The gas laws

What are the gas laws?
Squeeze a gas and it pushes back, heat it and it swells, and one short rule, PV = nRT, explains both. Push a piston, breathe like Boyle, light diesel with a squeeze, whistle a pressure cooker, fly a hot-air balloon, and turn CO₂ into a liquid.

▶ Play with it  ·  Read the 60-second explainer  ·  Watch the 40-second video

In 60 seconds

1. Pressure is molecules hitting walls. Air molecules fly at about 500 m/s and bounce off everything. Each bounce is a tiny push; billions of billions a second add up to pressure. More molecules, faster ones or less room all mean more pushes.
2. Four laws, one rule. Boyle: half the volume, twice the pressure. Charles: heat it and it swells. Gay-Lussac: heat it sealed and the pressure climbs. Avogadro: twice the gas needs twice the room. Together they are PV = nRT, with T in kelvin.
3. Squeeze it: lungs and engines. A bigger chest means lower pressure, so air flows in: that is breathing. Squeeze air fast, as an engine does, and it heats up: a diesel squeezes 18 times and the air passes 500 °C, enough to light the fuel.
4. Heat it: tyres, cookers and balloons. A hot tyre gains about 3.4 % pressure per 10 °C. A pressure cooker holds steam at 2 atm so water boils at about 120 °C. Heated air spreads thinner, so a balloon full of it floats.
5. Where the rule breaks. Squeeze CO₂ below 31 °C and it turns to liquid; LPG cylinders use this on purpose. For the air around you, though, PV = nRT is right to within 0.1 %. And gases don't fill jars because they are light: it is their motion.

Words worth knowing

Term	Meaning
Pressure	The push of a gas on each square metre of wall, from its molecules hitting it. Air around you is about 101 kPa.
Ideal gas law	PV = nRT: pressure × volume = moles × 8.314 × temperature in kelvin.
Kelvin	Temperature counted from absolute zero, −273.15 °C. The gas laws only work in kelvin.
Mole	A count of molecules: 6.022 × 10²³. A litre of air holds about 0.04 mol.
Boyle's law	At steady temperature, pressure × volume stays the same.
Charles's law	At steady pressure, volume grows in step with kelvin temperature.
Partial pressure	Each gas in a mixture pushes as if it were alone, and the pushes add up (Dalton's law).
Critical point	Above it, no squeeze can turn a gas into a liquid. For CO₂: 31 °C and 74 bar.

A short history

From a tube of mercury in Florence to a number fixed forever in 2019: 400 years of learning how gases push, stretch and move.

• 1662 · Squeeze the air and it pushes back harder (Robert Boyle, with Robert Hooke; helped by Richard Towneley and Henry Power, Oxford, England)
• 1783 · People fly for the first time, on hot air (Joseph-Michel and Jacques-Étienne Montgolfier; flown by Jean-François Pilâtre de Rozier and the Marquis d'Arlandes, Paris, France)
• 1802 · Charles's law goes public, with credit (Joseph Louis Gay-Lussac, Paris, France)
• 1811 · Equal volumes hold equal numbers (Amedeo Avogadro; later championed by Stanislao Cannizzaro, Turin, Kingdom of Sardinia (published in Paris))
• 1834 · One equation for every gas (Émile Clapeyron, Paris, France)
• 1873 · A law for real gases (Johannes Diderik van der Waals, Leiden, Netherlands)

The full story, with 20 moments, charts, people and 35 sources: glassbox.how/e/gaslawclear/history. The data lives in history.json.

Video and slides

Made with the Glassbox studio from this box's storyboard (window.glassbox.director). Free to reuse under CC BY 4.0.

File	What	Size
glassbox/reel.mp4	Reel / Short, with captions and soundtrack	1080×1920
glassbox/video.mp4	YouTube video, with captions and soundtrack	1920×1080
glassbox/slide-1…10.jpg	Instagram carousel	1080×1350
glassbox/thumb.jpg	YouTube thumbnail	1280×720
glassbox/cover.jpg	Share card and repo social preview	1200×630
glassbox/history-reel.mp4	“History in 10 moments” Reel / Short	1080×1920
glassbox/history-slide-*.jpg	History carousel	1080×1350
glassbox/post.json	Post copy and schedule used by the publish kit

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How it's built

File	What
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glassbox.json	Title, question, explainer beats, key terms, browser storage and credits shown on glassbox.how
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