States of Matter: Ideal Gases and Structure

This guide covers subtopics 4.1, The gaseous state: ideal and real gases and pV = nRT, and 4.2, Bonding and structure, from Topic 4 of Cambridge International AS & A Level Chemistry 9701, 2025–2027 series. Both are AS Level content. The two halves of this resource aren’t directly connected — they’re grouped only because they share a topic number — so treat them as two short, separate sections.

Before studying this

At IGCSE or O Level, you met the kinetic particle theory qualitatively and the basic solid/liquid/gas model. States of Matter and Kinetic Particle Theory covers that. This resource assumes it, and adds a genuinely quantitative treatment of gas behaviour (pV = nRT), plus a systematic classification of solid structures that draws on bonding ideas from Chemical Bonding: Electronegativity, Ionic and Metallic Bonds and the structure/bonding reasoning already used in The Periodic Table: Periodicity Across Period 3.

Syllabus coverage

CAMBRIDGE INTERNATIONAL AS & A LEVEL CHEMISTRY 9701 — AS Level, Topic 4

4.1 The gaseous state — explaining the origin of pressure in a gas in terms of molecular collisions with the container wall; understanding that ideal gases have zero particle volume and no intermolecular forces of attraction; stating and using the ideal gas equation, pV = nRT, in calculations, including determining Mr.

4.2 Bonding and structure — describing, in simple terms, the lattice structure of a crystalline solid that is giant ionic (including sodium chloride and magnesium oxide), simple molecular (including iodine, buckminsterfullerene C₆₀ and ice), giant molecular (including silicon(IV) oxide, graphite and diamond), or giant metallic (including copper); describing, interpreting and predicting the effect of structure and bonding on melting point, boiling point, electrical conductivity and solubility; deducing the type of structure and bonding present from given information.

The gaseous state

Pressure arises from gas molecules colliding with the walls of their container — each collision transfers a tiny amount of momentum, and the sum of billions of these collisions per second produces a steady, measurable force per unit area.

An ideal gas is a simplifying model: its particles are assumed to have zero volume (treated as points) and no intermolecular forces of attraction between them. Real gases only approximate this — they deviate most at high pressure (particles are forced close enough for their actual volume and their attractions to matter) and low temperature (particles move slowly enough for attractive forces to have a noticeable effect).

The ideal gas equation: pV = nRT, where p is pressure (Pa), V is volume (m³), n is number of moles, R is the gas constant (8.31 J K⁻¹ mol⁻¹), and T is temperature (K) — always convert °C to K by adding 273, and cm³ or dm³ to m³ before substituting.

Worked example. A 4.40 g sample of a gas occupies 2.24 dm³ at 101 kPa and 273 K. Calculate its Mr.

Convert units to SI: V = 2.24 × 10⁻³ m³, p = 101 000 Pa.

n = pV / RT = (101 000 × 2.24 × 10⁻³) / (8.31 × 273) = 226.2 / 2268.6 ≈ 0.0997 mol

Mr = mass / n = 4.40 / 0.0997 ≈ 44 g mol⁻¹ (consistent with CO₂).

Bonding and structure: four types of lattice

Structure	Examples	Bonding	Melting/boiling point	Conducts?	Typically soluble in water?
Giant ionic	NaCl, MgO	electrostatic attraction between ions, in all directions	high	only molten or in solution (mobile ions)	often, yes
Simple molecular	iodine, I₂; buckminsterfullerene, C₆₀; ice, H₂O	strong covalent bonds within each molecule; weak van der Waals (or, for ice, hydrogen bonding) between molecules	low	no (no mobile charged particles)	depends on polarity
Giant molecular (macromolecular)	silicon(IV) oxide, SiO₂; graphite; diamond	a continuous network of strong covalent bonds	very high	no, except graphite	no
Giant metallic	copper	electrostatic attraction between metal cations and delocalised electrons	high	yes, solid and molten	no

Giant ionic structures melt only at high temperature because melting means overcoming strong electrostatic attraction extending through the whole lattice, not just between one pair of ions. They don’t conduct as solids (the ions are fixed in place) but do once molten or dissolved, when the ions become free to move and carry charge.

Simple molecular structures are the odd one out in having strong bonds that are almost irrelevant to melting: the covalent bonds holding each I₂, C₆₀ or H₂O molecule together are strong, but melting only needs to overcome the much weaker forces between separate molecules. Ice is a special case worth knowing by name — it’s held together by hydrogen bonding, not just van der Waals forces, which gives ice an unusually open structure (and is why ice is less dense than liquid water and floats).

Giant molecular structures have the highest melting points of all, because melting a covalent network means breaking a great many strong bonds simultaneously. Diamond and graphite are both pure carbon, but behave very differently electrically — diamond doesn’t conduct, because every carbon atom uses all four outer electrons in four localised covalent bonds, leaving none free to move; graphite does conduct (along its layers), because each carbon only forms three bonds within its layer, leaving one electron per atom delocalised across the whole layer, free to carry charge.

Giant metallic structures conduct in both the solid and molten state, because the delocalised “sea” of electrons is mobile in either state — this is the same electrostatic model from Chemical Bonding: Electronegativity, Ionic and Metallic Bonds.

Worked example. An unknown white solid has a very high melting point, does not conduct electricity as a solid, but conducts well once molten, and dissolves readily in water to give a colourless solution. Deduce its type of structure and bonding.

A high melting point rules out simple molecular. Conducting only when molten (not as a solid) rules out giant metallic (which conducts in both states) and giant molecular (which, except graphite, doesn’t conduct at all). Conducting when molten, together with solubility in water, is the signature of mobile ions being released — the solid is giant ionic.

Common mistakes

• Saying I₂ or ice have “weak bonds” without specifying weak between molecules. The covalent bonds within each molecule are strong; only the intermolecular forces are weak — melting only has to overcome the latter.
• Assuming all giant molecular structures fail to conduct. Graphite is the standard exception, and the reason (delocalised electrons within each layer, from the one non-bonding outer electron per carbon atom) is worth being able to state, not just recall as an exception.
• Forgetting to convert units before using pV = nRT. Pressure must be in Pa, volume in m³, and temperature in K — using kPa, dm³ or °C directly gives an answer wrong by a power of ten or an offset of 273.
• Treating “ideal gas” as a real category of gas. It’s a simplifying model that real gases only approximate, most closely at low pressure and high temperature.

Quick revision checklist

• Origin of gas pressure: collisions with the container wall
• Ideal gas assumptions: zero particle volume, no intermolecular attraction
• pV = nRT with correct SI units, including finding Mr
• Four lattice types: giant ionic, simple molecular, giant molecular, giant metallic — their examples, bonding, and effect on melting point and conductivity
• Diamond vs graphite conductivity, and why they differ
• Deducing structure/bonding type from melting point, conductivity and solubility data

Related resources

• States of Matter and Kinetic Particle Theory — the IGCSE/O Level qualitative model this resource builds on
• Chemical Bonding: Electronegativity, Ionic and Metallic Bonds — the bonding models this resource applies to solid structures
• The Periodic Table: Periodicity Across Period 3 — the same four structure types applied to one specific period
• Cambridge AS & A Level Chemistry hub

Written against Cambridge International AS & A Level Chemistry 9701, 2025–2027 series. Always check the current syllabus for your examination year.