S1.1.2 The Kinetic Molecular Theory

States of Matter and Changes of State

1. You are holding a piece of ice in your hand.
2. As it begins to melt, the solid ice transforms into liquid water.
3. If you leave it out long enough, the water might evaporate, becoming an invisible gas in the air.

These changes illustrate the states of matter and their transformations—a cornerstone of chemistry.

The Three States of Matter: Solids, Liquids, and Gases

1. Matter exists in three primary states: solids, liquids, and gases.
2. The state of a substance depends on the arrangement and energy of its particles.

Solids: Fixed Shape and Volume

Key properties of solids:

• Shape: Fixed.
• Volume: Fixed.
• Particle Movement: Particles vibrate in place.

Liquids: Fixed Volume, Variable Shape

Key properties of liquids:

• Shape: Takes the shape of the container.
• Volume: Fixed.
• Particle Movement: Particles can move and slide past one another.

Gases: No Fixed Shape or Volume

Key properties of gases:

• Shape: Takes the shape of the container.
• Volume: Expands to fill the container.
• Particle Movement: Particles move rapidly and independently.

State Symbols in Chemical Equations

In chemical equations, the physical state of a substance is indicated by a state symbol in parentheses:

• (s): Solid
• (l): Liquid
• (g): Gas
• (aq): Aqueous (dissolved in water)

Changes of State: Transformations Between Solids, Liquids, and Gases

1. Changes of state occur when a substance absorbs or releases thermal energy.
2. These transformations are physical changes, meaning the chemical composition of the substance remains unchanged.
3. The energy involved affects the potential energy of the particles, not their temperature during the phase change.

1. Melting: Solid to Liquid

• When a solid absorbs thermal energy, its particles gain potential energy and vibrate more vigorously.
• Eventually, they gain enough energy to break free from their fixed positions, transitioning into a liquid. This process is called melting.

Ice melts into water at 0°C.

2. Freezing: Liquid to Solid

• The reverse of melting, freezing occurs when a liquid loses thermal energy.
• As particles release potential energy, they slow down and arrange themselves into a fixed, structured lattice.

Water freezes into ice at 0°C.

3. Vaporization: Liquid to Gas

• When a liquid absorbs enough thermal energy, its particles gain sufficient potential energy to overcome intermolecular forces and enter the gas phase.
• Vaporization can occur in two ways:
  1. Evaporation: Happens only at the surface of the liquid, at any temperature, when particles near the surface absorb enough energy to escape.
  2. Boiling: Happens throughout the entire liquid at a specific temperature where particles throughout the liquid gain enough energy to break free. (e.g., water boils at 100°C under standard pressure).

4. Condensation: Gas to Liquid

• Condensation occurs when a gas loses thermal energy, causing its particles to release potential energy and slow down.
• As a result, intermolecular forces draw the particles closer together, forming a liquid.

5. Sublimation: Solid to Gas

• Sublimation occurs when a solid absorbs thermal energy so rapidly that its particles gain enough potential energy to break free directly into the gas phase without becoming a liquid.
• A common example is dry ice (solid CO${}_2$), which sublimates at room temperature.

6. Deposition: Gas to Solid

• The reverse of sublimation, deposition occurs when a gas loses thermal energy, causing its particles to release potential energy so quickly that they transition directly into a solid without becoming a liquid.

Energy Changes During State Changes

• State changes involve the transfer of thermal energy, either as absorption (endothermic) or release (exothermic).
• The energy transferred affects the potential energy of particles, changing their arrangement but not their temperature during the phase change.

Endothermic Processes

1. In endothermic processes, thermal energy is absorbed from the surroundings, increasing the potential energy of the particles.
2. This energy weakens or breaks intermolecular forces, allowing particles to move more freely.

• Melting: Thermal energy is absorbed to break the bonds holding particles in a solid structure.
• Vaporization: Thermal energy is absorbed to overcome intermolecular forces, separating liquid particles into a gas.
• Sublimation: Thermal energy is absorbed to allow a solid to transition directly into a gas without passing through the liquid state.

Exothermic Processes

1. In exothermic processes, thermal energy is released to the surroundings as particles lose potential energy.
2. Intermolecular forces strengthen, drawing particles closer together.

• Freezing: Thermal energy is released as particles lose potential energy and bond to form a solid.
• Condensation: Thermal energy is released as gas particles lose energy and bond to form a liquid.
• Deposition: Thermal energy is released as gas particles lose energy rapidly and transition directly into a solid.

Reflection