Chemistry in Everyday LifeNEET Chemistry · Class 12 · NCERT Chapter 12

Both antiseptics and disinfectants kill or inhibit microorganisms, but they differ in where they are used and their strength. Antiseptics are safe to apply on living tissue: wounds, skin, mucous membranes. They are used at low concentrations so they do not damage human cells. Examples: 0.2% phenol solution (antiseptic), dilute hydrogen peroxide, iodoform, Dettol (chloroxylenol + terpineol). Disinfectants are used only on non-living surfaces like floors, drains, surgical instruments, and water supplies. They are used at higher concentrations that would be harmful to human skin. Examples: 1% phenol, chlorine gas (0.2–0.4 ppm in drinking water), ozone, sodium hypochlorite. The key NEET test point: the same chemical can act as either an antiseptic or a disinfectant depending on its concentration. Phenol at 0.2% is an antiseptic; at 1% it is a disinfectant.

A bactericidal antibiotic kills bacteria directly. A bacteriostatic antibiotic does not kill bacteria immediately but stops them from multiplying, allowing the body's immune system to clear the infection. Bactericidal antibiotics: penicillin (inhibits cell wall synthesis by blocking transpeptidase enzyme, so bacteria burst due to osmotic pressure), streptomycin (irreversibly binds 30S ribosomal subunit, misreads mRNA, kills bacteria), ofloxacin (inhibits DNA gyrase, prevents DNA replication). Bacteriostatic antibiotics: erythromycin and chloramphenicol (bind 50S ribosomal subunit, block protein synthesis but do not kill), tetracycline (binds 30S subunit reversibly, blocks aminoacyl-tRNA binding). NEET tip: "bactericidal" contains "cidal" meaning to kill; "bacteriostatic" contains "static" meaning to stop or freeze. Chloramphenicol is a NEET favourite because it is broad spectrum and bacteriostatic.

Antacids neutralise excess hydrochloric acid (HCl) in the stomach, raising the gastric pH and relieving heartburn or acidity. Simple chemical antacids react with HCl: sodium bicarbonate (NaHCO3) + HCl → NaCl + H2O + CO2. Magnesium hydroxide Mg(OH)2 and aluminium hydroxide Al(OH)3 do the same without producing CO2 gas. So why not always use sodium bicarbonate? The problems are: (1) it releases CO2, causing belching; (2) sodium bicarbonate is absorbed into the bloodstream and can cause alkalosis (excess base in blood) with prolonged use; (3) it can stimulate even more acid production by "acid rebound." Mg(OH)2 and Al(OH)3 are poorly absorbed, act more slowly, and are gentler on the system. For deeper acid suppression, H2 receptor blockers like ranitidine prevent parietal cells from secreting acid in response to histamine. Proton pump inhibitors like omeprazole block the H+/K+ ATPase pump directly and are the most powerful acid suppressors.

An analgesic relieves pain without causing loss of consciousness. An antipyretic reduces fever (elevated body temperature). These effects can overlap. Paracetamol (acetaminophen) is both an analgesic and an antipyretic: it relieves mild to moderate pain AND reduces fever. Its mechanism involves inhibition of prostaglandin synthesis in the central nervous system (CNS), which is why it reduces fever and pain but has little anti-inflammatory effect in peripheral tissues. Aspirin is also both analgesic and antipyretic, plus it has anti-inflammatory effects because it irreversibly inhibits COX-1 and COX-2 enzymes, blocking prostaglandin synthesis everywhere. Ibuprofen is analgesic, antipyretic, and anti-inflammatory (non-selective COX inhibitor). Paracetamol is the preferred antipyretic for children because aspirin in children with viral infections can cause Reye's syndrome. NEET typically tests: "Which is a non-narcotic analgesic?" Answer: aspirin, paracetamol, ibuprofen are all non-narcotic.

Soaps are sodium or potassium salts of long-chain fatty acids (e.g. sodium stearate, C17H35COONa). Each soap molecule has a hydrophilic (water-loving) ionic head (the carboxylate group, -COO- Na+) and a long hydrophobic (water-fearing) tail (the fatty acid carbon chain). When soap is added to greasy water, the hydrophobic tails cluster around oil droplets (pointing inward toward the oil) while the hydrophilic heads point outward into the water. This cluster is called a micelle. Micelles suspend the oil in water, allowing it to be washed away. Hard water contains dissolved calcium (Ca2+) and magnesium (Mg2+) ions. These ions react with soap to form insoluble calcium and magnesium salts: 2 RCOONa + Ca2+ → (RCOO)2Ca (precipitate) + 2 Na+. This insoluble "scum" wastes soap, clogs fabrics, and the soap becomes ineffective for cleaning. Detergents avoid this problem because their sulfonate or sulfate head groups form soluble salts with Ca2+ and Mg2+.

Soaps are sodium or potassium salts of natural fatty acids (made from fats + NaOH via saponification). Detergents are synthetic cleaning agents, usually sodium salts of sulfonic acids or sodium alkyl sulfates. The key structural difference is the head group: soap has a carboxylate head (-COO- Na+); detergent has a sulfonate head (-SO3- Na+) or sulfate head (-OSO3- Na+). Why detergents work in hard water: when calcium or magnesium ions are present, calcium sulfonates and sulfates remain soluble in water (unlike insoluble calcium soaps). So detergents form micelles normally and clean effectively in hard water. Detergents also have a wider pH range of activity. However, older detergents (with branched alkyl chains) were non-biodegradable, causing foam build-up in rivers. Modern detergents use linear alkylbenzene sulfonates (LAS) with straight chains, which are biodegradable. This is a key NEET distinction: branched-chain detergents = non-biodegradable; linear-chain detergents = biodegradable.

Histamine is a chemical released by mast cells and basophils during an allergic reaction. It binds to H1 receptors on blood vessels (causing dilation, redness), mucous membranes (causing runny nose, itching), and smooth muscle (causing bronchoconstriction). The symptoms you feel during an allergy (sneezing, watery eyes, itching, hives) are caused by histamine. Antihistamines block H1 receptors competitively, preventing histamine from binding. With less H1 activation, the allergy symptoms reduce. Examples of antihistamines: diphenhydramine (Benadryl, first generation, causes drowsiness because it crosses the blood-brain barrier), terfenadine and cetirizine (second generation, non-sedating, do not cross BBB as much). H2 antihistamines (ranitidine, cimetidine) block H2 receptors in the stomach lining, reducing acid secretion. So "antihistamine" without a qualifier usually means H1 blocker (allergy); H2 blockers are used for acidity. NEET tests the drug names and the receptor type.

Sulfonamides are synthetic antibiotics that work by competitive inhibition of an enzyme in bacteria called dihydropteroate synthase. This enzyme is needed for bacteria to synthesise folic acid (dihydrofolic acid) from para-aminobenzoic acid (PABA). Sulfonamides are structural analogues of PABA: their molecular shape is similar to PABA, so they compete with PABA for the active site of dihydropteroate synthase. When sulfonamide occupies the active site, PABA cannot bind, folic acid synthesis stops, and bacteria cannot make purines or certain amino acids. Bacteria die or stop growing. Why are humans unaffected? Human cells do not synthesise folic acid; we absorb it from our diet. We do not have the enzyme dihydropteroate synthase at all. So sulfonamides selectively target bacterial metabolism without harming human cells. This selectivity is what makes them effective antibiotics. Sulphamethoxazole is the most widely used sulfonamide (often combined with trimethoprim as co-trimoxazole).