Pure substances vs mixtures

In chemistry, a pure substance is made of only one element or one compound. This is stricter than everyday English, where "pure orange juice" just means nothing added.

• A pure substance melts and boils at a single, sharp, fixed temperature.
• A mixture (an impure substance) melts over a range of temperatures, and its melting point is lowered by the impurity.

Watch out: "pure" does not mean "a single element" — a pure compound (like distilled water) is pure too. Pure means one substance, melting at one sharp temperature.

Pure or not?

Useful mixtures: formulations

A formulation is a mixture made to a precise recipe, where each component is present in a measured amount and has a job to do. Getting the proportions right gives the product the properties we want.

• Alloys — e.g. steel, brass (a metal mixed with other elements).
• Paints — pigment + binder + solvent + additives.
• Fuels, fertilisers, medicines, cleaning products, food.

Key idea: a formulation is still a mixture — the components are not chemically bonded, so it has no fixed formula and can be separated again.

Relative formula mass & empirical formula

Add up the relative atomic masses (A_r) of every atom in a formula to get the relative formula mass, M_r.

The empirical formula is the simplest whole-number ratio of atoms in a compound. For example C₆H₁₂O₆ has empirical formula CH₂O (divide each by 6).

Your turn — relative formula mass

Four separation techniques

Because the parts of a mixture are not chemically bonded, we can separate them using physical methods that exploit a difference in their properties:

• Filtration — separates an insoluble solid from a liquid (e.g. sand from water).
• Crystallisation — gets a dissolved solid back from a solution by evaporating the solvent so crystals form.
• Simple distillation — separates a solvent from a solution (e.g. pure water from salty water), using a difference in boiling point.
• Fractional distillation — separates a mixture of liquids with different boiling points (e.g. crude oil, or ethanol from water).

Choose the method

Match mixture to method

Tap a mixture on the left, then the technique on the right that separates it.

Paper chromatography

Chromatography separates a mixture of soluble, coloured substances (like dyes). Every chromatography method has two parts:

• A mobile phase — the moving solvent that carries substances up the paper.
• A stationary phase — the paper itself, which holds substances back.

Substances that are more attracted to the solvent (and less to the paper) travel further. Separation depends on this distribution between the two phases.

Watch out: R_f is a ratio, so it is always less than 1 and has no units (never write "cm"). A pure substance gives one spot; a mixture gives several.

Your turn — calculate R_f

Metals and non-metals

The position of an element in the periodic table tells you whether it is a metal or a non-metal. Metals sit to the left and centre; non-metals to the top right.

A metal's outer shell has few electrons (easy to lose); a non-metal's outer shell is nearly full (easy to gain). Their reactions e.g. with oxygen follow from this.

Electrons and position

Electrons occupy shells (energy levels) filling up 2, 8, 8…. An atom's electron arrangement is set by its atomic (proton) number, and it pins down its place in the table:

• Group number = number of electrons in the outer shell (for the main groups).
• Period number = number of occupied shells.

Watch out: the group number gives the outer-shell electrons, not the total. Sodium has 11 electrons but only 1 in its outer shell, so it is in Group 1.

Ionic bonding — transfer

An ionic bond forms between a metal and a non-metal. Electrons are transferred from the metal to the non-metal, making charged ions held together by strong electrostatic forces.

Ionic compounds form giant ionic lattices: a regular 3-D arrangement of alternating + and − ions, held by strong forces in every direction.

Covalent bonding — sharing

A covalent bond forms between non-metal atoms. Instead of transferring electrons, the atoms share a pair of electrons, so each reaches a full outer shell.

Covalent substances form either simple molecules (small groups like H₂O, CO₂) or giant covalent structures (huge networks — coming next).

Watch out — the classic mix-up: ionic = transfer of electrons (metal + non-metal); covalent = sharing of electrons (non-metal + non-metal). Don't swap them!

Predict the bonding

Classify the bonding

Tap the type of bonding present in each substance.

Metallic bonding — the electron sea

In a metal, the atoms pack into a giant lattice of positive ions. The outer electrons become delocalised — free to move through the whole structure as a "sea" of electrons.

The delocalised electrons carry charge → metals conduct electricity. The layers can slide → metals are malleable.

Carbon's structures

Carbon forms four covalent bonds, building very different giant structures from the same element. Compare diamond and graphite:

Graphite conducts electricity because each carbon uses only 3 of its 4 outer electrons for bonding — the 4th electron is delocalised and free to move along the layers.

Also know graphene (a single graphite layer — strong, conducts) and fullerenes (carbon cages/tubes, e.g. C₆₀ and nanotubes).

Explain a property

Properties from structure & bonding

The bulk properties of a material come from its structure, the strength of its bonds and how they are arranged. Learn this table:

Key idea: to melt or boil you must overcome forces. Intermolecular forces (between simple molecules) are weak; covalent and ionic bonds are strong — that's why giant structures melt so much higher. A single atom does not have these bulk properties.

States of matter & state symbols

You can predict the state of a substance at a given temperature using its melting and boiling points: below the m.p. it's solid; between m.p. and b.p. it's liquid; above the b.p. it's gas.

• solid (s)  ·  liquid (l)  ·  gas (g)  ·  in solution (aq) (aqueous)

The relative strength of bonds and intermolecular forces explains why different substances change state at very different temperatures.

Your turn — predict the state

Nanoparticles

Nanoparticles are tiny — roughly 1–100 nm across (1 nm = 1×10⁻⁹ m), only a few hundred atoms wide. As particles get smaller, their surface-area-to-volume ratio grows dramatically.

A high surface-area-to-volume ratio makes nanoparticles very reactive and effective in small amounts. Uses: catalysts, sun creams, medicine delivery, antibacterial coatings, electronics. Risks: their tiny size means effects on health and the environment are still uncertain.

Building the periodic table

Mendeleev (1869) arranged the known elements mainly by atomic mass, but cleverly:

• He left gaps for undiscovered elements and predicted their properties.
• He swapped some elements out of strict mass order so they fit groups with similar properties.

The modern table is arranged by atomic (proton) number, which explained Mendeleev's swaps. Groups are columns (same outer electrons → similar reactions); periods are rows.

Group 1, Group 7 & Group 0

• Group 1 (alkali metals): Li, Na, K… soft, very reactive metals; react vigorously with water. Reactivity increases down the group (the outer electron is lost more easily).
• Group 7 (halogens): F, Cl, Br, I… reactive non-metals. Reactivity decreases down the group (the outer shell gains an electron less easily). A more reactive halogen displaces a less reactive one from solution.
• Group 0 (noble gases): He, Ne, Ar… unreactive because they already have full outer shells.

Watch out: the two trends go opposite ways. Metals (Group 1) want to lose an electron — easier down the group. Non-metals (Group 7) want to gain one — harder down the group.

Will it displace?

The transition elements

The block of metals in the centre (e.g. iron, copper, zinc). Compared with Group 1 metals, transition metals are:

• Harder, stronger, denser and have higher melting points.
• Less reactive (they don't react violently with water).
• Often coloured compounds; many are useful catalysts; many form ions with more than one charge (e.g. Fe²⁺ and Fe³⁺).

Contrast: a Group 1 metal like sodium is soft and fiercely reactive; a transition metal like iron is hard, strong and far less reactive — which is why we build with it.

Spot the trend

The big ideas of C2

You've covered all three parts of OCR Gateway C2 — purity & separating mixtures, bonding, and the properties of materials & the periodic table. Press Finish to see your score.