Separation and purity. Mixtures are separated by physical methods — filtration, evaporation, crystallisation, simple and fractional distillation, sublimation and chromatography. A pure substance has a sharp, fixed melting or boiling point.
Atomic structure. J.J. Thomson discovered the electron through cathode-ray experiments (1897); Rutherford's alpha-particle gold-foil experiment (1911) established the small, dense, positive nucleus; Chadwick discovered the neutron (1932). The atomic number Z is the number of protons; the mass number A is protons plus neutrons, so neutrons = A − Z; a neutral atom has equal electrons and protons. Isotopes have the same Z but different mass numbers: chlorine occurs as Cl-35 and Cl-37 in about a 3:1 ratio, so Ar(Cl) = (0.75 × 35) + (0.25 × 37) = 35.5. Relative atomic mass is defined on the carbon-12 scale, with ¹²C taken as exactly 12.
Electronic configuration. Master four rules:
Bonding. An electrovalent (ionic) bond forms by complete electron transfer from metal to non-metal, giving ions held by electrostatic attraction (NaCl); a covalent bond forms by sharing electron pairs (Cl₂, CH₄) — both give atoms the noble-gas octet. In a coordinate (dative) covalent bond one atom donates both shared electrons, as in NH₄⁺ and H₃O⁺. Hydrogen bonding — H on N, O or F attracted to a lone pair on another electronegative atom — explains water's anomalously high boiling point of 100 °C compared with H₂S.
The mole, gases and stoichiometry. One mole contains 6.02 × 10²³ particles (the Avogadro constant, exactly 6.02214076 × 10²³ mol⁻¹ since the 2019 SI redefinition); one mole of any ideal gas occupies 22.4 dm³ at s.t.p. (273 K, 1 atm). Balanced equations rest on the laws of conservation of matter (Lavoisier), definite proportions (Proust) and multiple proportions (Dalton); Gay-Lussac's law of combining volumes and Avogadro's law govern reacting gases. For an empirical formula, divide each mass percentage by the relative atomic mass, then by the smallest quotient; molecular formula = empirical formula × n, where n = Mr ÷ empirical formula mass.
Kinetic theory and radioactivity. Kinetic theory pictures matter as particles in constant motion, explaining the three states and the gas laws. Radioactivity is the spontaneous emission of alpha, beta or gamma radiation from unstable nuclei — a nuclear change, unlike ordinary reactions that only rearrange electrons.
1. Which technique is most suitable for separating a mixture of sand and water?
Sand is insoluble in water, so filtration through filter paper separates the insoluble solid residue from the liquid filtrate. (JAMB UTME Chemistry Syllabus, Topic: Separation of mixtures — filtration technique.)
2. A student has a solution of common salt in water and wants to recover the salt in solid form. Which technique should be used?
Since the salt is dissolved (not suspended) in water, evaporating the water to dryness leaves the solid salt behind. (New School Chemistry for Senior Secondary Schools, Ababio — separation of a dissolved solid from its solution.)
3. Which separation technique relies on a solid changing directly into vapour on heating, without passing through the liquid state?
Sublimation is used to purify substances such as iodine that pass directly from solid to gas on heating and resolidify on cooling, leaving non-sublimable impurities behind. (JAMB UTME Chemistry Syllabus — sublimation as a purification technique.)
4. Simple distillation is normally used to separate...
Simple distillation recovers a pure liquid solvent, such as water, from a solution containing a dissolved non-volatile solid by evaporation followed by condensation. (Standard SSCE/UTME chemistry texts — simple distillation.)
5. Which technique would best separate a mixture of ethanol and water, two miscible liquids with different boiling points?
Fractional distillation uses a fractionating column to separate miscible liquids of different boiling points based on their relative volatilities. (JAMB UTME Chemistry Syllabus — fractional distillation of miscible liquids.)
6. A separating funnel is the most appropriate apparatus for separating...
Immiscible liquids form distinct layers according to density; a separating funnel allows the denser lower layer to be run off through the tap, separating the two liquids. (JAMB UTME Chemistry Syllabus — separation of immiscible liquids using a separating funnel.)
7. In paper chromatography, the ratio of the distance moved by a solute spot to the distance moved by the solvent front is called the...
The Rf value is characteristic of a substance in a given solvent system and is used to identify components separated by chromatography. (JAMB UTME Chemistry Syllabus — paper chromatography and Rf value.)
8. Which of these is used as a criterion of purity for a crystalline solid substance?
A pure crystalline substance melts sharply at a fixed temperature, whereas the presence of impurities lowers and widens the melting point range. (Standard SSCE/UTME chemistry texts — criteria of purity (melting and boiling point).)
9. What is the most efficient way to separate a mixture of iron filings and sulphur powder?
Iron filings are attracted to a magnet while sulphur powder is not, allowing quick physical separation of the two solids. (JAMB UTME Chemistry Syllabus — magnetic separation of mixtures.)
