In this lesson, we will explore the three physical states of matter: gas, liquid, and solid.
Gases are the simplest form of matter. They have no definite shape or volume, which means they fill all available space. The forces between gas molecules are very weak, and both pressure and temperature significantly affect their volume.
Liquids have stronger intermolecular forces, giving them a definite volume but no fixed shape; they take the shape of their container. Liquids can evaporate, and their vapors exert pressure. When the vapor pressure of a liquid equals the external pressure, the liquid boils. Compared to gases, liquids are less mobile and diffuse more slowly.
Solids have both a definite shape and volume. They are rigid and denser than both liquids and gases. Solids can be either amorphous (without a defined structure) or crystalline (with a structured arrangement).
Throughout this lesson, we will also examine the effects of changes in pressure and temperature on gas volume using Boyle’s Law and Charles’ Law, and discuss various properties of gases, liquids, and solids.
GASEOUS STATE
Typical Properties of Gases
Gases share several common physical properties that make them unique.
Diffusion refers to the rapid mixing of gas molecules due to their random motion and collisions, leading to a homogeneous mixture. The rate of diffusion is influenced by the molecular mass of the gases; lighter gases diffuse more quickly than heavier ones. For instance, hydrogen (H₂) diffuses four times faster than oxygen (O₂).
Effusion is the process by which gas molecules escape through a tiny hole into a lower-pressure space. An everyday example is when air escapes from a punctured tire. Similar to diffusion, effusion rates are faster for lighter gases compared to heavier gases.
Pressure is generated when gas molecules, continuously in motion, collide with the walls of their container. Pressure (P) is defined as the force (F) exerted per unit surface area (A), represented by the formula ( P = \frac{F}{A} ). The SI unit of pressure is the Pascal (Pa), where 1 Pascal equals 1 Newton per square meter (N/m²). Standard atmospheric pressure is defined as the pressure exerted by a 760 mm column of mercury at sea level, equivalent to 1 atm or 101,325 Pa.
Compressibility is a key feature of gases, allowing them to be easily compressed due to the significant empty space between their molecules. When compressed, gas molecules move closer together and occupy less volume compared to their uncompressed state.
Mobility is another important property, as gas molecules are in constant motion. Their high kinetic energy enables them to move freely and mix with one another, resulting in a uniform mixture.
Finally, gases have a low density compared to liquids and solids due to their lighter mass and larger volume. Gas density is typically measured in grams per cubic decimeter (g/dm³), while liquids and solids are measured in grams per cubic centimeter (g/cm³), making them about 1000 times denser than gases. The density of gases increases when cooled, as their volume decreases. For example, at normal atmospheric pressure, the density of oxygen gas is 1.4 g/dm³ at 20°C and increases to 1.5 g/dm³ at 0°C.
LAWS RELATED TO GASES
Boyle’s Law
In 1662, Robert Boyle explored the relationship between the volume and pressure of a gas at constant temperature. He discovered that the volume of a given mass of gas is inversely proportional to its pressure when the temperature remains unchanged. This means that as the pressure (P) of a gas increases, its volume (V) decreases, and vice versa. Mathematically, this relationship can be expressed as:
[ P \times V = k ]
where ( k ) is a constant that remains the same for a specific amount of gas. This law indicates that the product of pressure and volume for a fixed mass of gas is constant at a constant temperature.
Experimental Verification of Boyle’s Law
Boyle’s law can be demonstrated through experiments. Consider a gas contained in a cylinder with a movable piston. When different pressures are applied to the gas, we can observe the changes in volume. For instance, when a pressure of 2 atmospheres (atm) is applied, the volume of the gas is 1 dm³. If the pressure is increased to 4 atm, the volume decreases to 0.5 dm³. As the pressure continues to rise to 6 atm, the volume drops further to 0.33 dm³, and at 8 atm, the volume reduces to 0.25 dm³.
By calculating the product of pressure and volume in these experiments, we can see that it remains constant, confirming Boyle’s law. This relationship illustrates how the volume of a gas decreases as pressure increases, as long as the temperature is kept constant.