9th Class Chemistry Chapter No. 5 Physical States of Matter Notes
According to 9th level Chemistry unit 5, matter exists in three primary physical states: solid, liquid, and gas. Each state is characterized by the arrangement and movement of its constituent particles. In a solid, the particles are tightly packed in a fixed, orderly pattern, and they vibrate in their positions. Solids have a definite shape and volume due to their strong intermolecular forces. Liquids, on the other hand, have particles that are closely packed but less rigidly arranged.
They have a definite volume but take the shape of their container as the particles are free to move and flow past one another. Liquids possess moderate intermolecular forces. In contrast, gases have particles that are widely spaced and have high kinetic energy, leading to rapid and random motion. They can fill the entire volume of their container and have no definite shape. Gases have weak intermolecular forces, allowing them to expand and contract with changes in temperature and pressure. These physical states play a crucial role in determining the properties and behaviors of different substances, and transitions between states are often accompanied by changes in energy and physical characteristics.
Effect on the Volume of a Gas by a Change in
a. Pressure
According to Boyle’s Law, at constant temperature, the volume of a given amount of gas is inversely proportional to its pressure. In other words, if the pressure of a gas increases, its volume will decrease, and vice versa, as long as the temperature remains constant.
b. Temperature
According to Charles’s Law, at constant pressure, the volume of a given amount of gas is directly proportional to its absolute temperature (measured in Kelvin). In other words, as the temperature of a gas increases, its volume will also increase, and as the temperature decreases, its volume will decrease, as long as the pressure remains constant
Comparing Physical States of Matter with Regard to Intermolecular Forces
Gases: In the gaseous state, molecules are widely separated, and the intermolecular forces are weak. Gases have the highest kinetic energy among the three states of matter and take the shape and volume of their container.
Liquids: In the liquid state, molecules are closer together compared to gases, and the intermolecular forces are stronger. Liquids have moderate kinetic energy and take the shape of their container but have a fixed volume.
Solids: In the solid state, molecules are tightly packed, and the intermolecular forces are very strong. Solids have the lowest kinetic energy among the three states of matter and have both a fixed shape and volume.
Pressure-Volume Changes in a Gas using Boyle’s Law
Boyle’s Law states that the pressure and volume of a gas are inversely proportional to each other at constant temperature. When the pressure on a gas increases (compression), its volume decreases. Conversely, if the pressure on a gas decreases (expansion), its volume increases. This law is applicable as long as the temperature and the amount of gas remain constant.
Temperature-Volume Changes in a Gas using Charles’s Law
Charles’s Law states that the volume of a gas is directly proportional to its absolute temperature at constant pressure. When the temperature of a gas increases, its volume increases, and when the temperature decreases, its volume decreases, as long as the pressure and the amount of gas remain constant.
Properties of Gases
Diffusion: Gases spontaneously mix and spread evenly throughout a container. The rate of diffusion is inversely proportional to the square root of the molar mass of the gas. Lighter gases diffuse faster than heavier ones.
Effusion: Effusion is the escape of gas molecules through a tiny hole into a vacuum. The rate of effusion is inversely proportional to the square root of the molar mass of the gas.
Pressure: Gas pressure is the force exerted by gas molecules per unit area of the container walls. It results from the continuous collisions of gas molecules with the container walls.
Properties of Liquids
Evaporation: Evaporation is the process by which liquid molecules at the surface gain enough energy to escape and become vapor (gas). Evaporation occurs at temperatures below the boiling point.
Vapor Pressure: Vapor pressure is the pressure exerted by the vapor molecules above a liquid in a closed container at equilibrium. It increases with temperature and depends on the intermolecular forces in the liquid.
Boiling Point: The boiling point is the temperature at which the vapor pressure of a liquid equals the atmospheric pressure, and the liquid changes into vapor throughout its bulk.
Effect of Temperature and External Pressure on Vapor Pressure and Boiling Point
As temperature increases, the vapor pressure of a liquid also increases. At higher temperatures, more molecules have enough energy to escape from the liquid phase to the vapor phase.
As external pressure increases, the boiling point of a liquid also increases. This is because a higher external pressure makes it more difficult for vapor pressure to overcome the external pressure and convert the liquid into vapor.
Physical Properties of Solids
Melting Point: The melting point is the temperature at which a solid changes into a liquid. It is a characteristic property of a substance and remains constant under a specific pressure.
Boiling Point: The boiling point of a solid is the temperature at which it changes into a gas. However, not all solids have a boiling point. Some solids undergo sublimation, where they directly change into a gas without passing through the liquid phase.
Differentiating between Amorphous and Crystalline Solids
Amorphous Solids: Amorphous solids lack a well-defined and regular atomic or molecular arrangement. They have a disordered structure, and their atoms or molecules are randomly arranged. Examples include glass, rubber, and some plastics.
Crystalline Solids: Crystalline solids have a regular and repeating three-dimensional arrangement of their atoms or molecules, forming a crystal lattice. They have a definite geometric shape and well-defined melting points. Examples include salt, diamonds, and metals.
Allotropic Forms of Solids
Allotropic forms refer to different structural arrangements of the same element. Some elements can exist in multiple forms with different properties. For example: Carbon exists as diamond (hard, transparent) and graphite (soft, black) forms.
Oxygen exists as diatomic O2 and ozone O3. Phosphorus exists in various allotropes, such as white phosphorus and red phosphorus.
These different allotropes arise due to variations in the bonding and arrangement of atoms or molecules within the solid structure.
Allotropy
Allotropy refers to the phenomenon where an element can exist in more than one form in the same physical state. Allotropes are different structural arrangements of atoms or molecules of the same element, leading to distinct physical properties while retaining the same chemical properties. Two main reasons account for allotropy:
- Different Number of Atoms or Molecules: Some elements can form different kinds of molecules with varying numbers of atoms. For example, oxygen exhibits allotropy with two common allotropes: oxygen (O2) and ozone (O3).
- Different Arrangement of Atoms or Molecules: In some cases, the atoms or molecules of an element can have different arrangements in the crystal lattice of the solid. Sulphur is an example of this, showing allotropy with two main forms: rhombic sulfur and monoclinic sulfur.
Allotropes of the same element have unique physical properties, such as density, color, and hardness, but their chemical properties remain the same. The transition from one allotrope to another can occur at specific temperatures known as transition temperatures.
For instance, sulfur has a transition temperature of 96 °C. Below this temperature, the rhombic form is stable, while above this temperature, the molecules rearrange to give the monoclinic form. Tin and phosphorus are other examples of elements that exhibit allotropy, with different forms having varying properties.
Allotropy has practical significance in various fields, including industry and chemistry. For instance, white phosphorus is a waxy and reactive solid, while red phosphorus is less reactive and non-poisonous. This difference in properties makes red phosphorus suitable for specific applications.
As science progresses, the instrumentation used in scientific observation has also evolved. Instruments act as mediators between the world and human senses, enhancing the power of observation and making induction processes easier. They play a crucial role in validating or changing established theories, as they allow for more accurate measurements and analysis of various phenomena. In fields like chemistry and physics, the advancement of instrumentation has revolutionized research and exploration, opening up new possibilities for understanding the natural world.