Models of the Particulate Nature of Matter

S1. Structure 1: Models of the particulate nature of matter

All matter originated from the Big Bang, approximately 13.8 billion years ago.
The Big Bang theory posits that the universe began as a tiny dot that underwent rapid expansion.
About 380,000 years post-expansion, hydrogen, the first element, started to form.
Over billions of years, stars and galaxies formed, leading to the universe as we know it.

Learning outcomes:
- Distinguish the different states of matter.
- Use state symbols in chemical reactions (s, l, g, aq).
Experiments can often lead to new discoveries. Scientists have recently discovered a new state of matter, known as liquid glass. This new state of matter has properties between that of a solid and a colloid (a type of gel) and could have important implications for the future development of liquid crystal displays (LCDs).

To understand the properties of the states of matter, we apply the kinetic molecular theory (KMT).
KMT's core principles:
- All matter consists of small particles.
- Particles possess kinetic energy, causing constant motion.
- Kinetic energy is proportional to temperature: higher temperatures mean greater motion (straight-line motion), while lower temperatures mean lesser motion (vibrational motion).
- Collisions between particles are elastic, conserving kinetic energy.

The properties of solids, liquids, and gases are closely related to the arrangement of the particles in the substance.
Solids:
- Cannot be compressed because particles are close together.
- Strong forces of attraction hold particles together, giving them a fixed shape and volume.
- Cannot flow.
Liquids:
- Can flow because particles can move more freely.
- Weaker forces of attraction compared to solids.
- No fixed shape; take the shape of the container.
- Particles are still close together, so they cannot be compressed.
Gases:
- No fixed shape or volume; they take the shape of the container, and their volume depends on temperature and pressure.
- Weakest forces of attraction between particles.
- Can be compressed because particles are far apart.
Kinetic Energy:
- Solids have the least kinetic energy.
- Gases have the most kinetic energy.

Density is a substance's mass per unit volume.
Higher density implies a 'heavier' feel for the same volume.
Density formula: d = \frac{m}{V}, where:
- d is density
- m is mass
- V is volume
General density trend: Solids > Liquids > Gases.
Examples:
- Air: 0.0018 \text{ g cm}^{-3}
- Water: 1.00 \text{ g cm}^{-3}
- Solid Iron: 7.87 \text{ g cm}^{-3}

Changes of state (phase changes) occur when a substance transitions between physical states.
Example: Melting ice (solid to liquid).
Heat energy transfers from the surroundings to the ice.
The chemical composition of the water remains unchanged (H_2O).
Representing melting ice: H2O(s) \rightarrow H2O(l)
State symbols:
- (s): solid
- (l): liquid
- (g): gas
- (aq): aqueous solution

State symbols are also used in chemical equations.
Example: Reaction between sodium metal and water:
- 2Na(s) + 2H2O(l) \rightarrow 2NaOH(aq) + H2(g)

Vaporization includes:
- Evaporation: liquid to gas at the surface; occurs below boiling point.
- Boiling: liquid to gas throughout the liquid at a specific temperature; bubbles form.

'Aqueous' means a homogeneous mixture formed by dissolving a substance in water.
Including state symbols is good practice in chemical equations. So, make sure that you look out for them in future sections.

Water is one of the few substances that we can observe all changes in state in our everyday life. To what extent do you rely on your experience of macroscopic changes to understand what is happening on a microscopic level?