Solids, liquids and gases

The particle model explains the three states by the arrangement, movement and energy of particles:

The changes of state each have a name. Melting and boiling need energy in; freezing and condensing give energy out:

• solid → liquid = melting; liquid → gas = boiling / evaporating
• gas → liquid = condensing; liquid → solid = freezing; solid → gas = sublimation

Diffusion: particles spread from high to low concentration because they are moving randomly. Lighter particles diffuse faster (e.g. ammonia diffuses faster than hydrogen chloride).

Naming a change of state

Elements, compounds & mixtures

• An element is made of only one type of atom (e.g. copper, oxygen).
• A compound contains two or more elements chemically bonded in fixed proportions (e.g. H₂O, NaCl).
• A mixture contains substances not chemically joined — they keep their own properties and can be physically separated.

A pure substance is a single element or compound, with a sharp, fixed melting point. An impurity lowers and spreads out the melting point.

Watch out: air is a mixture of elements and compounds; a compound like CO₂ cannot be separated by physical means — only by chemical reactions.

Separating mixtures

You must know which technique separates which kind of mixture:

• Filtration — insoluble solid from a liquid (residue stays in the filter paper).
• Crystallisation — a soluble solid from its solution by evaporating the solvent.
• Simple distillation — a solvent from a solution (collects the liquid that boils off).
• Fractional distillation — two miscible liquids with different boiling points (e.g. ethanol/water).
• Paper chromatography — coloured substances in a solution, by how far each travels.

In chromatography the more soluble a substance is in the solvent, the further it travels, so the higher its R_f value.

Your turn — R_f value

Why is oxygen easy to get from air, not water?

Why is chlorine 35.5 on the periodic table?

Inside the atom

An atom has a tiny central nucleus (protons + neutrons) surrounded by electrons in shells:

Atomic (proton) number Z = number of protons. Mass number A = protons + neutrons. In a neutral atom, electrons = protons.

Isotopes are atoms of the same element (same protons) with different numbers of neutrons — so a different mass number. They react identically because they have the same electron arrangement.

Your turn — counting neutrons

Relative atomic mass (A_r)

Because an element is a mixture of isotopes, its A_r is the weighted mean mass of its atoms, compared with 1/12 of the mass of a ¹²C atom:

Watch out: A_r is a weighted mean, not a simple average — it leans toward the more abundant isotope. That's why chlorine's A_r is 35.5, closer to 35 than to 37.

Your turn — relative atomic mass

Why is chlorine 35.5 on the periodic table?

The periodic table

Elements are arranged in order of increasing atomic number. Groups are the columns; periods are the rows.

Elements in the same group have the same number of outer electrons, so they react in similar ways. The period tells you how many electron shells are in use.

How do you count atoms by weighing?

Writing balanced equations

A balanced symbol equation must have the same number of each atom on both sides — mass is conserved. State symbols show the physical state:

The relative formula mass M_r is the sum of the A_r values of every atom in the formula.

Balancing only changes the big numbers in front (the coefficients) — never change a formula's small subscripts to "make it balance".

Your turn — relative formula mass

The mole & Avogadro's constant

A mole is the amount of substance containing 6.02 × 10²³ particles (the Avogadro constant). One mole of a substance has a mass equal to its M_r in grams.

Watch out: the equation is moles = mass ÷ M_r (not mass × M_r). A bigger M_r means fewer moles for the same mass. Rearrange it as mass = moles × M_r.

Your turn — moles from mass

How do you count atoms by weighing?

Reacting masses, empirical formulae & yield

The balanced equation gives the mole ratio, which lets you work out reacting masses:

The empirical formula is the simplest whole-number ratio of atoms (found from masses ÷ A_r). The molecular formula is the actual number of atoms. Percentage yield compares what you got to the theoretical maximum:

Your turn — reacting masses

Concentration, titration & molar volume

Concentration can be given in g/dm³ or mol/dm³ (note 1 dm³ = 1000 cm³):

In a titration you measure the volume of one solution that exactly reacts with a known volume of another, then use the mole ratio to find an unknown concentration.

For gases, one mole occupies 24 dm³ (24 000 cm³) at room temperature and pressure (rtp):

Your turn — concentration

Why does molten salt light the bulb?

Ionic bonding

Ionic bonding happens between a metal and a non-metal: electrons are transferred from the metal to the non-metal, forming oppositely charged ions held by strong electrostatic attraction.

Ionic compounds form a giant lattice: a regular 3-D arrangement of alternating + and − ions. This gives them high melting points, and they conduct electricity when molten or dissolved (the ions are then free to move) but not when solid.

Spotting ionic bonding

Covalent bonding

Covalent bonding happens between non-metal atoms: they share pairs of electrons so each gets a full outer shell.

Simple molecular substances (e.g. H₂O, CO₂) have low melting/boiling points because the weak forces between molecules are easily broken — they don't conduct (no charged particles free to move).

Giant covalent structures (e.g. diamond, graphite, silicon dioxide) have millions of strong bonds, so very high melting points. Graphite conducts (delocalised electrons); diamond does not.

Transfer or share?

Tap a property, then tap the bonding type it belongs to.

Why does molten salt light the bulb?

Metallic bonding

In a metal, the atoms are packed in a lattice of positive ions surrounded by a "sea" of delocalised electrons (the outer electrons, free to move).

This explains metals' properties: they conduct electricity and heat (electrons carry charge/energy), and they are malleable (layers of ions can slide without breaking the bond).

Name the bonding

Tap the bonding type that matches each substance.

Electrolysis

An electrolyte is an ionic compound that is molten or dissolved, so its ions are free to move. Passing electricity through it decomposes it:

At the electrodes, ions are discharged. We write half-equations using electrons (e⁻):

Watch out: at the cathode, positive ions GAIN electrons (reduction); at the anode, negative ions lose electrons (oxidation). Remember OIL RIG and "RED CAT" — reduction at the cathode. In aqueous solutions H⁺ or the metal, and OH⁻ or the halide, compete to be discharged. This electrolysis is also how reactive metals such as aluminium are extracted.

At the cathode

The key ideas to know

You've covered the key ideas in Edexcel iGCSE Section 1 — Principles of chemistry. Press Finish to see your score.