The properties of solids, liquids, and gases can be explained by the movement and arrangement of
particles.
Comparing the States
Feature
Solid \((s)\)
Liquid \((l)\)
Gas \((g)\)
Arrangement
Fixed lattice. Regular.
Random. Close together.
Random. Far apart.
Motion
Vibrate only.
Slide past each other.
Rapid random motion.
Spacing
Very close (touching).
Close but irregular.
Very far apart.
Shape
Fixed.
Takes shape of container.
Fills entire container.
Volume
Fixed.
Fixed.
Variable (compressible).
Changes of State
Terminology is key.
Why does the temperature of boiling water stay flat?
This phenomenon highlights the distinction between Kinetic and Potential energy.
Definition: Temperature is a measure of average Kinetic Energy (speed of
particles).
The Mechanism: During a phase change (boiling), the added heat energy is
not used to speed up the particles. Instead, it performs work to overcome
intermolecular forces, increasing the distance between particles.
Conclusion: This energy is stored as Potential Energy. Since the speed
doesn't change, the thermometer reading flatlines.
Worked Example: Cooling Curve Logic
A student records the cooling curve of a pure substance X. The graph shows a temperature drop, a
horizontal plateau at 55°C, and then a further drop. Explain the change in particle arrangement at 55°C.
1. Identify: Cooling means removing energy. A plateau during cooling represents an
exothermic phase change.
2. Analyze Plateau: At 55°C, the substance is transitioning from Liquid to Solid
(Freezing).
3. Particle Description: The particles transition from a disordered state where they
slide past one another (Liquid) to a fixed lattice structure where they only vibrate in place (Solid).
The system is losing Potential Energy.