10. Crystallization is preferred to evaporation to dryness for recovering a dissolved solid mainly when...
Crystallization involves gently concentrating the solution and allowing it to cool slowly so crystals form, avoiding the decomposition that prolonged strong heating to complete dryness could cause. (JAMB UTME Chemistry Syllabus — crystallization as a purification technique.)
11. Which sequence of operations would correctly separate and recover pure dry salt from a mixture of common salt, sand and water?
Filtration first removes the insoluble sand, and the salt solution (filtrate) is then evaporated or crystallized to recover solid salt. (JAMB UTME Chemistry Syllabus — combined separation techniques for multi-component mixtures.)
12. Fractional distillation of liquefied air can separate nitrogen from oxygen mainly because...
Liquid nitrogen (boiling point about -196 C) boils off before liquid oxygen (boiling point about -183 C) as liquefied air is warmed, so the two gases can be separated according to their differing boiling points. (JAMB UTME Chemistry Syllabus — industrial fractional distillation of liquid air.)
13. Relative atomic mass of an element is defined as the mass of an atom compared with...
By international convention, relative atomic mass is measured against one-twelfth of the mass of one atom of carbon-12, which is taken as exactly 12. (JAMB UTME Chemistry Syllabus — relative atomic mass based on 12C = 12; IUPAC definition.)
14. The number of elementary entities (atoms, molecules or ions) contained in one mole of a substance is known as...
The Avogadro constant, approximately 6.02 x 10^23 per mole, is the fixed number of elementary entities present in one mole of any substance. (SI Brochure, 9th edition (BIPM, 2019); JAMB UTME Chemistry Syllabus — mole concept and Avogadro's number.)
15. At standard temperature and pressure (273 K, 1 atm), one mole of any ideal gas occupies a volume of...
The standard molar volume of an ideal gas at s.t.p. is 22.4 dm3, a value used throughout UTME gas stoichiometry calculations. (JAMB UTME Chemistry Syllabus — gas laws and mole concept; standard molar volume.)
16. Which law states that the total mass of the reactants in a chemical reaction equals the total mass of the products?
Lavoisier's law of conservation of matter requires that mass is neither created nor destroyed in a chemical reaction, which is why chemical equations must be balanced. (JAMB UTME Chemistry Syllabus — laws of chemical combination (conservation of matter).)
17. According to the law of definite (constant) proportions, a given pure compound...
Proust's law of definite proportions states that a pure compound always contains the same elements combined in the same fixed ratio by mass, regardless of its source or method of preparation. (JAMB UTME Chemistry Syllabus — law of definite proportions (Proust).)
18. Which of the following is the correctly balanced equation for the complete combustion of methane?
Balancing carbon, hydrogen and oxygen atoms on both sides shows that one mole of methane requires two moles of oxygen to yield one mole of CO2 and two moles of H2O. (JAMB UTME Chemistry Syllabus — balancing chemical equations and conservation of matter.)
19. Chlorine occurs naturally as isotopes Cl-35 and Cl-37 in an approximate abundance ratio of 3:1. What is the relative atomic mass of chlorine?
Ar = (0.75 x 35) + (0.25 x 37) = 26.25 + 9.25 = 35.5, matching the accepted relative atomic mass of chlorine. (JAMB UTME Chemistry Syllabus — isotopy and relative atomic mass calculations from isotopic abundance.)
20. Gay-Lussac's law of combining volumes states that gases react together in volumes that...
Gay-Lussac observed that the volumes of reacting gases and their gaseous products are always in simple whole-number ratios when measured at the same temperature and pressure. (JAMB UTME Chemistry Syllabus — Gay-Lussac's law of combining volumes.)
21. A compound contains 40% carbon, 6.7% hydrogen and 53.3% oxygen by mass. What is its empirical formula? (C = 12, H = 1, O = 16)
Dividing each percentage by its relative atomic mass gives mole ratios C:H:O of about 3.33:6.7:3.33, which simplify to 1:2:1, giving the empirical formula CH2O. (JAMB UTME Chemistry Syllabus — determination of empirical formula from percentage composition.)
22. A compound has the empirical formula CH2O and a relative molecular mass of 180. What is its molecular formula? (C = 12, H = 1, O = 16)
The empirical formula mass of CH2O is 30, so n = 180/30 = 6, giving the molecular formula (CH2O)6 = C6H12O6. (JAMB UTME Chemistry Syllabus — n = relative molecular mass divided by empirical formula mass.)
23. What mass of calcium oxide is produced when 50 g of calcium carbonate is completely decomposed by heat, CaCO3 -> CaO + CO2? (Ca = 40, C = 12, O = 16)
Molar mass of CaCO3 = 100 g/mol and of CaO = 56 g/mol; 50 g CaCO3 is 0.5 mol, which by the 1:1 mole ratio gives 0.5 mol CaO, that is 0.5 x 56 = 28 g. (JAMB UTME Chemistry Syllabus — stoichiometric mass calculations from balanced equations.)
