Distinguishing Properties of Solids, Liquids, and Gases:
Solids:
Shape: Fixed shape (does not change unless forced).
Volume: Fixed volume (doesn’t change unless compressed or expanded).
Liquids:
Shape: Takes the shape of its container (not fixed).
Volume: Fixed volume (doesn’t change unless under pressure).
Gases:
Shape: No fixed shape (fills the shape of the container).
Volume: No fixed volume (can expand to fill any space).
Changes of State:
Melting: Solid → Liquid
Freezing: Liquid → Solid
Boiling (or evaporation): Liquid → Gas
Condensation: Gas → Liquid
Important:
You don’t need to learn about sublimation (solid to gas) or deposition (gas to solid) for this part.
2.1.2 Particle Structure of Solids, Liquids, and Gases
1. Solids:
Arrangement: Particles are tightly packed in a regular, fixed pattern (like bricks in a wall).
Separation: Particles are very close together with little space between them.
Motion: Particles only vibrate in place; they do not move around.
2. Liquids:
Arrangement: Particles are randomly arranged but still quite close to each other.
Separation: Particles are close together but have small gaps, allowing them to move.
Motion: Particles move and slide past each other slowly, which is why liquids can flow.
3. Gases:
Arrangement: Particles are randomly arranged and spread far apart.
Separation: Particles are very far apart with large spaces between them.
Motion: Particles move quickly in all directions, colliding with each other and the walls of their container.
Relationship Between Particle Motion and Temperature:
Motion and Temperature:
As temperature increases, the particles move faster.
This happens because the particles gain kinetic energy (the energy of movement) when they are heated.
Lower temperature means slower-moving particles with less kinetic energy.
Absolute Zero (-273°C):
The lowest possible temperature is called absolute zero (−273°C or 0 Kelvin).
At absolute zero, particles have no kinetic energy and stop moving completely.
Absolute zero is a theoretical point, meaning it’s not possible to reach, but it’s useful for understanding the limit of particle motion.
Pressure and Changes in Pressure of a Gas:
Gas Pressure:
Gas pressure is caused by collisions of gas particles with the walls of their container.
When gas particles move, they hit the container walls, creating pressure.
Changes in Pressure:
If the temperature increases, gas particles move faster and collide more often and harder with the container walls, which increases pressure.
If the volume of the container decreases (with the same number of particles), the particles collide more often with the walls because they have less space to move, which also increases pressure.
More particles in the same space also means more collisions, leading to higher pressure.
Evidence for the Kinetic Particle Model (Random Motion of Microscopic Particles):
The random motion of very small particles in a fluid (liquid or gas) provides evidence for the kinetic particle model.
This random movement is caused by collisions with invisible particles of the fluid.
For example, in water, tiny particles like pollen or dust move in unpredictable directions because water molecules are constantly colliding with them.
Brownian Motion:
Brownian motion is the random movement of microscopic particles (like pollen) in a liquid or gas.
This happens because the microscopic particles are hit by the moving particles of the fluid around them (like water molecules or air particles).
The motion is random because the particles in the fluid collide from different directions with varying forces, which makes the microscopic particles move unpredictably.
Forces and Distances Between Particles and Their Effect on Properties:
The forces between particles (atoms, molecules, ions, and electrons) and the distances between them play a key role in determining whether a substance is a solid, liquid, or gas.
Solids:
Particles are closely packed with strong forces holding them together.
This results in a rigid structure with a fixed shape and volume.
Liquids:
Particles are close but can move past each other.
Forces are weaker than in solids, so liquids can flow and take the shape of their container but still have a fixed volume.
Gases:
Particles are far apart, with very weak forces between them.
The large distance between particles allows them to move freely and spread out, resulting in gases having no fixed shape or volume.
The motion of particles is also crucial:
Faster-moving particles (higher temperature) can overcome the forces holding them together, changing from a solid to a liquid or gas.
Slower-moving particles (lower temperature) are more tightly bound by the forces, which keeps them in solid or liquid states.
Gas Pressure and Changes in Pressure in Terms of Forces:
Gas Pressure:
Pressure in a gas is due to the force exerted by gas particles when they collide with surfaces (such as the walls of a container).
