States of matter

1. Overview

Matter on Earth exists in three primary physical forms: solids, liquids, and gases. Understanding the arrangement and behavior of particles in these states is fundamental to explaining how substances react to changes in temperature and pressure.

Key Definitions

• Matter: Anything that has mass and occupies space.
• Melting: The process where a solid turns into a liquid.
• Boiling: The process where a liquid turns into a gas at a specific temperature (throughout the liquid).
• Condensing: The process where a gas turns into a liquid.
• Freezing/Solidifying: The process where a liquid turns into a solid.
• Evaporation: The process where a liquid turns into a gas at the surface (can happen at any temperature below boiling).

Core Content

Properties of Solids, Liquids, and Gases

The behavior of matter is determined by the arrangement and motion of its particles (atoms or molecules).

Property	Solid	Liquid	Gas
Shape	Fixed shape	Takes the shape of the container bottom	Fills the entire container
Volume	Fixed volume	Fixed volume	No fixed volume (expands)
Density	High	High (slightly lower than solids*)	Low
Compressibility	Almost impossible to compress	Very difficult to compress	Easy to compress
Arrangement	Regular lattice, particles touching	Random arrangement, particles touching	Random arrangement, far apart
Motion	Vibrate about fixed positions	Slide over one another	Move rapidly and randomly

Changes of State

When energy (heat) is added to or removed from a substance, it can change state.

• To go from Solid → Liquid → Gas: Energy must be supplied. This energy is used to break or overcome the attractive forces between particles.
• To go from Gas → Liquid → Solid: Energy must be released. Particles lose kinetic energy and move closer together as attractive forces pull them into a more ordered structure.

Extended Content (Extended Only)

There is no additional Supplement content for this specific sub-topic (2.1.1).

Key Equations

While this topic is descriptive, it relates closely to the density formula:

$$\text{Density} (\rho) = \frac{\text{Mass} (m)}{\text{Volume} (V)}$$

• $\rho$ (Rho): Density (measured in $kg/m^3$ or $g/cm^3$)
• $m$: Mass (measured in $kg$ or $g$)
• $V$: Volume (measured in $m^3$ or $cm^3$)

Common Mistakes to Avoid

• ❌ Wrong: Thinking that $0\ ^\circ\text{C}$ is only the temperature of ice.
  • ✅ Right: $0\ ^\circ\text{C}$ is the temperature of the melting point of ice and the freezing point of water. Both solid ice and liquid water can exist at $0\ ^\circ\text{C}$ simultaneously during the state change.
• ❌ Wrong: Thinking particles in a liquid are far apart because they can flow.
  • ✅ Right: Particles in a liquid are still in constant contact (touching). They flow because they have enough energy to slide past one another, unlike solids.
• ❌ Wrong: Suggesting that molecules gain energy to become "more ordered."
  • ✅ Right: Molecules lose energy to become more ordered (e.g., as a gas cools to a liquid). Energy is required to break order and create disorder.
• ❌ Wrong: Believing you can see individual molecules under a standard school microscope.
  • ✅ Right: Molecules are far too small to be seen. We can only observe the effects of their motion (like Brownian Motion) on larger, visible particles like smoke or pollen.

Exam Tips

1. Use the word "Touching": When describing solids and liquids, always state that the particles are touching. Many students lose marks by drawing liquid particles with gaps between them.
2. Describe the Motion: If asked about a gas, use the phrase "rapid, random motion." For solids, use "vibrate about fixed positions."
3. State Change Temperature: Remember that during a change of state (melting or boiling), the temperature of the substance stays constant, even though heat is still being added.