Subjects and lessons to explore.

Law

Chemistry 2e · Gases

Gas Pressure

7 min read
6 key termsSourcesReport an error
Standard constants used (1 atm = 760 mmHg = 101.325 kPa, g = 9.81 m/s², mercury density ≈ 13.6 g/mL) are widely published reference values; verify against current primary sources (NIST, IUPAC) before formal citation.
Want it in plain words first? Jump to Eli explains — the same idea, no jargon.
On this page 9 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Worked example
  6. Key takeaway
  7. Check yourself
  8. Study tools
  9. Sources & references

In 30 seconds

is the force a substance exerts on a surface divided by the area of that surface. Gases push on every wall, lid, and corner of their container — that push is gas pressure. You live inside this phenomenon: the air presses on your skin with roughly 101,000 N per square meter at sea level, yet you feel nothing because the pressure inside your body pushes outward to match, and every breath works because air pressure outside your lungs slightly exceeds the pressure inside them.

Atmospheric pressure is not constant: it comes from the weight of the air column above, decreasing with altitude and increasing below sea level or underwater. This chapter opens with gas pressure because it is the measurable variable that lets us study the other gas properties — volume, temperature, and amount.

Why this matters

  • Weather and altitude: Falling barometric pressure often signals an approaching storm.
  • Healthcare: Blood pressure cuffs use mmHg — the same unit chemists use — and ventilators and oxygen delivery rely on gas pressure.
  • Diving and aviation: Divers withstand about 1 atm of added pressure per 10 m of seawater; aircraft cabins are pressurized at altitude.
  • Lab and industry: Manometers and gauges are used in reactions, natural gas delivery, tire inflation, and aerosol cans — which warn against heat because it can rupture them.
  • Exams: Pressure unit conversions and / readings are near-guaranteed test items.

The college version

Core Concepts

Pressure is force spread over an area

Pressure is defined as force per unit area:

P = FA

where P is pressure, F is the force applied perpendicular to the surface, and A is the area. The SI unit is the pascal (Pa), equal to one newton per square meter (1 Pa = 1 N/m2). Because a pascal is small, chemists usually use kilopascals (kPa). A sharp knife cuts better than a dull one because the same force spread over a tiny area produces far more pressure — pressure, not force alone, determines whether something cuts, punctures, or supports.

Atmospheric pressure and the barometer

The atmosphere is a thick layer of gas held to Earth by gravity. At any point, atmospheric pressure equals the weight of the air column above divided by its cross-sectional area. In 1643, Evangelista Torricelli built the first barometer: a glass tube closed at one end, filled with mercury, and inverted into a dish of mercury. The column falls until its weight balances the atmosphere's push. At sea level it stands about 760 mm tall, so atmospheric pressure is commonly reported as 760 mmHg.

The column height works because a liquid column's pressure is:

P = ρg h

where ρ is the liquid's density, g is gravitational acceleration (about 9.81 m/s2 at Earth's surface), and h is the column height. Mercury is used because its density is about 13.6 times water's — a water barometer would need a column roughly 10.3 m (34 ft) tall.

Units of pressure and how they connect

All of the following equal one standard atmosphere:

UnitEquivalent to 1 atm
atmosphere (atm)1 atm (by definition)
millimeters of mercury (mmHg)760 mmHg
torr760 torr (1 torr ≈ 1 mmHg)
kilopascals (kPa)101.325 kPa
bar1.01325 bar
pounds per square inch (psi)14.7 psi (about 14.696)

Because these units measure the same quantity, any pressure can be expressed in any unit by multiplying by conversion factors equal to 1, such as 760 mmHg1 atm.

Manometers: measuring gas pressure in the lab

A manometer is a U-shaped tube containing a liquid (usually mercury), one arm connected to the gas sample, the other open to the atmosphere or sealed. In an open-end manometer, the height difference h between the arms shows how the gas compares with atmospheric pressure. Mercury higher on the atmospheric side means the gas pushes harder:

Pgas = Patm + h

Mercury higher on the gas side means the atmosphere pushes harder:

Pgas = Patm - h

The height h must be in the same pressure units as Patm. A closed-end manometer has an evacuated sealed arm, so the gas pressure equals the height difference directly: Pgas = h.