Each collision applies a small force to the surface. The combined effect of many particles colliding creates the total pressure.
Force per Unit Area (Pressure):
Pressure is defined as force per unit area.
The more frequently and harder particles hit the surface, the greater the pressure.
Changes in Gas Pressure:
When the temperature increases, the gas particles gain more kinetic energy and move faster, resulting in more frequent and stronger collisions with the walls of the container, which increases pressure.
Reducing the volume of the gas also increases pressure, because particles have less space and will collide more often with the container walls.
If the number of particles increases in a fixed volume, pressure increases because more particles are colliding with the surface.
Example 01:
The temperature of a fixed mass of gas at constant volume is decreased. State and explain, in terms of particles, how the pressure of the gas changes. [3]
Ans:
As the temperature decreases, the gas particles move more slowly, reducing their kinetic energy.
This leads to fewer and less forceful collisions with the walls of the container,
causing the pressure to decrease.
Example 02:
Explain, in terms of the momentum of particles, how a gas exerts a pressure. [4]
Ans:
Gas particles constantly collide with the container walls.
During each collision, their momentum changes, exerting a force on the wall.
$$F={{mv-mu}\over{t}}$$
The combined forces from these collisions per unit area result in gas pressure.
$$P={{F}\over{A}}$$
Microscopic Particles Moved by Collisions with Fast-Moving Molecules:
Microscopic Particles (e.g., dust particles or pollen in the air) can be moved by collisions with light, fast-moving molecules of gases or liquids.
These tiny particles experience random motion (as seen in Brownian motion) because they are constantly bombarded by smaller atoms or molecules in the surrounding fluid.
The term “microscopic particles” refers to larger visible particles like dust or pollen, while atoms or molecules are the much smaller particles (atoms are the smallest units of elements, and molecules are made up of atoms).
2.1.3 Gases and the absolute scale of temperature
Effect on the Pressure of a Fixed Mass of Gas:
(a) Change of Temperature at Constant Volume:
When the temperature increases, the gas particles gain more kinetic energy and move faster.
This leads to more frequent and harder collisions with the walls of the container, which increases the pressure.
At constant volume, higher temperature always results in higher pressure because the particles are confined to the same space and collide more often with more force.
(b) Change of Volume at Constant Temperature:
When the volume increases (container expands), the gas particles have more space to move, so they collide with the walls less often, resulting in lower pressure.
When the volume decreases (container contracts), particles collide with the walls more frequently, leading to higher pressure.
If the temperature remains constant, increasing the volume will reduce the pressure, and decreasing the volume will increase the pressure.
P1 = initial pressure exerted by the gas V1 = initial volume occupied by the gas P2 = final pressure exerted by the gas V2 = final volume occupied by the gas
Converting Temperatures Between Kelvin and Celsius:
Kelvin (K) is the absolute temperature scale, starting from absolute zero (−273°C).
The relationship between Kelvin and Celsius is:
T(K)=θ(°C)+273
To convert from Celsius to Kelvin: Add 273 to the Celsius temperature.
Example: 25 °C+273=298 K
To convert from Kelvin to Celsius: Subtract 273 from the Kelvin temperature.
Example: 300 K−273=27 °C
Relationship Between Pressure and Volume of a Gas (Boyle’s Law):
Boyle’s Law states that for a fixed mass of gas at constant temperature, the pressure p and the volume V are inversely proportional. This means when the volume increases, pressure decreases and vice versa, provided temperature remains constant.
The relationship is expressed mathematically as:
pV=constant
The graphs above visually represent Boyle’s Law:
The left graph shows the relationship between pressure (p) and volume (V). As volume increases, pressure decreases, creating a curved line (hyperbola).
The right graph shows the relationship between pressure (p) and the inverse of volume (1/V). Here, the plot gives a straight line, indicating that pressure is inversely proportional to volume.
Example 01:
A piston slowly compresses a gas from 340cm3 to 40cm3 Note that temperature remains the same. The initial pressure was 150KPa, find out the final pressure
