The Gaseous State

Lesson

Part 1 — gas laws and the ideal gas equation. This chapter is formative assessment only: it is not in the annual theory paper, but it is taught here at textbook depth, with simple numericals, for school tests.

In this lesson you will

Describe the role of intermolecular forces in deciding the state of matter
State Boyle, Charles, Gay-Lussac and Avogadro laws and interpret their graphs
Use pV = nRT with R in different units to find pressure, volume, moles, density and molar mass

Table of the four variables that describe a gas with symbols, units and conversions
Variable	Symbol	Common units	Link to SI
Pressure	p	atm, bar, mmHg, kPa	1 atm = 101.325 kPa = 760 mmHg; 1 bar = 100 kPa
Volume	V	L, dm³, mL, m³	1 L = 1 dm³ = 10⁻³ m³
Temperature	T	kelvin (K)	T = t + 273.15
Amount	n	mol	n = mass ÷ molar mass

Concept

Intermolecular forces and the state of matter

A substance is a gas when the thermal energy of its molecules is much greater than the intermolecular attractions. Cooling or compressing a gas reduces thermal energy or brings molecules closer, and attraction wins: the gas liquefies.

Table of the main intermolecular forces with where they act, relative strength and examples
Force (in brief)	Acts between	Relative strength	Example
Dispersion (London)	All molecules, from temporary dipoles	Weak, grows with size	Noble gases, N₂, CH₄
Dipole–dipole	Polar molecules	Moderate	HCl, SO₂
Hydrogen bonding	H on N, O or F and a lone pair	Strongest of these three	NH₃, H₂O, HF
Ion–dipole	Ion and polar molecule	Strong	Na⁺ in water

Concept

Boyle’s law: pressure and volume

At constant temperature the volume of a fixed amount of gas is inversely proportional to its pressure: $pV = \text{constant}$ , or $p_1V_1 = p_2V_2$ .

Table of pressure, volume and the constant product for one mole of ideal gas at 300 kelvin
p / bar	V / L (1 mol, 300 K)	p × V / L bar
1	24.94	24.94
2	12.47	24.94
4	6.24	24.94
8	3.12	24.94

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