Understanding Chemistry: Principles of chemistry: Edexcel IGCSE Science Single Award Explained

The principles of chemistry section lays the groundwork for every other chemistry topic in the single award

If you are preparing for the Pearson Edexcel IGCSE Science Single Award (4SS0), the principles of chemistry section is where you build the conceptual toolkit that everything else in the chemistry paper depends on. States of matter, atomic structure, the Periodic Table, chemical formulae, and the two main types of bonding are all here. Get these right and the inorganic, physical and organic chemistry sections will make far more sense.

This article serves as your edexcel igcse science single award revision notes for the entire principles of chemistry section. Every specification point is covered below, with worked examples and common mistakes flagged so you can avoid them in the exam. Use these edexcel igcse science single award notes as a structured walkthrough of the content, then test yourself with the self-check questions at the end.

States of matter

The specification requires you to understand the three states of matter in terms of the arrangement, movement and energy of the particles.

Property	Solid	Liquid	Gas
Arrangement	Particles in a regular, closely packed pattern	Particles close together but irregularly arranged	Particles far apart and randomly arranged
Movement	Vibrate about fixed positions	Move around each other, sliding past	Move rapidly in all directions
Energy	Lowest kinetic energy	More kinetic energy than solids	Highest kinetic energy

The interconversions between states follow a predictable pattern. Heating a solid provides energy for particles to overcome the forces holding them in place, producing melting (solid to liquid). Further heating produces boiling or evaporation (liquid to gas). Cooling reverses these: condensation (gas to liquid) and freezing (liquid to solid). Sublimation occurs when a solid converts directly to a gas without passing through the liquid phase.

The specification also asks you to explain results from dilution and diffusion experiments. Dilution of a coloured solution (such as potassium permanganate in water) shows that particles are very small and spread out through the solvent. Diffusion of gases (such as the reaction between hydrogen chloride and ammonia gases producing a white ring of ammonium chloride in a glass tube) demonstrates that gas particles move randomly and that lighter particles diffuse faster.

Elements, compounds and mixtures

You need to classify substances into three categories:

Element: a substance made of only one type of atom. It cannot be broken down into simpler substances by chemical means. Examples: iron, oxygen, carbon.
Compound: a substance made of two or more different types of atom chemically bonded together. It can only be separated by chemical reactions. Examples: water (H₂O), sodium chloride (NaCl).
Mixture: two or more substances (elements or compounds) that are not chemically bonded and can be separated by physical methods.

A pure substance has a fixed, sharp melting point and boiling point. A mixture melts or boils over a range of temperatures. This distinction is tested regularly in edexcel igcse exams.

Separation techniques

The specification requires you to describe five separation techniques:

Technique	What it separates	How it works
Simple distillation	A solvent from a solution	The solution is heated until the solvent boils. The vapour is condensed and collected, leaving the solute behind.
Fractional distillation	A mixture of liquids with different boiling points	The mixture is heated. Liquids boil at different temperatures and are condensed and collected separately using a fractionating column.
Filtration	An insoluble solid from a liquid	The mixture is poured through filter paper. The solid remains on the paper; the liquid passes through.
Crystallisation	A dissolved solid from a solution	The solution is heated to evaporate some solvent, then allowed to cool. The solute forms crystals as the solution becomes saturated.
Paper chromatography	The components of a mixture of soluble substances	A spot of the mixture is placed on chromatography paper. A solvent moves up the paper, carrying the components at different rates. More soluble components travel further.

For chromatography, you also need to understand how to calculate R_f values:

R_f = distance moved by substance / distance moved by solvent front

Each substance has a characteristic R_f value under the same conditions, which allows you to identify unknown components by comparing them to known standards.

Atomic structure

An atom consists of a nucleus (containing protons and neutrons) surrounded by electrons. The specification requires you to know the relative masses and charges of these sub-atomic particles:

Particle	Relative mass	Relative charge	Location
Proton	1	+1	Nucleus
Neutron	1	0	Nucleus
Electron	1/1836 (negligible)	-1	Shells (energy levels) around the nucleus

The atomic number is the number of protons in an atom. The mass number is the total number of protons and neutrons. Isotopes are atoms of the same element (same number of protons) with different numbers of neutrons and therefore different mass numbers.

The relative atomic mass (A_r) of an element is the weighted average of the masses of its isotopes, taking into account their relative abundances. For example, chlorine has two isotopes: Cl-35 (75% abundant) and Cl-37 (25% abundant). The A_r is calculated as:

A_r = (35 x 75 + 37 x 25) / 100 = (2625 + 925) / 100 = 3550 / 100 = 35.5

Common mistake: Writing A_r = (35 + 37) / 2 = 36. This simple average ignores the fact that Cl-35 is three times more abundant than Cl-37. Always use the weighted average formula.

