Molecular Nature of Matter

The kinetic particle theory states that:

• All matter is made up of small particles that cannot be seen directly with the naked eye.
• There are spaces between particles in all matter, but the amount of space differs in each state.
• Particles are in constant random motion, but their speed varies in different states.
• Particles possess kinetic energy that changes with temperature.
• The average kinetic energy of all particles is directly proportional to the absolute temperature.

Matter and Its Properties

Matter is defined as anything that has mass and occupies space. All substances are composed of matter, which exists in three states:

• Solid: Examples include ice, brick, metal, concrete, and wood.
• Liquid: Examples include water, milk, and oil.
• Gas: Examples include oxygen, nitrogen, and carbon dioxide (CO₂).

Structure of Matter

Matter is composed of discrete particles: atoms, molecules, and ions.

1. Atoms

An atom is the smallest unit of an element that can exist independently. It consists of:

• A central nucleus containing protons and neutrons.
• Electrons revolving around the nucleus in orbits or shells.
• Protons have a positive charge, electrons have a negative charge, and neutrons are neutral.

An atom is electrically neutral when the number of protons equals the number of electrons.

2. Molecules

A molecule consists of two or more atoms of the same or different elements chemically bonded together.

States of Matter

1. Solid

In solids, molecules are tightly packed due to strong intermolecular forces. They vibrate about fixed positions, giving the solid a definite shape. When heated, the molecules gain energy and vibrate more until they break free from their fixed positions.

2. Liquid

In liquids, molecules move freely within a fixed volume. Liquids have a definite volume but take the shape of their container. When heated, molecules gain kinetic energy and move faster, eventually escaping into the gaseous state.

3. Gas

Gas molecules are in constant motion and are widely spaced. They move at high speeds, colliding with each other and the walls of the container, exerting pressure.

Diffusion and Osmosis

Diffusion

Diffusion is the process by which different fluids mix due to the random motion of their molecules. The rate of diffusion depends on:

1. Density
2. Mass
3. Temperature
4. Pressure
5. Concentration

Graham’s Law of Diffusion states that at constant temperature, the rate of diffusion of a gas is inversely proportional to the square root of its molar mass:

\[ R \propto \frac{1}{\sqrt{M}} \] \[ R = \frac{K}{\sqrt{M}} \] \[ R_1 \sqrt{M_1} = R_2 \sqrt{M_2} \] \[ \frac{R_1}{R_2} = \frac{\sqrt{M_2}}{\sqrt{M_1}} \]

where:

• \( R \) = rate of diffusion
• \( M \) = relative molecular mass

Note:

• \( R = \frac{\text{volume of gas}}{\text{time}} \)
• Relative molecular mass is twice its vapor density.

Osmosis

Osmosis is the movement of water molecules from a region of higher concentration to a region of lower concentration through a semi-permeable membrane.

Factors affecting osmosis

Surface Area: A larger surface area increases the rate of osmosis.

Temperature: A higher temperature speeds up the rate of osmosis.

Viscosity: Higher viscosity (or density) slows down the rate of osmosis.

Concentration Gradient: A greater difference in concentration leads to a faster rate of osmosis.

Pressure: Increased pressure accelerates the rate of osmosis.

Distance: The farther the molecules must travel, the slower the osmosis rate.

Crystal Structure

Crystalline solids have an orderly arrangement of particles in parallel planes. These structures are formed from repeating units called unit cells.

Types of Crystal Systems

There are seven types of crystal systems:

1. Cubic
2. Rhombic
3. Monoclinic
4. Triclinic
5. Tetragonal
6. Rhombohedral
7. Hexagonal

X-ray analysis reveals that most crystal systems have three perpendicular axes, except for the hexagonal system. The arrangement of these axes determines the type of crystal system.

Cubic and Monoclinic Crystal Systems

• Cubic: All three axes are of equal length and at right angles. Found in copper, sodium chloride, silver, gold, iron, and potassium.
• Monoclinic: Axes are of different lengths, with only two at right angles. Found in sugar, washing soda, and ferrous sulfate.

Differences Between Crystalline and Amorphous Substances

Crystalline Substances	Amorphous Substances
Have a definite internal arrangement of particles.	Have a random distribution of particles.
Have cleavage planes along which they fracture when struck.	Do not have cleavage planes.
Melt sharply at a specific temperature.	Do not melt sharply at a definite temperature.
Considered as true solids.	Regarded as supercooled liquids.
Have a long-range order in particle arrangement.	Have a short-range order in particle arrangement.
Can exhibit efflorescence.	Do not exhibit efflorescence.