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General Chemistry I · Core Concept

Properties of Gases

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On this page 7 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Worked example
  6. Key takeaway
  7. Study tools

In 30 seconds

Gases have no fixed shape or volume: they expand to fill their container, are highly compressible, have low density, and mix completely with one another. The defining measurable property of a gas is — the force exerted per unit area — reported in atmospheres (atm), millimeters of mercury (), , or pascals (Pa). Atmospheric pressure is read with a , while a compares the pressure of a trapped gas against the atmosphere.

Why this matters

The mmHg unit is everywhere in medicine: blood pressure is reported as systolic over diastolic pressure in mmHg (e.g., 120/80 mmHg), and a sphygmomanometer is essentially a manometer plus an inflatable cuff. Partial-pressure readings in respiratory care, arterial blood-gas analysis, and oxygen-delivery equipment all trace back to gas-pressure measurements. Clinically, altitude matters too: at high elevations atmospheric pressure drops, so less oxygen is available per breath — a direct consequence of the barometer principle. In the laboratory, every vacuum pump, gas cylinder, and reaction vessel operated under a specific pressure depends on accurate pressure measurement and unit conversion.

The college version

1. What Makes Something a Gas

Of the three common states of matter, gases are the most "spread out." Their particles are far apart and move freely, which gives gases their signature behaviors. A gas expands to fill its container (no fixed shape or volume), is compressible (you can squeeze a large volume into a small tank), has low density (about a thousandth of the density of the corresponding liquid), and is miscible — two gases mix completely and uniformly in any proportion. These behaviors arise from the large empty space between particles and their weak mutual attractions.

2. Pressure

Pressure is the amount of force applied over a given area:

P = FA

The SI unit of force is the newton (N) and of area the square meter, so the SI unit of pressure is the newton per square meter, called the : 1 Pa = 1 N/m2. Because a pascal is small, gas pressures are often given in kilopascals (kPa) or in the older but still common units tied to mercury: atmospheres (atm), millimeters of mercury (mmHg), and torr. The key equivalences are:

1 atm = 760 mmHg = 760 torr = 101,325 Pa = 101.325 kPa

Note that mmHg and torr are numerically identical for all practical purposes (they differ by less than one part in a million).

3. Measuring Pressure: Barometers and Manometers

A barometer measures atmospheric pressure. In Torricelli's classic design, a tube closed at one end is filled with mercury and inverted into a dish of mercury. The mercury drops until the weight of the remaining column exactly balances the atmospheric push on the dish, leaving a vacuum above the column. At sea level the column stands about 760 mm high; on a mountain top, where the atmosphere presses less, it stands lower. Weather reports, altimeters, and many lab instruments rely on this principle.

A manometer measures the pressure of a gas sealed in a container. A U-shaped tube partially filled with mercury has one arm open to the atmosphere and the other connected to the gas sample. The difference in mercury height h between the two arms tells you how the gas pressure compares to atmospheric pressure. In a closed-end manometer the non-sample arm is sealed under vacuum, so the height difference gives the gas pressure directly. In an open-end manometer you add or subtract h (in mmHg) from atmospheric pressure, depending on which side the mercury is higher.

How it works

  1. Identify which pressure is being asked for (atmospheric, or a gas in a container).
  2. Choose the appropriate instrument: a barometer for the open atmosphere, a manometer for a sealed gas.
  3. Read the mercury column height difference and record it in mmHg (or torr).
  4. For a closed-end manometer, that height is the gas pressure.
  5. For an open-end manometer, compare the two arms: if mercury is higher on the gas side, add h to atmospheric pressure; if higher on the atmosphere side, subtract h.
  6. Convert to the requested unit using 1 atm = 760 mmHg = 101.325 kPa.

Common confusions

Do not confuseWithDifference
mmHgtorrThey are equal; only the name differs (1 mmHg = 1 torr)
BarometerManometerBarometer reads the open atmosphere; manometer reads a sealed gas relative to the atmosphere
PakPa1 kPa = 1000 Pa; never write "760 kPa" for standard pressure
Gauge pressureAbsolute pressureGauge pressure is the difference from atmospheric; gas laws use absolute pressure
°CKGas laws use Kelvin; °C has an arbitrary zero point

Memory aids

"Seven-sixty to one": 760 mmHg (or 760 torr) = 1 atm. Remember the two "seven-sixty" units are the same, and 101.325 kPa completes the set — 760 pairs, one atmosphere, 101.325 pascals-with-a-k.

Quick review

Topic Recap

Gases are characterized by expansion, , low density, and . Their key measurable property is pressure, P = F/A, with the essential conversion 1 atm = 760 mmHg = 760 torr = 101.325 kPa. Barometers measure atmospheric pressure; manometers measure a confined gas against the atmosphere. All subsequent gas-law work requires temperatures and consistent pressure units.

Knowledge Check

  1. Which of the following is NOT a characteristic property of gases? (a) low density (b) compressibility (c) fixed volume (d) complete mixing with other gases.
  2. Convert 0.850 atm to mmHg.
  3. A closed-end manometer attached to a gas flask shows a mercury height difference of 312 mm. What is the gas pressure in torr?
  4. An open-end manometer shows mercury 24 mmHg higher on the gas side; atmospheric pressure is 758 mmHg. What is the gas pressure?
  5. Convert 25.0 °C to kelvin, and 298.15 K to Celsius.

