavior. Example: Sodium (11) has configuration 2, 8, 1 → easily loses 1 electron.

• **Chemical bonding**: Ionic bonds form by electron transfer (metal + nonmetal), creating charged ions; covalent bonds form by electron sharing (nonmetal + nonmetal). Metallic bonding involves a "sea of electrons" among metal atoms.

• **Acids, bases, salts**: Acids taste sour, turn blue litmus red, release H⁺ in water (HCl, H₂SO₄). Bases taste bitter, turn red litmus blue, release OH⁻ (NaOH, Ca(OH)₂). Salts form when acid reacts with base (neutralization).

• **pH scale**: Measures acidity/alkalinity from 0–14. pH < 7 = acidic, pH = 7 = neutral, pH > 7 = basic. Stomach acid ~pH 2, pure water pH 7, soap ~pH 10.

• **Common compounds in daily life**: NaCl (table salt), NaHCO₃ (baking soda), CaCO₃ (limestone/marble), CaO (quicklime), Ca(OH)₂ (slaked lime), NaOH (caustic soda), H₂SO₄ (battery acid).

• **Metals vs nonmetals**: Metals are lustrous, malleable, ductile, good conductors; tend to lose electrons. Nonmetals are brittle, poor conductors (except graphite); tend to gain electrons. Metalloids (Si, Ge) have intermediate properties.

• **Periodic table basics**: Elements arranged by increasing atomic number. Groups (vertical columns) share similar properties; periods (horizontal rows) show gradual property changes. Group 1 = alkali metals, Group 17 = halogens, Group 18 = noble gases.

Formulas / Key Facts

• **Common chemical formulas**: NaCl (sodium chloride), H₂O (water), CO₂ (carbon dioxide), NaOH (sodium hydroxide), HCl (hydrochloric acid), H₂SO₄ (sulfuric acid), CaCO₃ (calcium carbonate), NH₃ (ammonia), CH₄ (methane), C₆H₁₂O₆ (glucose).

• **Valencies**: H = 1, O = 2, N = 3, C = 4, Na = 1, Ca = 2, Al = 3, Cl = 1, S = 2. Valency determines how atoms combine (e.g., H₂O because O needs 2, H provides 1 each).

• **Neutralization reaction**: Acid + Base → Salt + Water. Example: HCl + NaOH → NaCl + H₂O.

• **Metal + Acid reaction**: Metal + Acid → Salt + Hydrogen gas. Example: Zn + H₂SO₄ → ZnSO₄ + H₂↑.

• **Indicators**: Litmus (red in acid, blue in base), phenolphthalein (colorless in acid, pink in base), methyl orange (red in acid, yellow in base), turmeric (yellow in base).

• **Avogadro's number**: 6.022 × 10²³ particles per mole (not usually calculation-heavy in PET, but know it's the "dozen" of chemistry).

• **First 20 elements (symbol & atomic number)**: H(1), He(2), Li(3), Be(4), B(5), C(6), N(7), O(8), F(9), Ne(10), Na(11), Mg(12), Al(13), Si(14), P(15), S(16), Cl(17), Ar(18), K(19), Ca(20).

• **Strong acids**: HCl, HNO₃, H₂SO₄. Strong bases: NaOH, KOH.

Worked Examples

**Example 1: Determine the formula of aluminum oxide.**

• *Step 1*: Identify valencies: Aluminum (Al) = 3, Oxygen (O) = 2.
• *Step 2*: Cross-multiply valencies: Al₂O₃.
• *Step 3*: Check: 2 Al atoms (2×3 = 6) balance 3 O atoms (3×2 = 6). Formula = Al₂O₃.

**Example 2: What is the pH of lemon juice, and why?**

• *Step 1*: Lemon juice contains citric acid.
• *Step 2*: Acids have pH < 7.
• *Step 3*: Typical pH of lemon juice = 2–3 (strongly acidic). This pH kills bacteria, hence lemon's preservative property.

**Example 3: Why does sodium chloride conduct electricity when dissolved in water but not as a solid?**

• *Step 1*: NaCl is an ionic compound (Na⁺ and Cl⁻ ions).
• *Step 2*: In solid state, ions are locked in crystal lattice—cannot move.
• *Step 3*: When dissolved, ions separate and move freely in solution, carrying electric current. Hence, solution conducts, solid does not.

Common Mistakes

• **Confusing atomic number with mass number**: Atomic number = protons only; mass number = protons + neutrons. Students often mix these, especially when calculating neutrons (neutrons = mass number − atomic number).

• **Writing incorrect chemical formulas**: Writing NaCl₂ instead of NaCl. *Fix*: Always cross-check valency. Sodium has valency 1, chlorine 1 → NaCl (1:1 ratio).

• **Thinking all white powders are the same**: Confusing salt (NaCl), sugar (C₁₂H₂₂O₁₁), baking soda (NaHCO₃), washing soda (Na₂CO₃·10H₂O). *Fix*: Memorize specific uses and chemical names. Salt = seasoning, baking soda = baking/antacid, washing soda = cleaning.

• **Assuming pH 7 means "no ions"**: pH 7 = neutral, meaning equal H⁺ and OH⁻ concentration, not zero ions. Pure water has pH 7 but still contains 10⁻⁷ M H⁺ and OH⁻.

• **Believing all metals react with water**: Only reactive metals (Na, K, Ca) react vigorously with water. Less reactive metals (Fe, Zn) react with steam. Noble metals (Au, Ag) don't react with water at all. *Fix*: Reactivity series determines which metal reacts under what conditions.

Quick Reference

• **Atomic structure**: Proton (+), electron (−), neutron (neutral). Atomic number = protons. Mass number = protons + neutrons.
• **Ionic bond**: Electron transfer, forms between metal and nonmetal (e.g., NaCl). Covalent bond: Electron sharing, forms between nonmetals (e.g., H₂O).
• **Acid + Base → Salt + Water** (neutralization). **Metal + Acid → Salt + H₂ gas**.
• **pH scale**: 0–6 acidic, 7 neutral, 8–14 basic. Litmus: red → acid, blue → base.
• **Common compounds**: NaCl (salt), NaHCO₃ (baking soda), CaCO₃ (marble/limestone), NaOH (caustic soda), H₂SO₄ (battery acid).
• **Valency shortcut**: H=1, O=2, C=4, Na=1, Ca=2, Al=3, Cl=1. Cross-multiply to get formula.