Understanding Principles of chemistry: States of matter to Chemical formulae, equations and calculations: Edexcel IGCSE Chemistry Explained

The first five topics of the Edexcel IGCSE Chemistry specification build the toolkit you will use for the rest of the course. Get these right and everything else becomes easier.

Principles of chemistry is the largest section in the igcse 4ch1 specification, and the topics covered here, from states of matter through to chemical formulae, equations and calculations, appear in almost every exam session. These edexcel igcse chemistry revision notes walk you through the core ideas, show you how they connect, and flag the spots where students most commonly lose marks. If you are looking for edexcel igcse chemistry principles of chemistry: states of matter to chemical formulae, equations and calculations explained in plain language, you are in the right place.

States of matter

All matter exists in one of three states: solid, liquid or gas. The difference between them comes down to how the particles are arranged, how much energy they have, and how freely they move.

The interconversions have specific names. Melting turns solid to liquid. Boiling (or evaporating) turns liquid to gas. Condensing turns gas to liquid. Freezing turns liquid to solid. Sublimation goes directly from solid to gas. Each change involves either absorbing or releasing energy, but the temperature stays constant during the change itself because the energy goes into breaking or forming the forces between particles, not into raising the temperature.

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration. Gases diffuse faster than liquids because their particles move more quickly and have greater spaces between them. The classic experiment involves putting a drop of potassium manganate(VII) into water and watching the purple colour spread, or observing how bromine gas fills a jar over time.

You should also know the solubility vocabulary: a solvent is the liquid that does the dissolving, a solute is the substance that dissolves, a solution is the result, and a saturated solution is one that cannot dissolve any more solute at that temperature. Solubility is measured in grams of solute per 100 g of solvent, and solubility curves show how this changes with temperature.

Elements, compounds and mixtures

An element is a substance made of only one type of atom. A compound is a substance formed when two or more elements are chemically combined in fixed proportions. A mixture contains two or more substances that are not chemically bonded and can be separated by physical methods.

The distinction between a compound and a mixture trips up more students than you might expect. Water (H₂O) is a compound because hydrogen and oxygen are chemically bonded and can only be separated by chemical means. Salt water is a mixture because the salt and water can be separated by evaporation. Get comfortable with this distinction because it forms the basis for many exam questions on separation techniques.

The edexcel specification expects you to describe five separation techniques:

1. Simple distillation separates a solvent from a solution (e.g. pure water from salt water). Heat the solution, collect the vapour, cool it in a condenser.
2. Fractional distillation separates a mixture of liquids with different boiling points (e.g. crude oil). The fractionating column provides a temperature gradient.
3. Filtration separates an insoluble solid from a liquid. The liquid passes through the filter paper; the solid stays behind as the residue.
4. Crystallisation produces pure solid crystals from a solution. Heat the solution to evaporate some solvent, then leave it to cool slowly.
5. Paper chromatography separates the components of a mixture based on their different solubilities in a solvent.

For chromatography, you need to calculate R_f values:

A pure substance has a fixed, sharp melting point and boiling point. A mixture melts or boils over a range of temperatures. This is a clean way to test purity, and the exam often asks about it.

Atomic structure

An atom consists of a nucleus containing protons and neutrons, surrounded by electrons in energy levels (shells). The key numbers are:

The atomic number (Z) is the number of protons. This defines which element the atom is. The mass number (A) is the total number of protons and neutrons. The number of neutrons = mass number - atomic number.

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. Chlorine, for example, has two main isotopes: ³⁵Cl (17 protons, 18 neutrons) and ³⁷Cl (17 protons, 20 neutrons).

The relative atomic mass (A_r) is calculated from isotopic abundances. Here is how:

The Periodic Table

Elements are arranged in the Periodic Table in order of increasing atomic number. Columns are called groups and contain elements with similar chemical properties because they have the same number of electrons in their outer shell. Rows are called periods and correspond to the number of occupied electron shells.

