Chemistry for prelims: matter, reactions and the compounds in everyday life

Chemistry is the study of matter: what substances are made of, how they change, and the compounds hidden in everyday life. For prelims, chemistry questions cluster around a small set of recurring facts: why water dissolves so much, how to tell a physical change from a chemical one, which compounds appear in food and the human body, and the extremes of the material world. This article collects those facts with the exact details prelims has tested.

Water: the universal solvent

Water is called the universal solvent because it dissolves more substances than any other liquid. The reason is its polar structure: the water molecule is bent, with oxygen pulling electrons toward itself, so one end of the molecule is slightly negative and the other slightly positive. This lets water surround and separate the ions of salts and the molecules of many polar substances, pulling them into solution. Prelims tests this as a one-line fact: water's solvent power comes from its polarity.

Chlorination is the addition of chlorine or chlorine compounds to drinking water to kill disease-causing microbes. Chlorine destroys bacteria and viruses by oxidising their cell components, and a small residual amount keeps the water safe as it travels through pipes to the tap. In prelims terms: chlorination disinfects water; it does not remove dissolved salts or heavy metals.

Physical change vs chemical change

A physical change is a change in which no new substance is formed: only the state, shape or appearance of matter alters. Melting ice, boiling water, tearing paper and dissolving sugar are physical changes, and most are reversible. Prelims frames this as: the chemical identity of the substance stays the same.

A chemical change is a change in which one or more new substances with new properties are formed. Rusting of iron, burning of wood, cooking of food and curdling of milk are chemical changes, and they are usually irreversible. The test line: if the molecular composition changes, the change is chemical.

The iodine test is the classic test for starch: a drop of iodine solution turns blue-black in the presence of starch. The colour comes from iodine molecules slipping inside the coiled amylose chains of starch to form an inclusion complex. Prelims has asked this directly: a blue-black colour with iodine means starch is present.

Sweeteners and the chemistry of blood

Aspartame is an artificial sweetener, a methyl ester of the dipeptide made from aspartic acid and phenylalanine. It is about 200 times sweeter than sucrose, so tiny amounts sweeten food with almost no calories. In the body it breaks down into phenylalanine, which is why people with phenylketonuria (PKU), who cannot metabolise that amino acid, must avoid it. That breakdown detail is the prelims hook.

The pH of human blood is held between 7.35 and 7.45, slightly alkaline. This narrow range is maintained by buffer systems, chiefly the bicarbonate buffer (carbonic acid and bicarbonate ions), which absorb excess acid or base. If blood pH falls below the range the condition is acidosis; if it rises above, alkalosis. Prelims tests the number and the alkaline character.

Mercury vapour lamps: light from an electric discharge

A mercury vapour lamp is a gas-discharge lamp that produces light by passing an electric current through vaporised mercury. The current excites mercury atoms, and as they return to lower energy states they emit an intense bluish-white light. These lamps were widely used for street and industrial lighting because of their brightness and long life, and they need careful disposal because mercury is toxic. Prelims asks what produces the light: an electric discharge through mercury vapour.

The lightest and the densest metals

Lithium is the lightest metal, with a density of about 0.53 grams per cubic centimetre, so it floats on water. It is the least dense solid element under ordinary conditions. Prelims pairs this fact with its opposite.

Osmium is the densest naturally occurring element, at about 22.6 grams per cubic centimetre, roughly twice as dense as lead. This is the heavyweight of prelims: the standard pair tested is lithium as the lightest metal and osmium as the densest element.

Explosives: TNT and RDX

TNT (trinitrotoluene) is the reference explosive: the power of bombs and warheads is measured in kilotons or megatons of TNT equivalent. It is a stable, yellow crystalline compound that needs a detonator to set it off.

RDX (Research Department eXplosive, chemically cyclotrimethylene trinitramine) is a military high explosive that is more powerful and brisant than TNT. It is the main charge in many munitions and a key ingredient of the plastic explosive C-4. Prelims contrasts the two: TNT is the measuring standard, RDX is the more powerful military explosive.

