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

Solution

Chloramphenicol is a broad-spectrum bacteriostatic antibiotic. It inhibits bacterial protein synthesis by binding to the 50S ribosomal subunit and blocking the transpeptidation step during translation. Because it is bacteriostatic (not bactericidal), it stops bacterial growth without directly killing bacteria, relying on the immune system to clear the infection. It is effective against both gram-positive and gram-negative bacteria (broad spectrum). Option A is wrong: cell wall synthesis is inhibited by penicillin. Option C is wrong: it is broad-spectrum. Option D describes the mechanism of fluoroquinolones like ofloxacin.

Solution

Aspirin (acetylsalicylic acid) is a non-narcotic analgesic and antipyretic. It works by irreversibly inhibiting both COX-1 and COX-2 (cyclooxygenase enzymes). These enzymes catalyse the first step in prostaglandin synthesis. Prostaglandins are mediators of pain and fever. By blocking COX enzymes, aspirin reduces prostaglandin levels, relieving pain and lowering fever. The inhibition is irreversible because aspirin acetylates a serine residue in the active site of COX. This is why low-dose aspirin has an antiplatelet effect (platelets cannot regenerate COX). Option A is the mechanism of antihistamines. Option C is the mechanism of morphine and codeine. Option D is the mechanism of sulfonamide drugs.

Solution

Dettol is a commercial antiseptic whose active ingredient is chloroxylenol (a chlorinated phenol derivative) dissolved in terpineol (a terpenoid solvent). This combination gives Dettol its characteristic odour and broad antiseptic activity against bacteria, fungi, and some viruses. It is safe for use on skin and mucous membranes at the recommended dilution. Sodium hypochlorite (bleach) is a disinfectant used on surfaces, not typically on skin. Bithional is an antiseptic added to soaps, not the main ingredient of Dettol. 0.2% phenol is an antiseptic concentration, but plain phenol is not used in Dettol.

Solution

Sulfonamides are structural analogues of para-aminobenzoic acid (PABA). Bacteria synthesise folic acid from PABA using the enzyme dihydropteroate synthase. Sulfonamides mimic the shape of PABA closely enough to competitively inhibit this enzyme: they occupy the active site, PABA cannot bind, and folic acid synthesis is blocked. Without folic acid, bacteria cannot synthesise purines and certain amino acids, so they stop growing (bacteriostatic). Human cells are unaffected because we do not synthesise folic acid; we get it from our diet and lack the enzyme dihydropteroate synthase entirely. This selective toxicity makes sulfonamides safe and effective antibiotics.

Solution

Hard water contains dissolved Ca2+ and Mg2+ ions. When soap (sodium salt of a fatty acid, R-COONa) is added to hard water, Ca2+ reacts with the carboxylate head to form calcium soaps: 2 RCOONa + CaCl2 → (RCOO)2Ca (insoluble precipitate) + 2 NaCl. This insoluble grey scum reduces the cleaning ability and wastes soap. Detergents have a sulfonate head group (-SO3-Na+) or sulfate head group (-OSO3-Na+). Calcium and magnesium sulfonates are soluble in water. So detergents remain dissolved even in hard water, forming micelles normally and cleaning effectively. Option A is wrong: tail length is not the key difference. Option C is wrong: both can be made from either source. Option D is wrong: most common detergents are also ionic.

Solution

Bithional (2,2'-thiobis(4,6-dichlorophenol)) is a halogenated phenol derivative added to medicated soaps as an antiseptic. Its primary function is to inhibit the growth of bacteria on the skin surface. Many bacteria on skin metabolise sweat components and produce volatile odorous compounds (body odour). By killing these bacteria, bithional prevents body odour. It is not a lathering agent (that function comes from the soap molecule itself). It does not soften water (water softeners work by removing Ca2+ and Mg2+ ions). It does not stabilise fatty acids. Bithional-containing soaps are commonly called "antiseptic soaps" or "medicated soaps."

Solution

Morphine is a narcotic analgesic (also called an opioid analgesic). It acts on opioid receptors (mu, kappa, and delta receptors) in the central nervous system (CNS) and peripheral nervous system. By binding these receptors, morphine reduces the perception of pain, induces a sense of well-being (euphoria), and causes sedation. Because it acts on the CNS and can cause dependence (addiction) with prolonged use, it is classified as a narcotic. Non-narcotic analgesics (aspirin, paracetamol, ibuprofen) work peripherally by inhibiting prostaglandin synthesis and do not cause dependence at therapeutic doses. Morphine is derived from the opium poppy (Papaver somniferum) and is used medically for severe pain.

Solution

Antihistamines (H1 blockers) work by competitive antagonism at H1 histamine receptors. Histamine is released from mast cells during an allergic reaction and binds H1 receptors, causing vasodilation, increased vascular permeability, itching, and mucous secretion. Diphenhydramine and other H1 antihistamines have a similar molecular shape to histamine and compete for the H1 receptor binding site. When the antihistamine occupies the receptor, histamine cannot bind and the allergic response is suppressed. Antihistamines do NOT inhibit histamine synthesis (that would require a different drug class). They do not destroy histamine. They do not directly trigger corticosteroid release.

Solution

Ranitidine is an H2 receptor blocker (H2 antagonist). Parietal cells in the stomach lining have H2 receptors. When histamine binds these H2 receptors, the parietal cell is stimulated to secrete hydrochloric acid (HCl) into the stomach. Ranitidine competitively blocks H2 receptors, reducing the histamine-stimulated acid secretion. The result is lower acid production and a rise in gastric pH, relieving the ulcer. Option A describes simple antacids like Mg(OH)2 or Al(OH)3 that neutralise acid chemically. Option B describes omeprazole and lansoprazole (proton pump inhibitors, PPIs), which are even more potent acid suppressors. Option D describes sucralfate, a different drug class that forms a gel over ulcers.

Solution

Streptomycin is a bactericidal and broad-spectrum antibiotic. It is an aminoglycoside antibiotic that works by irreversibly binding to the 16S rRNA component of the 30S ribosomal subunit. This causes misreading of mRNA codons and incorporation of wrong amino acids into proteins, producing non-functional proteins and ultimately cell death. Because it kills bacteria (not just stops their growth), it is bactericidal. It is broad-spectrum, effective against many gram-negative bacteria (like the causative agent of tuberculosis, Mycobacterium tuberculosis) and some gram-positive bacteria. Streptomycin was the first antibiotic effective against tuberculosis. Narrow-spectrum antibiotics work against only a limited range of organisms.