School curriculum· 25 min· 21 questions
Chemistry is the science of substances: their composition, structure, properties and transformations into one another. Every chemistry problem rests on the ideas in this topic: atom, molecule, element, element versus compound, relative mass, valency and the basic laws of chemistry. Once you can tell these terms apart precisely, many test questions stop being "word games", and writing formulae and finding mass fractions become automatic. Although the topic is introductory, questions on it appear in the exam every year.
Not every substance is made of molecules. Ionic compounds (NaCl, CaO) consist of ions, metals of atoms and ions, and diamond and quartz (SiO₂) of a continuous network of atoms. For such substances we speak of a formula unit rather than a molecule.
A chemical symbol stands for an element and one of its atoms (O), while a formula shows the composition of a substance (O₂ is one oxygen molecule, 2O₂ two molecules). The number in front of a formula is a coefficient; the small number after a symbol is a subscript.
A simple substance (element) consists of one element (O₂, O₃, Fe, S₈); a compound consists of several elements (H₂SO₄, NaCl, C₂H₅OH).
The word "oxygen" has two meanings: "water contains oxygen" refers to the element; "we breathe oxygen" refers to the simple substance O₂. Likewise, "iron is part of blood" means the element, while "an iron nail" means the substance.
Allotropy is the ability of one element to form several simple substances: oxygen (O₂ and ozone O₃), carbon (diamond, graphite, carbyne, fullerenes), phosphorus (white, red, black), sulfur (rhombic, monoclinic, plastic), tin (white and grey). Allotropes differ in the make-up of their molecules (O₂ and O₃) or in the structure of their crystal lattice (diamond and graphite).
A pure substance has a fixed composition and properties (definite melting and boiling points). A mixture consists of several substances and its composition varies: air, sea water, crude oil, alloys. Mixtures are separated by physical methods: settling, filtration, evaporation, distillation and magnetic separation.
Burning, rusting, fermentation, milk going sour, digestion and photosynthesis are chemical changes. In a reaction atoms are neither destroyed nor created; they are only rearranged, which is why mass is conserved.
Law of conservation of mass: the mass of the reactants equals the mass of the products (M. V. Lomonosov, 1748; A. Lavoisier, 1789)
That is why equations are balanced: each element must have the same number of atoms on both sides.
Law of constant composition (J. Proust): a pure substance always has the same composition, however it is made
For example, in water m(H) : m(O) = 2 : 16 = 1 : 8, whatever the method of preparation.
Avogadro’s law (1811): equal volumes of gases at the same temperature and pressure contain equal numbers of molecules
It follows that at standard temperature and pressure (STP: 0 °C, 101.3 kPa) 1 mol of any gas occupies 22.4 L. Volumes of reacting gases are in the ratio of the coefficients in the equation (Gay-Lussac’s law of combining volumes).
Aluminium nitrate decomposes: 4Al(NO₃)₃ → 2Al₂O₃ + 12NO₂ + 3O₂. Find the average molar mass of the gas mixture released.
Relative atomic mass Ar is the ratio of an atom’s mass to 1/12 of the mass of a carbon-12 atom; it has no units. 1/12 of the mass of a carbon-12 atom is the atomic mass unit (1 u ≈ 1.66·10⁻²⁷ kg). The Ar values in the periodic table are averages over isotopes, so they are often not whole numbers.
Relative molecular mass Mr is the sum of the Ar values of all atoms in the molecule (each multiplied by its subscript).
Mr(H₂SO₄) = 2·1 + 32 + 4·16 = 98
Mass fraction of an element: ω(E) = n · Ar(E) / Mr(substance), where n is the number of E atoms in the formula
The mass fractions always add up to 1 (100 %).
Find the mass fraction of nitrogen in ammonium nitrate NH₄NO₃.
The mass fraction of iron in an iron oxide is 70 %. Find the formula of the oxide.
Valency is the number of chemical bonds an atom forms with other atoms; it is written in Roman numerals. Elements with fixed valency: H, Na, K, F (I); O, Mg, Ca, Ba, Zn (II); Al, B (III). Elements with variable valency: Fe (II, III), Cu (I, II), S (II, IV, VI), C (II, IV), N (I–V), P (III, V), Cl (I, III, V, VII).
To write the formula of a binary compound, find the lowest common multiple (LCM) of the valencies and divide it by each element’s valency. Al(III) and O(II): LCM = 6, 6/3 = 2 Al atoms and 6/2 = 3 O atoms → Al₂O₃. The reverse task: to find a valency from a formula, divide the total valency of one element by the number of atoms of the other; in Fe₂O₃, 2·3 = 6, so iron’s valency is 6/2 = III.