Ch 5States of Matter — Class 11 Chemistry, concept-first.

Intermolecular forces are the forces of attraction and repulsion that act between interacting particles — atoms or molecules.

An isolated atom or a non-polar molecule has its electron cloud distributed symmetrically, so on average it carries no dipole moment. Yet even such species can develop a fleeting dipole.

When molecules already carry a permanent dipole — like HCl, where chlorine pulls electron density towards itself — their oppositely-charged ends attract neighbouring molecules.

A third kind of attractive force operates between a molecule that has a permanent dipole and a neighbouring molecule that has no permanent dipole of its own.

Hydrogen bonding, introduced already in Unit 4, is best thought of as a particularly powerful variety of dipole-dipole interaction (Section 5.1).

Every atom and molecule in a substance is in constant motion, and thermal energy is simply the energy a body possesses because of that motion.

We have now met the two opposing tendencies that decide how matter behaves: intermolecular forces pull molecules together, while thermal energy keeps them apart by keeping them in constant, energetic…

The gaseous state is the simplest of the three states of matter. We live our entire lives immersed in a mixture of gases — air — specifically within the troposphere, the lowest layer of the atmosphere…

The gas laws we are about to study are the product of centuries of careful experimental work on the physical behaviour of gases.

From his experiments, Robert Boyle concluded that at constant temperature, the pressure of a fixed amount (fixed number of moles, ) of gas varies inversely with its volume. This is Boyle's Law.

Charles and Gay Lussac, working independently while trying to improve hot-air-balloon technology, ran a series of experiments on gases.

Anyone who has checked a car tyre knows the everyday version of this law: tyre pressure, nearly constant most of the time, rises noticeably on a hot day and can even burst the tyre, while on a cold mo…

In 1811, the Italian scientist Amedeo Avogadro combined ideas from Dalton's atomic theory with Gay Lussac's law of combining volumes (Unit 1) to state what is now known as Avogadro's Law: equal volume…

Boyle's, Charles' and Avogadro's laws each hold one pair of variables fixed while relating the other two:

Starting from the ideal gas equation and rearranging for the molar amount per volume:

John Dalton formulated this law in 1801: the total pressure exerted by a mixture of non-reacting gases equals the sum of the partial pressures of the individual gases — where the partial pressure of a…

Gas molecules are in ceaseless motion, constantly colliding with each other and with the container walls.

Laws like Boyle's and Charles' are concise summaries of what scientists observe in the lab. But observing that pressure rises on compression only tells us what happens — it doesn't explain why, at the…

The kinetic-molecular model matches experiment well overall, but trouble appears when we test how precisely reproduces the actual pressure-volume-temperature behaviour of real gases.

The first complete picture of how pressure, volume and temperature relate together in both the gaseous and liquid states of a substance came from Thomas Andrews, who plotted isotherms of carbon dioxid…

Intermolecular forces are stronger in liquids than in gases. Molecules in a liquid sit so close together that there is very little empty space between them — this is exactly why liquids are, under nor…

If an evacuated container is partly filled with a liquid, some of the liquid evaporates to fill the remaining space with vapour.

Liquids are supposed to take the shape of their container — so why does a small drop of mercury pull itself into a sphere instead of spreading flat? Why do soil particles at a riverbed stay separate u…

Viscosity measures a liquid's resistance to flow, arising from internal friction between layers of the fluid as they slide past one another.