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Gas properties and pressure units Knowledge-Based Study

Chemistry · States of Matter & Solutions · Chemistry · Gases

Chemistry topic explanations on Gas properties and pressure units.

Study notes

  1. General Characteristics of the Gaseous State

    Gases represent the most energetic and dispersed common state of matter. Unlike solids and liquids, gases possess distinctive physical characteristics governed by the large distances between their constituent particles:

    • Indefinite Shape and Volume: Gas particles are in constant, random motion and possess sufficient kinetic energy to overcome intermolecular attractions. Consequently, a gas expands spontaneously to completely and uniformly fill the shape and volume of any container.
    • High Compressibility: Because the actual volume of gas particles is negligible compared to the vast empty space between them, applying external pressure readily decreases the total volume of the gas sample.
    • Low Density: With relatively few particles per unit volume under standard conditions, gases have densities that are typically hundreds to thousands of times lower than those of liquids or solids.
    • Spontaneous Mixing and Diffusion: Due to continuous translational movement, different gases mix completely and homogeneously in all proportions without the need for mechanical stirring.

    How Gases Exert Pressure

    Pressure is defined physically as the force applied perpendicular to the surface of an object per unit area:

    Pressure = Force / Area

    In a microscopic context, gas pressure arises from the countless collisions that gas particles make with the interior walls of their container. Each collision imparts an impulse, transferring momentum to the container surface. The macroscopic manifestation of these billions of continuous impacts per second across a given surface area is observed as pressure.

    Atmospheric Pressure and the Barometer

    The Earth's atmosphere is held by gravity, creating a blanket of air that exerts downward force on all surface objects. This is known as atmospheric pressure.

    Evangelista Torricelli first measured atmospheric pressure quantitatively using a mercury barometer. At sea level and 0 °C, the weight of the atmosphere supports a column of liquid mercury exactly 760 millimeters high in an inverted, evacuated glass tube.

    Pressure Units in Chemistry

    Gas pressure can be quantified using various units depending on the historical context and experimental application:

    • Atmosphere (atm): A traditional unit historically defined by the average atmospheric pressure at sea level at 0 °C.
    • Millimeters of Mercury (mmHg) and Torr: Derived directly from the height of a mercury column. One millimeter of mercury equals one Torr, named in honor of Torricelli (1 mmHg = 1 Torr).
    • Pascal (Pa): The standard SI unit of pressure, equivalent to one newton per square meter (1 Pa = 1 N/m2). Because the pascal is a very small quantity, the kilopascal (kPa) is more frequently encountered in laboratory practice.
    • Bar: A metric unit defined as exactly 100,000 Pa (100 kPa), widely used in meteorology and thermodynamics.

    Standard Conversion Factors

    Calculations involving gas laws require fluency in converting between these units. The fundamental equivalences to memorize are:

    1 atm = 760 mmHg = 760 Torr = 101,325 Pa = 101.325 kPa = 1.01325 bar

    For example, to convert a measured pressure of 380 Torr into atmospheres:

    380 Torr × (1 atm / 760 Torr) = 0.50 atm