The Periodic Table

Elements in the Periodic Table are arranged in order of atomic number. Vertical columns are called groups; horizontal rows are called periods. Elements in the same group have similar chemical properties because they have the same number of electrons in their outermost shell.

You also need to be able to identify metals and non-metals from their position. Metals are found on the left and centre of the Periodic Table; non-metals are found on the right. The dividing line runs diagonally from boron to astatine.

Chemical formulae and equations

The specification requires you to write word equations and balanced chemical equations with state symbols. State symbols are: (s) for solid, (l) for liquid, (g) for gas, and (aq) for aqueous (dissolved in water).

Balancing an equation means ensuring the same number of atoms of each element appears on both sides. For example:

Mg(s) + 2HCl(aq) -> MgCl₂(aq) + H₂(g)

You also need to calculate relative formula masses (M_r) by adding up the A_r values of all the atoms in the formula. For example, M_r of CaCO₃ = 40 + 12 + (16 x 3) = 40 + 12 + 48 = 100.

Ionic bonding

Ions form when atoms gain or lose electrons. Metals lose electrons to form positive ions (cations); non-metals gain electrons to form negative ions (anions). The specification requires you to know the charges of common ions:

Group 1 metals: +1 (e.g. Na⁺, K⁺)
Group 2 metals: +2 (e.g. Mg²⁺, Ca²⁺)
Group 3 metals: +3 (e.g. Al³⁺)
Group 5 non-metals: -3 (e.g. N^3-)
Group 6 non-metals: -2 (e.g. O^2-, S^2-)
Group 7 non-metals: -1 (e.g. Cl^-, Br^-)

Ionic bonding involves the electrostatic attraction between oppositely charged ions. In a giant ionic lattice, each ion is surrounded by ions of the opposite charge, creating a strong, regular structure. This explains why ionic compounds have high melting and boiling points: a large amount of energy is needed to overcome the many strong electrostatic attractions throughout the lattice.

Covalent bonding

A covalent bond forms when two atoms share a pair of electrons. This typically occurs between non-metal atoms. The shared electrons are attracted to the nuclei of both atoms, holding them together.

Substances with simple molecular structures (such as water, methane, oxygen) are gases, liquids, or solids with low melting and boiling points. This is because, although the covalent bonds within each molecule are strong, the forces between molecules (intermolecular forces) are weak. It is these weak intermolecular forces that are overcome when the substance melts or boils, not the covalent bonds themselves.

Substances with giant covalent structures (such as diamond, graphite, silicon dioxide) are solids with very high melting and boiling points. In these structures, every atom is bonded to its neighbours by strong covalent bonds in a continuous network. Breaking these bonds requires a very large amount of energy.

Exam tip: When explaining the low boiling point of a simple molecular substance, make it clear that the weak forces being overcome are the intermolecular forces, not the covalent bonds. Writing "the covalent bonds are weak" is incorrect and will lose you marks. The covalent bonds are strong; it is the attractions between molecules that are weak.

Self-check questions

Describe the arrangement and movement of particles in a solid, liquid and gas.
Explain why a pure substance has a sharp melting point but a mixture melts over a range of temperatures.
Calculate the R_f value if a substance travels 4.2 cm and the solvent front travels 7.0 cm.
Copper has two isotopes: Cu-63 (69% abundant) and Cu-65 (31% abundant). Calculate the relative atomic mass of copper.
Write a balanced equation with state symbols for the reaction of magnesium with hydrochloric acid.
Explain why sodium chloride has a high melting point.
Explain why water has a low boiling point despite containing strong covalent bonds.
Calculate the M_r of Ca(OH)₂, given: Ca = 40, O = 16, H = 1.

Tying the principles together

The chemistry: principles of chemistry edexcel igcse section is dense, but every topic within it reappears in the sections that follow. Understanding states of matter helps you interpret rate of reaction experiments. Understanding bonding helps you explain the properties of metals and polymers. Understanding formulae and equations is essential for every calculation in the rest of the chemistry paper.

For your igcse 4ss0 chemistry: principles of chemistry revision, prioritise the areas that carry calculation marks: R_f values, relative atomic mass from isotopic abundances, relative formula mass, and balancing equations. These are the places where precise working and correct units translate directly into marks.

Once you are confident with these edexcel igcse science single award notes, put them to the test with edexcel igcse science single award practice questions. The edexcel igcse science single award explained approach in this article gives you the framework, but exam readiness comes from applying it under timed conditions. Mark your answers against the official mark scheme, note where you lose marks, and revisit those specific areas. The Green Bridge CBT platform offers chemistry questions organised by topic, so you can target the principles section directly and track your improvement over time.