Molecular Theory of Matter
Molecular Behavior in Solids, Liquids, and Gases
- Matter exists in three primary states: solids, liquids, and gases.
- The behavior of molecules in each state is distinct and is determined by the intermolecular forces and the energy of the particles.
Solids
- Arrangement: Molecules are tightly packed in a fixed, orderly structure.
- Movement: Particles vibrate around fixed positions but do not move freely.
- Forces: Strong intermolecular forces keep particles in place.
- Properties: Solids have a definite shape and volume.
Liquids
- Arrangement: Molecules are close together but not in a fixed pattern.
- Movement: Particles can slide past each other, allowing liquids to flow.
- Forces: Moderate intermolecular forces allow movement but keep particles relatively close.
- Properties: Liquids have a definite volume but take the shape of their container.
- Don't confuse "definite volume" with "definite shape."
- Liquids have a definite volume but take the shape of their container, while solids have both definite volume and shape.
Gases
- Arrangement: Molecules are far apart and randomly distributed.
- Movement: Particles move freely and rapidly in all directions.
- Forces: Negligible intermolecular forces except during collisions.
- Properties: Gases have no definite shape or volume and are highly compressible.
- Intermolecular forces determine the state of matter.
- Stronger forces lead to solids, while weaker forces result in liquids or gases.
Remember, the distance between particles increases from solids to liquids to gases, which affects their density and compressibility.
Density ($\rho$)
Definition and Calculation
Density is a measure of how much mass is contained in a given volume. It is defined by the formula:
$$\rho = \frac{m}{V}$$
where:
- $\rho$ = density ($\text{kg m}^{-3}$)
- $m$ = mass ($\text{kg}$)
- $V$ = volume ($\text{m}^3$)
- A block of iron has a mass of $15 \text{ kg}$ and a volume of $0.002 \text{ m}^3$.
- Its density is:
$$\rho = \frac{15 \text{ kg}}{0.002 \text{ m}^3} = 7500 \text{ kg m}^{-3}$$
Density in Different States of Matter
- Solids: High density due to tightly packed particles.
- Liquids: Slightly lower density than solids but still compact.
- Gases: Very low density because particles are far apart.
- Density is an intrinsic property, meaning it does not depend on the amount of substance.
- A small piece of iron has the same density as a large block.
Temperature Scales
Celsius and Kelvin Scales
The Celsius and Kelvin scales are two commonly used temperature scales.
- Celsius Scale:
- Based on the freezing point (0°C) and boiling point (100°C) of water.
- Kelvin Scale:
- An absolute temperature scale starting at absolute zero (0 K), the lowest possible temperature where molecular motion stops.
