Properties of Gases


This is part of Year 11 HSC Chemistry course under the topic of Gas Laws.

HSC Chemistry Syllabus

  • Conduct investigations and solve problems to determine the relationship between the Ideal Gas Law and:
    • Gay-Lussac's Law (Temperature)
    • Boyle's Law
    • Charles' Law
    • Avogadro's Law

    Properties of Gases

    Gases possess unique characteristics that differentiate them from solids and liquids. These properties are shaped by the free-moving nature of their particles. Below are the main properties of gases:


    • Low Density:

    Gases contain scattered molecules dispersed across a given volume, making them much less dense than solids and liquids. Their low density leads to fluidity, allowing gases to move rapidly within a space. They can expand and contract without fixed positioning. Since the molecules are widely spaced, there's often minimal interaction affecting their motion. The densities of gases, usually calculated in grams per litre (g/L), are about 1000 times less dense than liquids and solids. As the formula for density is `d=m/V` , density increases as a gas cools since the volume decreases.


    • Freely Flowing/Forming:

    The random movement of gases means they are not confined to any particular shape. However, this free-flowing nature can cause gases to leak from improperly sealed systems.


    • Compressibility and Expandability:

    Gases' low densities and large spacing between molecules enable them to be compressed or expanded. They can occupy any available space in a closed system.


    • Volume Change with Temperature:

    When heated, a gas sample expands, and when cooled, it contracts. The volume change is 50 to 100 times greater for gases than for liquids or solids. This occurs as the gas molecules vibrate, gaining energy to move around.


    • Diffusivity:

    The large gaps between gas molecules allow different gases to mix easily, forming a homogeneous mixture. An example of such a mixture is air, mainly consisting of nitrogen and oxygen gases.



    Kinetic Molecular Theory of Gases

    The kinetic molecular theory of gases offers insight into how the macroscopic properties of gases are shaped by microscopic attributes. This theory posits that:

    • Gases consist of particles (molecules or atoms) moving in random straight-line motions.
    • Collisions between gas particles are elastic, conserving kinetic energy.
    • Gas particles are small, and their actual volume is negligible relative to the container's total volume.
    • There are negligible interactive forces, such as attraction or repulsion, between gas particles.
    • The average kinetic energy of gas particles is proportional to the absolute temperature.


    The kinetic energy of a gas particle is calculated using the equation:

    $$KE = \frac{1}{2} mv^2$$ 


    • KE is the kinetic energy in joules (J)
    • m is the mass in grams (g)
    • v is the velocity in meters per second 


    According to the kinetic theory, gas particles' constant and high-velocity collisions create pressure. Pressure is defined as:

     $$\text{Pressure} = \frac{\text{Force}}{\text{Area}}$$


    Pressure units vary, and common conversions include:

    • 1 Pa = 1 `Nm^-2`
    • 1 atm = 101.325 kPa
    • 1 torr = 1 mmHg = 1/760 atm = 133.322 Pa
    • 1 bar = `1×10^2` kPa