3.2 General Science — Chemistry & Everyday Materials

Chemistry — Key Concepts

Matter and Its States

State changes: melting (solid→liquid), evaporation (liquid→gas), condensation (gas→liquid), freezing (liquid→solid), sublimation (solid→gas directly).

Atoms and Molecules

• Atom — smallest particle of an element that retains its properties
• Molecule — two or more atoms chemically bonded
• Atomic number (Z) = number of protons
• Mass number (A) = protons + neutrons
• Isotopes — same atomic number, different mass number (e.g., ¹H, ²H, ³H)

Acids, Bases, Salts and pH

• Strong acids: HCl, H₂SO₄, HNO₃
• Strong bases: NaOH, KOH
• Common salts: NaCl (common salt), CaCO₃ (limestone), NaHCO₃ (baking soda)

Chemical Reactions

Periodic Table Basics

• Groups = vertical columns (1–18); elements in a group have similar outer electron configuration
• Periods = horizontal rows (1–7); atomic number increases left to right
• Alkali metals (Group 1): Li, Na, K — highly reactive
• Halogens (Group 17): F, Cl, Br, I — reactive non-metals
• Noble gases (Group 18): He, Ne, Ar — inert

Carbon Compounds

• Allotropes of carbon — diamond (hardest natural), graphite (conductor), fullerene, graphene
• Hydrocarbons — alkanes (single bond), alkenes (double bond), alkynes (triple bond)
• Functional groups — alcohol (-OH), aldehyde (-CHO), carboxylic acid (-COOH)
• Ethanol (C₂H₅OH) — used in alcoholic drinks and as fuel
• Ethanoic acid (CH₃COOH) — vinegar (5–8% solution)

Metals and Non-Metals

• Reactivity series (most to least reactive): K, Na, Ca, Mg, Al, Zn, Fe, Pb, Cu, Ag, Au. A more reactive metal displaces a less reactive one from its salt solution — iron nails placed in copper sulphate solution turn brown and the blue solution fades.
• Rusting of iron needs both oxygen and moisture; it is prevented by painting, oiling, galvanising (zinc coating) or alloying into stainless steel.
• Alloys to remember: brass (copper + zinc), bronze (copper + tin), steel (iron + carbon), stainless steel (iron + chromium + nickel), solder (lead + tin).
• Noble metals — gold, silver, platinum — resist corrosion, which is why they occur in native form.

Everyday Chemicals — Common Name to Formula

Water Quality — Directly Relevant to Village Work

• Hard water contains dissolved calcium and magnesium salts. It wastes soap by forming scum and deposits scale in kettles and boilers. Soft water lathers easily.
• Temporary hardness (bicarbonates) is removed by boiling; permanent hardness (sulphates and chlorides) needs washing soda or an ion-exchange resin.
• Drinking water is disinfected by chlorination (bleaching powder) or boiling. Excess fluoride in groundwater causes fluorosis — a real problem in parts of north Karnataka — and excess arsenic or nitrate contamination also arises from groundwater.
• Water pollution indicators: high BOD (biochemical oxygen demand) means heavy organic load; eutrophication is nutrient enrichment causing algal blooms, as seen in Bengaluru's lakes.

Fertilisers, Soil Chemistry and Crops

• Plants need three primary nutrients — nitrogen (N) for leaf growth, phosphorus (P) for roots and flowering, potassium (K) for overall vigour and disease resistance.
• Urea — CO(NH₂)₂, about 46% nitrogen — is the most widely used nitrogen fertiliser in India. DAP supplies nitrogen and phosphorus; MOP (muriate of potash) supplies potassium.
• Acidic soil is corrected by adding lime; alkaline or saline soil is treated with gypsum. A VAO recording soil type and irrigation source on the RTC is, in effect, recording the inputs a soil-health question would ask about.
• Biofertilisers — Rhizobium in legume root nodules, Azotobacter, blue-green algae — fix atmospheric nitrogen biologically. Compost and green manure improve organic carbon.

Exam Angle for Chemistry

Chemistry questions in this paper are overwhelmingly definitional or common-name based. Three habits cover most of them: memorise the pH and litmus rules in one line each; keep a running common name to formula to use table like the one above; and learn the reactivity series as a spoken sentence so displacement questions become instant. Balancing equations and stoichiometry are effectively never asked at this level, so do not spend time there.