24. What volume of carbon(IV) oxide gas, measured at s.t.p., is produced when 5 g of calcium carbonate reacts completely with excess dilute hydrochloric acid? CaCO3 + 2HCl -> CaCl2 + H2O + CO2 (Ca = 40, C = 12, O = 16; molar volume = 22.4 dm3 at s.t.p.)
Molar mass of CaCO3 = 100 g/mol, so 5 g equals 0.05 mol, which produces 0.05 mol CO2 by the 1:1 ratio in the equation; volume = 0.05 x 22.4 = 1.12 dm3 at s.t.p. (JAMB UTME Chemistry Syllabus — gas volume stoichiometry using molar volume at s.t.p.)
25. The relative atomic mass of an element is defined as the average mass of one atom of the element compared with what fraction of the mass of one atom of carbon-12?
Relative atomic mass is measured on the carbon-12 scale, where one atom of carbon-12 is taken as exactly 12 units, so the reference fraction is one-twelfth of its mass. (JAMB UTME Chemistry Syllabus, Topic: Chemical combination — relative atomic mass based on 12C = 12; IUPAC definition)
26. A sodium atom has an atomic number of 11 and a mass number of 23. How many neutrons does this atom contain?
The number of neutrons equals the mass number minus the atomic number: 23 − 11 = 12. (JAMB UTME Chemistry Syllabus, Topic: Atomic structure and bonding — neutrons = A − Z)
27. Naturally occurring chlorine consists of two isotopes, ³⁵Cl and ³⁷Cl, in an approximate abundance ratio of 3:1. What relative atomic mass does this data give for chlorine?
Weighting the masses by abundance gives (0.75 × 35) + (0.25 × 37) = 26.25 + 9.25 = 35.5, matching the accepted relative atomic mass of chlorine. (JAMB UTME Chemistry Syllabus, Topic: Atomic structure and bonding — isotopy calculations; Ar(Cl) = 35.5)
28. Which scientist is credited with discovering the electron through cathode-ray tube experiments in 1897?
J. J. Thomson's cathode-ray experiments of 1897 identified the electron as a negatively charged sub-atomic particle. (JAMB UTME Chemistry Syllabus, Topic: Atomic structure and bonding — contributions of scientists; Thomson, 1897)
29. Ernest Rutherford's alpha-particle scattering (gold-foil) experiment of 1911 led to the conclusion that the atom contains what feature?
Most alpha particles passed straight through the gold foil, but a few were deflected sharply, showing that positive charge and mass are concentrated in a tiny, dense nucleus. (JAMB UTME Chemistry Syllabus, Topic: Atomic structure and bonding — contributions of scientists; Rutherford, 1911)
30. James Chadwick's experiments in 1932 led to the discovery of which sub-atomic particle?
Chadwick identified the neutron, a neutral particle in the nucleus, completing the basic model of atomic structure. (JAMB UTME Chemistry Syllabus, Topic: Atomic structure and bonding — contributions of scientists; Chadwick, 1932)
31. Using the 2n² rule for maximum electron capacity of a principal energy level, what is the maximum number of electrons that can occupy the third shell (n = 3)?
Applying 2n² with n = 3 gives 2 × 3² = 18, the maximum number of electrons the third shell can hold. (JAMB UTME Chemistry Syllabus, Topic: Atomic structure and bonding — rules guiding arrangement of electrons; 2n² rule)
32. According to the Pauli exclusion principle, what is the maximum number of electrons a single atomic orbital can hold, and how must their spins be oriented?
The Pauli exclusion principle states no two electrons in an atom can share the same set of four quantum numbers, so an orbital holds at most two electrons and they must have opposite spins. (Pauli exclusion principle (1925); JAMB UTME Chemistry Syllabus, Topic: Atomic structure and bonding — electron configuration rules)
33. In accordance with Hund's rule, how are the three electrons in the 2p sub-level of a nitrogen atom (2p³) arranged among the three degenerate 2p orbitals?
Hund's rule requires electrons to occupy degenerate orbitals singly with parallel spins before any pairing occurs, so each of the three 2p orbitals gets one unpaired electron. (Hund's rule of maximum multiplicity; JAMB UTME Chemistry Syllabus, Topic: Atomic structure and bonding — electron configuration rules)
34. Following the Aufbau principle, which is the correct electron configuration of a potassium atom (atomic number 19)?
Orbitals fill in order of increasing energy, and the 4s sub-level fills before 3d, giving potassium the configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹. (Aufbau principle; JAMB UTME Chemistry Syllabus, Topic: Atomic structure and bonding — electron configuration of elements Z = 1 to 20)
35. In the modern periodic table, elements are arranged in order of increasing what property?
The modern periodic law arranges elements by increasing atomic number, which resolved earlier anomalies found in arrangements based on atomic mass. (Modern Periodic Law (Moseley, 1913); JAMB UTME Chemistry Syllabus, Topic: Atomic structure and the periodic table)