Answers and Rationales

  1. (c) fixed volume. Gases have no fixed shape or volume; they expand to fill any container. Low density, compressibility, and complete mixing are all genuine gas properties.
  2. 646 mmHg. 0.850 atm × 760 mmHg/atm = 646 mmHg.
  3. 312 torr. In a closed-end manometer the vacuum side exerts no pressure, so the mercury height directly equals the gas pressure; torr and mmHg are identical.
  4. 782 mmHg. Mercury higher on the gas side means the gas pushes harder than the atmosphere, so Pgas = 758 + 24 = 782 mmHg.
  5. 298.15 K and 25.0 °C. 25.0 + 273.15 = 298.15 K, and 298.15 - 273.15 = 25.0 °C.
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

Picture a room full of tiny bouncy balls flying in every direction and constantly smashing into the walls. Each time one hits a wall it gives a tiny push; add up all those pushes spread over the whole wall and you get pressure. A gas is just an enormous crowd of fast-moving particles doing exactly this all the time. Squeeze the same gas into a smaller box and the particles hit the walls more often, so the pressure rises; cool it down and they slow down, so the pressure falls. We measure this "push" with two tools: a barometer reads how hard the whole atmosphere presses on a column of liquid, and a manometer compares the push of a trapped gas to the push of the outside air. This bouncy-ball picture stops being exact because real gas particles do have some size and do attract each other weakly — but under ordinary laboratory conditions it is a very good model.

Simple Example

A barometer at sea level holds a column of mercury 760 mm high. The weight of that column exactly balances the push of the atmosphere, so we say atmospheric pressure is "760 mmHg" — the same thing as 1 atmosphere (1 atm), or 760 torr.

Worked example

The conversions you must be fluent with are:

1 atm = 760 mmHg = 760 torr = 101.325 kPa

T(K) = T(°C) + 273.15

Worked Example 1 — Unit conversion. Convert 745 mmHg to atm, torr, and kPa.

745 mmHg × 1 atm760 mmHg = 0.980 atm

745 mmHg = 745 torr  (mmHg and torr are equivalent)

745 mmHg × 101.325 kPa760 mmHg = 99.3 kPa

Worked Example 2 — Open-end manometer. A gas sample is connected to an open-end manometer. The mercury level is 18 mmHg higher on the arm open to the atmosphere, and atmospheric pressure is 762 mmHg. Find the gas pressure.

Because the atmosphere's side is higher, the atmosphere is pressing down harder, so the gas pressure is less than atmospheric by 18 mmHg:

Pgas = Patm - h = 762 mmHg - 18 mmHg = 744 mmHg

Common setup errors. (1) Forgetting to convert Celsius to Kelvin before using any gas law — only Kelvin works. (2) Mixing units: 760 is only correct when the pressure unit is mmHg or torr; a value of "760" with kPa is wrong. (3) Adding h when you should subtract it in a manometer — always ask which side's mercury is higher and whether that means the gas is above or below atmospheric pressure. (4) Using "torr" and "mmHg" as if they were different units; they are interchangeable.

Key takeaways

  • High yield: 1 atm = 760 mmHg = 760 torr = 101,325 Pa = 101.325 kPa.
  • High yield: mmHg and torr are interchangeable; never treat them as different units.
  • High yield: Gas-law problems require Kelvin: T(K) = T(°C) + 273.15.
  • Gases expand to fill their container, are compressible, and have low density.
  • A barometer measures atmospheric pressure; a manometer measures a confined gas against the atmosphere.
  • In an open-end manometer, a higher gas-side mercury level means the gas pressure is higher than atmospheric.
  • At sea level, a mercury barometer stands at 760 mm.
  • The pascal is the SI unit of pressure; kPa (not Pa) is usually more convenient.
  • Standard atmospheric pressure is exactly 1 atm by definition.

Keep learning

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Practice General Chemistry I

This lesson has no separate scored set. Practice draws from the subject’s question bank.

Study tools & related lessonsYou’ll learn to · Key vocabulary · Related

You’ll learn to

  • Describe the characteristic physical properties that distinguish gases from liquids and solids.
  • Define pressure as force per unit area and express it in each common unit (atm, mmHg, torr, Pa, kPa).
  • Explain how a barometer measures atmospheric pressure and how a manometer measures the pressure of a confined gas.
  • Convert between Celsius and Kelvin and between pressure units using dimensional analysis.

Key vocabulary

Pressure
Force applied per unit area
Pascal (Pa)
SI unit of pressure; 1 N/m²
Atmosphere (atm)
Unit equal to normal sea-level air pressure
mmHg
Pressure that supports a 1 mm column of mercury
Torr
Unit numerically equal to mmHg
Barometer
Device that measures atmospheric pressure
Manometer
Device that measures a gas pressure vs. the atmosphere
Kelvin
Absolute temperature scale (K = °C + 273.15)
Compressibility
How much a substance's volume shrinks under pressure
Miscibility
Ability of substances to mix in any proportion

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