The Gas Phase
Gases uniformly fill their container, compress easily, and exert pressure — force per unit area — on their surroundings.
Gases behave differently from solids and liquids in one defining way: a gas expands to uniformly fill whatever container it's in, compresses easily, and mixes completely with any other gas present. That behavior is what makes a gas exert pressure on its surroundings — the air inside an inflated balloon pushes outward against the elastic walls and keeps it firm. This article covers the basic vocabulary of gas behavior — temperature, volume, and pressure — the units used to express pressure, and the two classic instruments used to measure it: the manometer and the barometer.
Key Takeaways
A gas uniformly fills its container, is easily compressed, mixes completely with other gases, and exerts pressure on its surroundings.
Temperature measures the average kinetic energy of a system's particles; volume is the 3D space enclosed; pressure is force per unit area, with SI unit pascal (Pa = N/m²).
Common pressure units — pascal, bar, torr, mmHg, atm — all tie back to standard atmospheric pressure: 1 atm = 101,325 Pa = 1.01325 bar = 760 mmHg = 760 torr.
A manometer measures the pressure of a gas in a closed container by comparing it to atmospheric pressure via a mercury-filled U-tube.
A barometer measures atmospheric pressure using a mercury column balanced against a vacuum, in a tube inverted in a mercury reservoir; standard atmospheric pressure at sea level is 760 mmHg.
Properties of a Gas
A gas has three defining physical properties:
It uniformly fills any container it occupies.
It's easily compressed.
It mixes completely with any other gas.
Because gas particles are in constant motion and collide with the walls of their container, every gas exerts pressure on its surroundings. This is why an inflated balloon stays firm: the air inside is constantly pushing against the elastic sides.
Temperature, Volume, and Pressure
Three quantities describe the basic physical state of a gas:
Temperature: a physical quantity that expresses hot and cold, or a measure of the average kinetic energy of the atoms or molecules in a system.
Volume: a scalar quantity expressing the amount of three-dimensional space enclosed by a closed surface.
Pressure: defined as force per unit area. For a given force, the smaller the area it's spread over, the higher the pressure.
The SI unit of pressure is the pascal (Pa), equal to newtons per square meter (N/m²) — one pascal is one newton of force exerted over one square meter of area.
Units of Pressure
Pressure shows up in several different units on the MCAT, and converting between them is a core skill. The most common are pascal, bar, torr, millimeters of mercury (mmHg), and atmosphere (atm). Standard atmospheric pressure is the reference point that ties them all together:
1 atm = 101,325 Pa = 101.325 kPa
1 atm = 1.01325 bar
1 atm = 760 mmHg
1 atm = 760 torr
Worked example — converting a gas pressure of 2 atm to other units:
mmHg / torr: 2 atm × 760 mmHg/atm = 1,520 mmHg (= 1,520 torr)
kPa: 2 atm × 101.325 kPa/atm = 202.65 kPa
Measuring Pressure: Manometers and Barometers
Two classic instruments are used to measure pressure:
Manometer: measures the pressure of a gas in a closed container. One common design attaches the gas container to a glass U-tube that's open to the atmosphere and partially filled with liquid mercury. The mercury responds to the pressure exerted by the gas in the container on one side and by the atmosphere on the other, and the difference in mercury height across the two arms of the tube reflects the gas pressure.
Barometer: measures atmospheric pressure. It consists of a long glass tube, closed at one end and filled with mercury. To set it up, the open end is temporarily covered, then inverted and placed in a reservoir of mercury; when the cover is removed, some mercury flows out into the reservoir, but a column remains standing in the tube.
The space above the mercury column inside the tube is essentially a vacuum, so nothing pushes down on top of the column from inside the tube.
Atmospheric pressure instead pushes down on the surface of the mercury in the open reservoir.
The weight of the mercury column (a downward force from gravity) is balanced by that atmospheric pressure. If atmospheric pressure increases, it pushes more mercury up into the tube; if it decreases, the mercury column drops.
The height of the mercury column — usually measured in millimeters — corresponds directly to atmospheric pressure. Standard atmospheric pressure at sea level is 760 mmHg.
Common MCAT Mistakes
Confusing pressure with force. Pressure is force per unit area, not force alone — the same force spread over a smaller area produces a higher pressure, not the same pressure.
Treating mmHg and torr as needing a conversion factor. They're numerically identical (1 mmHg = 1 torr); both equal 1/760 of standard atmospheric pressure, so no conversion is needed between them.
Mixing up what a manometer and a barometer each measure. A manometer measures the pressure of a gas in a closed container relative to the atmosphere; a barometer measures atmospheric pressure itself, with nothing else involved.
Converting directly between mmHg/torr and Pa/kPa without going through atm. Since the given conversion factors are all defined relative to standard atmospheric pressure (1 atm), the safest path is converting to atm first, then to the target unit.
MCAT-Style Concept Check
Question: A gas has a pressure of 0.5 atm. What is this pressure in mmHg?
A) 195 mmHg
B) 380 mmHg
C) 570 mmHg
D) 760 mmHg
Answer: B
Explanation: Using the conversion factor 1 atm = 760 mmHg: 0.5 atm × 760 mmHg/atm = 380 mmHg. Option D restates the conversion factor itself (1 atm), not the pressure of the gas in question; the other options don't correspond to the correct arithmetic.
FAQ
What is the SI unit of pressure?
The SI unit of pressure is the pascal (Pa), equal to newtons per square meter (N/m²) — one pascal is one newton of force exerted over one square meter of area.
What is standard atmospheric pressure?
Standard atmospheric pressure is 1 atm, which equals 101,325 Pa (101.325 kPa), 1.01325 bar, 760 mmHg, and 760 torr.
What's the difference between a manometer and a barometer?
A manometer measures the pressure of a gas in a closed container by comparing it to atmospheric pressure via a mercury-filled U-tube. A barometer measures atmospheric pressure itself, using a mercury column balanced against a vacuum in a tube inverted in a mercury reservoir.
Why does a gas exert pressure on its container?
Gas particles are in constant motion and continually collide with the walls of their container; those collisions are what produce the outward-pushing force known as pressure.
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