- Chemistry questions in SSC and RRB papers lean heavily on common names and formulas, ores and alloys — almost pure memory, so they are quick marks.
- The pH scale runs from 0 to 14: below 7 is acidic, 7 is neutral and above 7 is basic. Each step is a tenfold change.
- Air is about 78% nitrogen, 21% oxygen, 0.93% argon and about 0.04% carbon dioxide.
Natural Sources of Acids
| Source | Acid |
|---|---|
| Lemon, orange | Citric acid |
| Vinegar | Acetic (ethanoic) acid |
| Tamarind, grapes | Tartaric acid |
| Tomato, spinach | Oxalic acid |
| Curd, sour milk | Lactic acid |
| Ant and bee sting | Formic (methanoic) acid |
| Apple | Malic acid |
| Stomach | Hydrochloric acid |
Typical pH values: stomach acid about 1.5–3.5; pure water 7; human blood about 7.4 (slightly basic). Tooth decay starts when the pH in the mouth falls below about 5.5. An ant sting is soothed by a mild base such as baking soda.
Common Names and Formulas
| Common name | Chemical name | Formula |
|---|---|---|
| Common salt | Sodium chloride | NaCl |
| Baking soda | Sodium hydrogen carbonate | NaHCO₃ |
| Washing soda | Sodium carbonate decahydrate | Na₂CO₃·10H₂O |
| Bleaching powder | Calcium oxychloride | CaOCl₂ |
| Plaster of Paris | Calcium sulphate hemihydrate | CaSO₄·½H₂O |
| Gypsum | Calcium sulphate dihydrate | CaSO₄·2H₂O |
| Quicklime / slaked lime | Calcium oxide / calcium hydroxide | CaO / Ca(OH)₂ |
| Limestone, chalk, marble | Calcium carbonate | CaCO₃ |
| Dry ice | Solid carbon dioxide | CO₂ |
| Laughing gas | Nitrous oxide | N₂O |
Blood pH~7.4
Liquid metalMercury
Liquid non-metalBromine
Most abundant in crustOxygen
Most abundant metal in crustAluminium
LPG mainlyButane (+ propane)
Metals and Their Ores
| Metal | Main ore |
|---|---|
| Aluminium | Bauxite |
| Iron | Haematite, magnetite |
| Copper | Copper pyrites (chalcopyrite) |
| Zinc | Zinc blende (sphalerite) |
| Lead | Galena |
| Mercury | Cinnabar |
Alloys
| Alloy | Made of | Use |
|---|---|---|
| Brass | Copper + zinc | Utensils, fittings |
| Bronze | Copper + tin | Statues, medals |
| Stainless steel | Iron + chromium + nickel (+ carbon) | Cutlery; chromium prevents rusting |
| Solder | Lead + tin | Joining electrical wires (low melting point) |
| German silver | Copper + zinc + nickel | Utensils — it contains no silver |
| Amalgam | Any alloy of mercury | Dental fillings (historically) |
Record-Holding Elements
- Mercury is the only metal liquid at room temperature; bromine the only liquid non-metal.
- Hydrogen is the lightest element; lithium the lightest metal.
- Tungsten has the highest melting point of all metals; silver is the best conductor of heat and electricity; gold is the most malleable and ductile.
- In the Earth's crust, oxygen is the most abundant element and aluminium the most abundant metal.
Polymers and Fuels
- Teflon (PTFE) makes non-stick coatings; PVC pipes and cables; nylon was the first fully synthetic fibre; Bakelite is a thermosetting plastic used in electrical switches because it does not soften on heating.
- Natural rubber is a polymer of isoprene; vulcanisation with sulphur (Charles Goodyear) makes it tougher.
- LPG is mainly butane with propane; a strong-smelling compound, ethyl mercaptan, is added so leaks can be detected. CNG and biogas are mainly methane.
- Petrol quality is rated by its octane number, diesel by its cetane number.
- Use CO₂ or dry-powder extinguishers on oil and electrical fires — water spreads burning oil and conducts electricity.
- Common name ↔ formula — especially the calcium compounds (gypsum vs plaster of Paris, quicklime vs slaked lime).
- Ore ↔ metal and alloy ↔ composition, often as "match the following".
- Odd facts: "German silver contains…" (no silver), "Which is a liquid non-metal?" (bromine).
Q1. The chemical formula of baking soda is:
✅ Answer: (b) NaHCO₃
💡 Explanation: Baking soda is sodium hydrogen carbonate, NaHCO₃.
Q2. Bauxite is the main ore of:
✅ Answer: (c) Aluminium
💡 Explanation: Aluminium is extracted from bauxite.
Q3. Which acid is present in an ant sting?
✅ Answer: (a) Formic acid
💡 Explanation: Ants and bees inject formic (methanoic) acid.
Q4. Brass is an alloy of:
✅ Answer: (d) Copper and zinc
💡 Explanation: Brass is copper and zinc; bronze is copper and tin.
Q5. The main component of CNG is:
✅ Answer: (a) Methane
💡 Explanation: Compressed natural gas is mostly methane.
🔁 Revise in 60 Seconds
- Lemon citric, vinegar acetic, tamarind tartaric, tomato oxalic, curd lactic, ant formic.
- NaHCO₃ baking soda; Na₂CO₃·10H₂O washing soda; CaOCl₂ bleaching powder; CaSO₄·½H₂O plaster of Paris.
- Bauxite → Al, haematite → Fe, galena → Pb, cinnabar → Hg.
- Brass Cu+Zn, bronze Cu+Sn, solder Pb+Sn; LPG butane, CNG methane.
📌 Prelims Traps to Avoid
- ✘ Gypsum has 2 water molecules; plaster of Paris has ½.
- ✘ German silver contains no silver.
- ✘ Pure LPG is odourless — the smell comes from added ethyl mercaptan.
What is the chemical name of washing soda?
Sodium carbonate decahydrate, Na₂CO₃·10H₂O. Baking soda is a different compound: sodium hydrogen carbonate, NaHCO₃.
Which acid is found in tamarind?
Tartaric acid. Tomatoes contain oxalic acid, lemons citric acid and vinegar acetic acid.