Chemistry in Everyday Life

This chapter applies chemistry to daily life — the medicines we take, the chemicals in our food, and the agents we clean with. Drugs are chemicals (usually of low molecular mass, 100–500 u) that interact with biological molecules to produce a response; when used to treat disease they are medicines, and their use is called chemotherapy. The same chemical can be a medicine in the right dose and a poison in excess, so dose matters.

Drugs are classified in four overlapping ways, and NEET expects you to recognise each. By pharmacological effect groups drugs by what they do (analgesics relieve pain, antibiotics fight bacteria). By drug action groups them by the mechanism (for example, drugs that inhibit a particular enzyme). By chemical structure groups drugs with similar skeletons (which often share effects, like the sulpha drugs). By molecular target groups them by the biomolecule they act on — and this last classification is the most chemically meaningful.

The two main molecular targets are enzymes and receptors. Enzymes are protein catalysts; a drug can act as an inhibitor that blocks the enzyme. A competitive inhibitor resembles the natural substrate and competes for the active site, while a non-competitive (allosteric) inhibitor binds elsewhere and changes the enzyme's shape so the substrate no longer fits. Blocking a key enzyme of a pathogen is a powerful way to stop disease.

Receptors are proteins, usually embedded in cell membranes, that receive chemical messages (such as hormones or neurotransmitters). Drugs that act on receptors are either agonists, which mimic the natural messenger and switch the receptor on, or antagonists, which bind the receptor and block the natural messenger (for example, many antihistamines and antacids work this way). Understanding 'enzyme inhibitor' versus 'receptor agonist/antagonist' is exactly the conceptual framework NEET tests in this topic before moving to specific drug classes.

Drugs are grouped into therapeutic classes by the effect they produce, and NEET expects you to know a representative example or two from each. Starting with the digestive system, antacids relieve acidity by countering excess stomach $\text{HCl}$ — mild ones (sodium bicarbonate, metal hydroxides) simply neutralise it, while modern drugs such as ranitidine and omeprazole reduce acid production, working more effectively.

Antihistamines (such as brompheniramine and terfenadine) treat allergies by acting as antagonists to histamine, the chemical that causes allergic symptoms. Drugs acting on the nervous system include tranquilisers and antidepressants (such as equanil and the barbiturates), used for anxiety, stress and sleep disorders, and analgesics, which relieve pain. Analgesics are of two kinds: non-narcotic (aspirin, paracetamol — also reduce fever and inflammation, non-addictive) and narcotic (morphine and related opioids — powerful painkillers but addictive, used under strict control).

The antimicrobials are a large and important group. Antibiotics (such as penicillin, the first antibiotic) are substances that kill or inhibit micro-organisms; they may be bactericidal (kill bacteria, like penicillin) or bacteriostatic (stop their growth, like erythromycin), and broad-spectrum (act on many types) or narrow-spectrum (act on a few). A crucial NEET distinction sits within antimicrobials: antiseptics versus disinfectants.

Antiseptics are applied to living tissue — skin, wounds and the mouth — and include Dettol (a mixture of chloroxylenol and terpineol), savlon, and tincture of iodine. Disinfectants are used on non-living surfaces such as floors and instruments, and are too harsh for the body. The classic example is phenol, which acts as an antiseptic at about 0.2% concentration but as a disinfectant at about 1% — the same chemical, different use and strength. Finally, antifertility drugs (such as norethindrone and ethynylestradiol) are synthetic hormones used to control reproduction. Knowing one example per class, and the antiseptic-vs-disinfectant rule, covers the bulk of this topic's questions.

A range of chemicals are added to food to keep it safe, attractive and suitable for special diets. The main groups NEET asks about are artificial sweeteners, preservatives, antioxidants, and edible colours and flavours.

Artificial sweetening agents taste sweet but supply little or no energy, so they help diabetics and people limiting calories. The common ones are saccharin (about 550 times sweeter than sugar, excreted unchanged — safe for diabetics), aspartame (about 100 times sweeter, but unstable when heated, so used only in cold foods and soft drinks), sucralose (about 600 times sweeter, stable at cooking temperatures), and alitame (very sweet, high-potency). The stability-on-heating point about aspartame is a favourite NEET detail.

Food preservatives stop spoilage by preventing the growth of micro-organisms. The oldest are common salt and sugar (used in pickles and jams), along with oils and vinegar. The most widely used chemical preservative is sodium benzoate, which the body metabolises readily; salts of sorbic acid and propanoic acid are also used. Antioxidants are a related group that prevent the oxidation of fats and oils (which causes rancidity and 'off' flavours) — the main ones are BHA (butylated hydroxyanisole) and BHT (butylated hydroxytoluene), which are more easily oxidised than the food and so are sacrificed first.

Together these additives illustrate how chemistry keeps food edible and varied: sweeteners replace sugar for health, preservatives extend shelf life, antioxidants protect fats, and colours and flavours improve appeal. For NEET, the must-remember facts are the well-known sweeteners and their relative sweetness (especially aspartame's heat-instability), sodium benzoate as the common preservative, and BHA/BHT as antioxidants that prevent rancidity. These small, factual points are easy marks if learned as a short list.

Cleansing agents remove dirt and grease, and the two main kinds — soaps and synthetic detergents — share the same physical principle but differ chemically. Both work because each molecule has a hydrophilic (water-loving) head and a hydrophobic (water-hating) hydrocarbon tail, which lets it bridge between water and grease.

Soaps are the sodium or potassium salts of long-chain fatty acids (such as sodium stearate). They are made by saponification — boiling a fat or oil with sodium hydroxide, which gives soap plus glycerol. Soaps clean well, are biodegradable and gentle, but they have one big drawback: they do not work in hard water. Hard water contains $\text{Ca}^{2+}$ and $\text{Mg}^{2+}$ ions, which react with soap to form an insoluble scum (curdy precipitate), wasting the soap and leaving deposits.

Synthetic detergents were developed to overcome this. They are typically the sodium salts of long-chain alkylbenzenesulphonic acids (or sulphates), and crucially their calcium and magnesium salts are soluble, so detergents do not form scum and clean effectively even in hard water. Detergents are classified as anionic (most common, e.g. sodium alkylbenzenesulphonates, used in washing powders), cationic (quaternary ammonium salts, used in fabric softeners and hair conditioners), and non-ionic (e.g. those used in liquid dishwashing detergents). One environmental concern is biodegradability: detergents with straight hydrocarbon chains are biodegradable, but heavily branched ones resist breakdown and cause water pollution — so straight-chain detergents are preferred.

The cleansing action of both is the same and is a classic NEET diagram. In water, the molecules arrange into spherical clusters called micelles, with the hydrophobic tails pointing inward and the hydrophilic heads facing the water. When grease is present, the tails embed into the grease droplet while the heads stay in the water, so the droplet is surrounded, lifted off the surface, and carried away in the rinse water. So the take-home points for NEET are: soap = fatty-acid salt by saponification, fails in hard water (scum); detergent = sulphonate salt, works in hard water; both clean via micelles; and straight-chain detergents are biodegradable.