Methane: firedamp in coal mines

Methane (CH4) is the simplest hydrocarbon: colourless, odourless and highly flammable. In coal mines, methane seeping from coal seams mixes with air to form firedamp, an explosive mixture behind historic mine disasters. Miners' safety lamps, such as the Davy lamp, were designed to detect this gas without igniting it. Prelims asks the name: methane in mines is called firedamp.

Corundum: ruby and sapphire are twins

Corundum is crystalline aluminium oxide (Al2O3), the second hardest natural mineral after diamond, with a Mohs hardness of 9. Both ruby and sapphire are varieties of corundum: the gemstone's identity is decided by trace impurities.

Ruby is red corundum, coloured by traces of chromium, while blue sapphire is corundum coloured by iron and titanium. Sapphires in any other colour are called fancy sapphires. Prelims tests the pair: ruby and sapphire are chemically the same mineral, aluminium oxide.

Metal-organic frameworks: crystals full of holes

Metal-organic frameworks (MOFs) are crystalline materials built by linking metal ions or clusters with organic molecules, called linkers, into open, lattice-like frameworks riddled with pores. A single gram of a MOF can unfold an internal surface area larger than a football field, so the material behaves like a sponge designed molecule by molecule. Susumu Kitagawa, Richard Robson and Omar Yaghi shared the 2025 Nobel Prize in Chemistry for developing them; Yaghi's method of designing such structures is called reticular chemistry, the stitching of molecular building blocks into extended frameworks.

Why they matter. Because pore size and chemistry can be tuned, MOFs can trap carbon dioxide from exhaust gases, harvest drinking water from desert air, store hydrogen and methane, and remove toxic PFAS chemicals from water. For the exam, MOFs are the clearest recent case of a Nobel-winning idea moving towards energy and environment applications.

Key Terms

• Universal solvent: water is called the universal solvent because its polar, bent molecules surround and separate ions and polar molecules, dissolving more substances than any other liquid.
• Physical change: a change with no new substance formed; only state, shape or appearance alters. Examples: melting, boiling, tearing paper.
• Chemical change: a change producing new substances with new properties; usually irreversible. Examples: rusting, burning, cooking.
• Chlorination: adding chlorine or chlorine compounds to drinking water to kill microbes; residual chlorine keeps piped water safe to the tap.
• Iodine test: iodine solution turns blue-black in the presence of starch, from iodine trapped inside coiled amylose chains.
• Aspartame: artificial sweetener, a methyl ester of the aspartic acid-phenylalanine dipeptide, about 200 times sweeter than sucrose; people with phenylketonuria must avoid it.
• Blood pH: human blood is held between 7.35 and 7.45, slightly alkaline, chiefly by the bicarbonate buffer system.
• Bicarbonate buffer: carbonic acid and bicarbonate ions in blood that absorb excess acid or base to hold pH steady.
• Mercury vapour lamp: gas-discharge lamp producing intense bluish-white light from electric current through mercury vapour; widely used in street lighting.
• Lithium: the lightest metal, density about 0.53 g/cm3; it floats on water.
• Osmium: the densest naturally occurring element, about 22.6 g/cm3, roughly twice as dense as lead.
• TNT: trinitrotoluene, the reference explosive; explosive power is measured in kilotons or megatons of TNT equivalent.
• RDX: Research Department eXplosive (cyclotrimethylene trinitramine), a military high explosive more powerful than TNT and the main charge in C-4.
• Firedamp: the explosive mixture of methane and air found in coal mines; detected with miners' safety lamps.
• Corundum: crystalline aluminium oxide (Al2O3), Mohs hardness 9; ruby (red, from chromium) and sapphire (blue, from iron and titanium) are both corundum.
• Metal-organic frameworks (MOFs): Metal-organic frameworks are crystalline, porous materials built by linking metal ions or clusters with organic molecules into lattice-like structures with enormous internal surface areas. Example: MOFs can capture carbon dioxide, harvest drinking water from desert air and store gases such as hydrogen; their development won the 2025 Nobel Prize in Chemistry for Susumu Kitagawa, Richard Robson and Omar Yaghi.

Prelims practice