Chemistry: Elements, Compounds, Acids, Bases, Salts, Fertilizers & Pesticides
Overview
Chemistry forms a core component of the General Science section in TNPSC Group II/IIA Prelims. Questions typically test fundamental concepts rather than complex calculations—expect direct factual questions on properties of acids and bases, common chemical compounds, pH values, and applications of fertilizers and pesticides in agriculture.
This topic connects strongly with other areas like Botany (plant nutrition), Environment (pollution from chemicals), and Health (pesticide toxicity). Mastering the basics here gives you easy marks and builds a foundation for interdisciplinary questions. Focus on definitions, classification, everyday applications, and the chemical names of common substances.
Key Concepts
- Element vs Compound: An element contains only one type of atom (e.g., oxygen, iron). A compound contains two or more elements chemically bonded in a fixed ratio (e.g., water H₂O, salt NaCl).
- Acids: Substances that release hydrogen ions (H⁺) in water. They taste sour, turn blue litmus red, and have pH less than 7. Examples: hydrochloric acid (HCl), sulphuric acid (H₂SO₄), acetic acid (vinegar).
- Bases: Substances that release hydroxide ions (OH⁻) in water. They taste bitter, feel soapy, turn red litmus blue, and have pH greater than 7. Examples: sodium hydroxide (NaOH), calcium hydroxide (lime water).
- Salts: Formed when an acid reacts with a base (neutralization). The general equation: Acid + Base → Salt + Water. Example: HCl + NaOH → NaCl + H₂O.
- pH Scale: Measures acidity or alkalinity on a scale of 0–14. pH 7 is neutral (pure water). Below 7 is acidic; above 7 is basic/alkaline.
- Fertilizers: Chemical or organic substances added to soil to supply essential nutrients (N, P, K) for plant growth. They enhance crop yield but can cause water pollution if overused.
- Pesticides: Chemicals used to kill or control pests. Categories include insecticides, herbicides, fungicides, and rodenticides. Overuse leads to bioaccumulation and environmental damage.
Formulas / Key Facts
| Category | Name | Chemical Formula | Key Use/Property |
|---|---|---|---|
| Acid | Hydrochloric acid | HCl | Digestive juice in stomach |
| Acid | Sulphuric acid | H₂SO₄ | "King of chemicals," batteries |
| Acid | Nitric acid | HNO₃ | Fertilizer production, explosives |
| Acid | Acetic acid | CH₃COOH | Vinegar (5% solution) |
| Base | Sodium hydroxide | NaOH | Caustic soda, soap making |
| Base | Calcium hydroxide | Ca(OH)₂ | Slaked lime, whitewashing |
| Base | Ammonium hydroxide | NH₄OH | Cleaning agents |
| Salt | Sodium chloride | NaCl | Common/table salt |
| Salt | Sodium bicarbonate | NaHCO₃ | Baking soda, antacid |
| Salt | Sodium carbonate | Na₂CO₃ | Washing soda |
| Salt | Calcium carbonate | CaCO₃ite | Limestone, marble, chalk |
| Fertilizer | Urea | CO(NH₂)₂ | 46% nitrogen, most used N-fertilizer |
| Fertilizer | Ammonium sulphate | (NH₄)₂SO₄ | Nitrogen + sulphur source |
| Fertilizer | Superphosphate | Ca(H₂PO₄)₂ | Phosphorus fertilizer |
| Pesticide | DDT | Dichlorodiphenyltrichloroethane | Banned insecticide, persistent |
| Pesticide | Malathion | Organophosphate | Mosquito control |
pH values to remember: Lemon juice ≈ 2, Vinegar ≈ 3, Pure water = 7, Blood ≈ 7.4, Milk of magnesia ≈ 10, Household ammonia ≈ 11.
NPK ratio: Fertilizer bags show N:P:K ratio—Nitrogen promotes leaf growth, Phosphorus aids root/flower development, Potassium improves disease resistance.
Worked Examples
Example 1: What happens when zinc reacts with dilute hydrochloric acid?
Solution: Zinc displaces hydrogen from the acid. Zn + 2HCl → ZnCl₂ + H₂↑ Products: Zinc chloride (salt) and hydrogen gas (released as bubbles). This is a displacement reaction and also produces a salt.
Example 2: Identify whether the following are acidic, basic, or neutral: (a) Tomato juice (pH 4), (b) Blood (pH 7.4), (c) Soap solution (pH 9).
Solution: (a) pH 4 < 7 → Acidic (b) pH 7.4 > 7 → Slightly basic (c) pH 9 > 7 → Basic
Example 3: Why is urea preferred over ammonium sulphate as a fertilizer?
Solution: Urea contains 46% nitrogen compared to only 21% in ammonium sulphate. Higher nitrogen content means less quantity needed per hectare, reducing transport and application costs. Urea is also highly water-soluble, making it easily absorbed by plants.
Common Mistakes
- Confusing acids and bases by litmus test: Students often reverse this. Remember: Acids turn blue litmus RED (think "A-RED" for Acid-Red), bases turn red litmus BLUE.
- Mixing up baking soda and washing soda: Baking soda is NaHCO₃ (sodium bicarbonate)—used in cooking. Washing soda is Na₂CO₃ (sodium carbonate)—used for cleaning. The "bi" in bicarbonate indicates one hydrogen.
- Assuming all fertilizers are harmful: Fertilizers themselves are essential; the problem is overuse causing eutrophication. Organic fertilizers (compost, manure) are alternatives but not substitutes for all nutrients.
- Believing DDT is still in use: DDT is banned in agriculture in India (since 1989) but restricted use continues for malaria control. Questions may test whether you know its banned status.
- Forgetting that pH 7 is only neutral at room temperature: For exam purposes, always treat pH 7 as neutral. Don't overcomplicate with temperature effects unless specifically asked.
Quick Reference
- Acid = H⁺ donor, pH < 7, sour, blue litmus → red
- Base = OH⁻ donor, pH > 7, bitter/soapy, red litmus → blue
- Neutralization: Acid + Base → Salt + Water
- NPK: Nitrogen (growth), Phosphorus (roots), Potassium (immunity)
- Urea = 46% N, most common nitrogen fertilizer
- DDT = banned persistent organic pollutant (bioaccumulation)
- Plaster of Paris = CaSO₄·½H₂O; Gypsum = CaSO₄·2H₂O