You need to deduce electronic configurations for the first 20 elements. The shells fill in order: first shell holds up to 2, second holds up to 8, third holds up to 8 (at IGCSE level). So sodium (atomic number 11) has the configuration 2,8,1 and calcium (atomic number 20) has 2,8,8,2.

Elements on the left of the Periodic Table and in the centre are metals. Elements on the right are non-metals. You can classify them by their physical properties (electrical conductivity) and by the acid-base character of their oxides: metal oxides are bases (some dissolve in water to form alkalis), while non-metal oxides tend to be acidic.

The noble gases (Group 0) have full outer shells, which is why they do not readily react. They already have a stable electronic configuration.

Chemical formulae, equations and calculations

This is the topic that threads through every other part of the specification. If you can handle mole calculations, balancing equations and reacting mass problems, you unlock marks across all four sections.

Balancing equations: The number of atoms of each element must be the same on both sides. State symbols show the physical state: (s) for solid, (l) for liquid, (g) for gas, (aq) for aqueous solution.

Example: magnesium burning in oxygen.

2Mg(s) + O₂(g) → 2MgO(s)

Relative formula mass (M_r) is the sum of the relative atomic masses of all atoms in the formula. For CaCO₃: M_r = 40 + 12 + (16 x 3) = 100.

The mole is the unit for amount of substance. One mole of any substance contains the same number of particles. The key formula is:

moles = mass / M_r

Reacting masses: Use the balanced equation to find the mole ratio, then convert between moles and mass.

Percentage yield = (actual yield / theoretical yield) x 100. The actual yield is always less than the theoretical yield because of incomplete reactions, side reactions, or losses during transfer and purification.

Empirical formula is the simplest whole-number ratio of atoms. Molecular formula shows the actual number of atoms in one molecule. To find the empirical formula from experimental data, divide each element's mass by its A_r, then divide all results by the smallest.

Concentration calculations: moles = concentration (mol/dm³) x volume (dm³). Remember to convert cm³ to dm³ by dividing by 1000.

Gas volumes: At room temperature and pressure (rtp), one mole of any gas occupies 24 dm³ (or 24,000 cm³). So volume = moles x 24 dm³.

Common mistakes in these topics

• Confusing atoms and molecules. An atom is a single particle of an element. A molecule is two or more atoms bonded together. O₂ is a molecule; O is an atom.
• Forgetting state symbols. The edexcel mark scheme often requires state symbols for full marks. If a question says "write a balanced equation including state symbols," leaving them off costs you.
• Mixing up mass number and atomic number. The atomic number is the smaller number (protons only). The mass number is the larger number (protons + neutrons).
• Rounding errors in A_r calculations. Use the exact values given in the question or the Periodic Table, not rounded figures.
• Forgetting to convert units in concentration calculations. If volume is given in cm³, divide by 1000 before using it with concentration in mol/dm³.

Self-check questions

1. Describe the arrangement and movement of particles in a gas compared to a solid.
2. A chromatogram shows substance X moved 4.5 cm and the solvent front moved 9.0 cm. Calculate the R_f value of substance X.
3. Copper has two isotopes: ⁶³Cu (69%) and ⁶⁵Cu (31%). Calculate the relative atomic mass of copper.
4. Write a balanced equation, with state symbols, for the reaction of calcium carbonate with dilute hydrochloric acid.
5. Calculate the mass of carbon dioxide produced when 5.0 g of calcium carbonate reacts with excess hydrochloric acid. (A_r values: Ca = 40, C = 12, O = 16, H = 1, Cl = 35.5)

With all of the edexcel igcse chemistry explained above, if you can answer all five without looking anything up, you are in strong shape for these principles of chemistry: states of matter to chemical formulae, equations and calculations edexcel igcse topics. If you stumbled on any, go back and work through the relevant section again. The point of edexcel igcse chemistry practice questions is not to score well on the first attempt. It is to find the gaps and fill them before the exam does it for you.

For more edexcel igcse chemistry notes and topic-by-topic practice, check the Green Bridge CBT platform. The igcse 4ch1 principles of chemistry: states of matter to chemical formulae, equations and calculations material here maps directly to the specification, so nothing gets missed.