The Gas Phase
High-Yield Summary
- A gas uniformly fills its container, is easily compressed, mixes completely with other gases, and exerts pressure via constant particle collisions with container walls.
- Pressure = force per unit area; SI unit is the pascal (Pa = N/m²).
- Common pressure units all tie to standard atmospheric pressure: 1 atm = 101,325 Pa = 1.01325 bar = 760 mmHg = 760 torr.
- Manometer measures the pressure of a gas in a closed container (vs. atmosphere); barometer measures atmospheric pressure directly (760 mmHg at sea level).
Key Terms
- Pressure
- Force per unit area; SI unit pascal (Pa = N/m²).
- Temperature
- A measure of the average kinetic energy of a system's particles.
- Volume
- A scalar quantity expressing the 3D space enclosed by a closed surface.
- Manometer
- Instrument measuring the pressure of a gas in a closed container via a mercury-filled U-tube open to the atmosphere.
- Barometer
- Instrument measuring atmospheric pressure via a mercury column balanced against a vacuum.
Pressure Unit Conversions
1 atm = 101,325 Pa = 101.325 kPa = 1.01325 bar = 760 mmHg = 760 torr
- atm = standard atmosphere
- Pa = pascal, SI unit (N/m²)
- mmHg / torr = millimeters of mercury / torr (numerically equivalent)
- Worked example: 2 atm = 1,520 mmHg = 1,520 torr = 202.65 kPa.
How a Barometer Measures Atmospheric Pressure
- 1Fill a long glass tube (closed at one end) with mercury, temporarily cover the open end.
- 2Invert the tube and place the open end in a reservoir of mercury.
- 3Remove the cover — some mercury flows out into the reservoir, but a column remains standing.
- 4The space above the column inside the tube is essentially a vacuum, so nothing pushes down from inside.
- 5Atmospheric pressure pushes down on the reservoir's surface, balancing the weight of the standing mercury column.
- 6Column height (mm) corresponds directly to atmospheric pressure; standard sea-level pressure = 760 mmHg.
Must-Know Points
- Gas pressure arises from constant, random particle collisions with container walls — this is the mechanistic link to kinetic molecular theory (see Kinetic Molecular Energy).
- A manometer compares a closed container's gas pressure to atmospheric pressure; a barometer measures atmospheric pressure alone.
- Standard atmospheric pressure at sea level = 760 mmHg = 1 atm — memorize this conversion chain cold.
Common MCAT Trap
- Don't confuse a manometer (measures a gas sample's pressure relative to atmosphere) with a barometer (measures atmospheric pressure itself).
- Pressure unit conversions are a frequent MCAT calculation trap — always double-check which unit a problem wants before finalizing an answer (atm vs. mmHg vs. kPa vs. torr).
Quick Recall
Convert 0.5 atm to mmHg.
What creates the vacuum space at the top of a barometer's tube?
What is the SI unit of pressure, and what does it equal?