Chapter 11 · Gases
11.1Extra-Long Straws
7 min · two checks
Predict
Why can’t a straw, however long, lift water higher than about 10 m?
The idea
- Describe pressure as a push from collisions.
- Explain the height limit of a barometer.
Sucking on a straw lowers the pressure of the gas inside it. The higher pressure of the atmosphere on the drink’s surface pushes liquid up the straw. You are not hauling the liquid with a vacuum rope. If the straw were taller than the column the atmosphere can support, the liquid would stop. For water that height is near 10 m. For mercury, much denser, it is 760 mm. That mercury column is the historical definition behind the pressure unit mmHg.
Gases exert pressure because their particles collide with surfaces. More collisions, or harder ones, mean higher pressure. The rest of the chapter turns that picture into laws relating pressure, volume, temperature, and moles — and every temperature in those laws is absolute.
Keep these
- Atmospheric pressure pushes; a straw does not lift.
- 760 mmHg = 1 atm = 101.325 kPa.
- Gas pressure is particle collisions with the walls.