A gas is a state of matter characterized by particles that are far apart and move randomly, unlike the fixed arrangement found in solids or the close contact in liquids. A gas completely fills any container it occupies, meaning it has neither a fixed shape nor a fixed volume. Because a gas’s state depends heavily on its surroundings, its behavior and quantity must be quantified by measuring specific physical properties. This measurement involves using specialized units to define the space, force, energy, and number of particles in the gaseous sample.
Measuring Volume: The Space Occupied
The volume of a gas refers to the total three-dimensional space it occupies, which is identical to the volume of the vessel containing it. For small-scale measurements, such as in a laboratory setting or for a scuba tank, the unit of the liter (L) is commonly used. Liters are an accepted metric unit convenient for dealing with manageable quantities of gas.
For large-scale industrial or engineering applications, the standard unit shifts to the cubic meter (\(m^3\)), which is the derived International System of Units (SI) unit for volume. This unit is frequently encountered when measuring natural gas consumption in homes or calculating the capacity of large storage facilities. One cubic meter is exactly equivalent to 1,000 liters, allowing for easy conversion. Smaller units like the milliliter (mL) or cubic centimeter (\(cm^3\)) are also used in precise scientific work.
Measuring Pressure: The Force Exerted
Gas pressure is the force exerted by gas molecules colliding with the interior walls of their container, calculated as force distributed over a specific area. The standard SI unit for pressure is the Pascal (Pa), equivalent to one Newton of force applied over one square meter. Because the Pascal is a very small unit, it is often expressed as kilopascals (kPa) in many scientific and engineering contexts.
The atmosphere (atm) is a widely used unit representing the average air pressure at sea level, defined precisely as 101,325 Pascals. This unit provides a reliable baseline for comparing gas behavior in chemistry and physics experiments. Another common unit is the bar, which is very close to one atmosphere, defined as exactly 100,000 Pascals.
Other Pressure Units
The bar is often used in meteorological reports and for measuring the pressure in scuba tanks. Units like the torr, which is practically identical to the millimeter of mercury (mmHg), persist in specific fields, such as medical settings to measure blood pressure. Both the torr and mmHg are based on historical measurements using mercury barometers, with 760 torr or mmHg equaling one standard atmosphere. In the United States, the imperial unit of pounds per square inch (psi) is prevalent for practical applications like measuring car tire pressure.
Measuring Temperature and Amount: Defining the Energy and Quantity
Temperature in a gas is a measure directly related to the average kinetic energy of its moving molecules. The unit most commonly used for everyday and weather measurements is the Celsius scale (°C), which sets the freezing and boiling points of water as its main references. However, for all scientific calculations involving gases, the Kelvin (K) scale is the preferred absolute temperature unit.
The Kelvin scale is absolute because its zero point, known as absolute zero (0 K), corresponds to the theoretical temperature at which gas particles would have their minimal possible kinetic energy. Zero Kelvin is equivalent to approximately -273.15 degrees Celsius, and temperatures on the Kelvin scale will never be negative. Using Kelvin is necessary for gas law equations because it ensures that the calculated relationship between temperature and other properties accurately reflects the gas’s true energy state.
Measuring Quantity: The Mole
Defining the total quantity of gas uses a unit that counts the number of particles present, which is the mole (mol). The mole is the SI unit for the amount of substance and serves as a counting number for the extremely small particles that make up a gas, such as atoms or molecules. One mole of any substance contains exactly \(6.022 \times 10^{23}\) particles, known as Avogadro’s number. This unit allows scientists to relate the microscopic world of molecules to the macroscopic properties, such as volume, in a standardized way.