It treated molecules as real — before anyone could see them — and made their size and stickiness measurable.
Van der Waals’s 1873 equation was the first thermodynamic model to treat fluids as composed of real molecules — with finite size and mutual attraction — and it successfully described both liquid and gas phases, predicted critical-point behaviour, and yielded quantitative estimates of molecular size and attraction strength. It did not prove molecular existence, nor does it hold across all conditions. Its value lies in making molecular assumptions productive — long before they were directly observable.
Van der Waals derived the equation in 1873, assuming molecules exist, have finite size, and attract each other.
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First molecular fluid model
It was the first successful thermodynamic model to treat fluids as composed of molecules with finite size and intermolecular interactions.
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Critical point prediction
It accurately predicted fluid behaviour around the critical point, with qualitative and quantitative agreement with experiments.
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From equation to molecules
It enabled estimation of molecular size and attraction strength, and showed gas and liquid phases are of the same nature.
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Immediate scientific impact
The thesis was immediately recognised as a hallmark in physics — notably by James Clerk Maxwell — for explaining non-ideality via intermolecular forces.
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One equation, two phases
It describes both liquid and gas states, based on the then-controversial idea that fluids are composed of discrete particles.
Worth your time?
Yes. Study the whole thing.
4.5/ 5
What works
explains critical point behaviour
enables estimation of molecular size and attraction
Fluids are composed of discrete particles with finite volume and mutual attraction. Gas non-ideality arises from those properties. The gas and liquid phases are not fundamentally distinct but continuous manifestations of the same molecular system.
How it was done
Van der Waals derived the equation in 1873 as part of his doctoral thesis at the University of Leiden. He assumed molecules exist, have finite size, and attract each other. He modelled intermolecular forces as hard repulsion plus weak attraction at distance. He introduced molecular volume and molecular attraction as physical parameters. He attributed gas non-ideality to intermolecular interactions.
What holds up
It describes both liquid and gas states. It accurately predicted fluid behaviour around the critical point. It showed qualitative and quantitative agreement with experiments. It enabled quantitative estimation of molecular size and attraction strength from experimental data. It unified gas and liquid phases as continuous states of the same nature.
What does not
It does not quantitatively predict fluid behaviour across all temperatures, pressures, or substances. It does not describe molecular motion, kinetics, or quantum effects. It does not establish molecular existence as fact — only that assuming it yields predictive power.
Why it matters beyond the lab
It shifted thermodynamics from phenomenological description to molecular explanation. It gave experimentalists a way to estimate molecular size and interaction strength decades before direct observation. It made molecular reality plausible to sceptics — including James Clerk Maxwell, who lauded the thesis immediately.
Is it worth your time
Yes. It is the first thermodynamic model to treat fluids as composed of real molecules — not ideal points — and it remains foundational for teaching phase behaviour, critical phenomena, and molecular-scale reasoning in physics and chemistry.