Common Confusions

Do Not ConfuseWithDifference
PressureForceForce is a push (N); pressure is force per area (N/m²). Same force on less area = more pressure
mmHgtorrNumerically equal in practice (760 mmHg = 760 torr); torr is named after Torricelli, mmHg is a measurement unit
ManometerBarometerBarometer measures atmospheric pressure; manometer measures a gas sample's pressure against a reference
Gas pressureGas volumePressure is force per area from collisions; volume is the space the gas occupies — related but distinct
High altitudeHigh pressureAltitude lowers atmospheric pressure; depth underwater raises it
psiPapsi is an engineering unit; Pa is SI. 1 atm ≈ 14.7 psi = 101,325 Pa
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

Imagine tiny invisible balls bouncing off the inside of a balloon. Every smack gives the wall a tiny push, and gas pressure is all those pushes added together. More balls, faster balls, or a smaller room mean more pushes per second — higher pressure. A barometer is like a straw of heavy liquid in a dish: the air pushes down on the liquid and holds it up in the straw, so a taller column means higher air pressure.

Worked example

Example 1: Convert pressure between units (dimensional analysis)

A weather report says the pressure is 29.9 inHg. Convert to mmHg and atm.

Step 1 — convert inches to mmHg (25.4 mm per inch):

P = 29.9 inHg × 25.4 mmHg1 inHg = 759 mmHg

Step 2 — convert mmHg to atm using 1 atm = 760 mmHg:

P = 759 mmHg × 1 atm760 mmHg = 0.999 atm

About 1 atm, as expected for a sea-level reading; units cancel at each step.

Example 2: Open-end manometer reading

A gas sample is connected to an open-end mercury manometer. Atmospheric pressure is 755 mmHg. The mercury level is 42 mmHg higher on the open (atmospheric) side than on the gas side. Find the gas pressure.

Mercury higher on the open side means the gas pushes harder than the atmosphere:

Pgas = Patm + h = 755 mmHg + 42 mmHg = 797 mmHg

Check: the gas pushed mercury up against the atmosphere, so its pressure must exceed 755 mmHg — and 797 > 755, consistent.

Example 3: What "760 mmHg" really means

Verify that a 760-mm mercury column exerts about one atmosphere, using P = ρg h.

Step 1 — gather values: ρ= 13.6 × 103 kg/m3, g = 9.81 m/s2, h = 0.760 m.

Step 2 — substitute:

P = (13.6 × 103 kg/m3)(9.81 m/s2)(0.760 m) = 1.01 × 105 Pa = 101 kPa

This matches 101.325 kPa within rounding — the physical basis of the mmHg unit. Units combine: kg/m3 × m/s2 × m = N/m2 = Pa.

Key takeaways

  • Pressure = force ÷ area; SI unit: pascal (1 Pa = 1 N/m2).
  • Sea-level pressure ≈ 760 mmHg = 760 torr = 1 atm = 101.325 kPa = 1.01325 bar ≈ 14.7 psi.
  • Barometer = atmospheric pressure; manometer = gas sample vs a reference.
  • Gas pressure rises when molecules collide with walls more often or harder.
  • Pressure decreases with altitude because less air is above.
  • Write unit conversions as fractions and cancel units.
  • Mercury keeps barometer columns short; a water barometer would need ~10 m.

Check yourself

5 review questions from the chapter. Try each one, then open the answer.

  1. Define pressure and give its SI unit. Why is it force per area rather than just force?

    Show answer

    Pressure is force divided by the area it acts on, P = F/A. SI unit: pascal (Pa = N/m²). It is force per area because the same force on a small area creates much larger pressure (e.g., a knife edge).

  2. List four units equal to 1 atm.

    Show answer

    760 mmHg, 760 torr, 101.325 kPa, 1.01325 bar (also ≈ 14.7 psi).

  3. Convert 0.650 atm to mmHg using dimensional analysis.

    Show answer

    0.650 atm × 760 mmHg1 atm = 494 mmHg.

  4. In an open-end manometer, mercury is 25 mmHg higher on the gas side than the open side; atmospheric pressure is 760 mmHg. What is the gas pressure?

    Show answer

    Mercury higher on the gas side means the atmosphere pushes harder: Pgas = 760 - 25 = 735 mmHg.

  5. Why does atmospheric pressure decrease with altitude?

    Show answer

    Atmospheric pressure is the weight of the air column above; at higher altitude there is less air, so the weight (and pressure) is smaller.

Keep learning

Ready to build on this? Continue to the next lesson.

Study tools & related lessonsKey vocabulary · Related

Key vocabulary

Pressure
Force applied perpendicular to a surface, divided by the area of that surface
Pascal (Pa)
SI unit of pressure: one newton per square meter
Atmosphere (atm)
Average sea-level pressure of Earth's atmosphere
mmHg / torr
Pressure equal to the force of a 1-mm mercury column
Barometer
Instrument measuring atmospheric pressure with a liquid column
Manometer
U-shaped tube comparing gas pressure to a reference

Sources & references

  1. openstax.org — Chemistry 2e

This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.

Educational content only. It is not medical, legal or professional advice. Found an error? Tell us.