States of Matter & Vapor Pressure
Objectives
- Explain the three states of matter and phase changes using particle models
- Understand the relationship between vapor pressure and boiling point
- Read and interpret phase diagrams
Three States of Matter
Matter exists in three states — solid, liquid, gas — determined by temperature and pressure.
| Change | Name | Energy |
|---|---|---|
| Solid → Liquid | Melting | Endothermic |
| Liquid → Gas | Vaporization | Endothermic |
| Solid → Gas | Sublimation | Endothermic |
| Gas → Liquid | Condensation | Exothermic |
| Liquid → Solid | Freezing | Exothermic |
| Gas → Solid | Deposition | Exothermic |
Heating curve: When a pure substance is heated at a constant rate, the temperature plateaus at the melting and boiling points (latent heat is being absorbed).
Vapor Pressure
In a closed container, liquid evaporates until the rate of evaporation = rate of condensation — this is liquid-vapor equilibrium.
The pressure exerted by the vapor at this point is the saturated vapor pressure (vapor pressure).
- Vapor pressure increases with temperature
- Boiling point = temperature at which vapor pressure equals external pressure
- At high altitude (lower atmospheric pressure), the boiling point is lower
Phase Diagrams
In a phase diagram:
- Triple point: all three phases coexist
- Critical point: beyond this temperature, liquid and gas become indistinguishable
- Lines = boundaries where two phases coexist
- Water’s anomaly: the solid-liquid boundary slopes to the left (ice is less dense than water)
Check Your Understanding
Q1 The boiling point is the temperature at which vapor pressure equals...
Q2 What does a plateau on a heating curve indicate?
Q3 What is the triple point on a phase diagram?
Exercises
Q1. Explain why water boils below 100°C at the summit of Mt. Fuji (atmospheric pressure ≈ 0.63 atm).
Solution
Boiling occurs when the vapor pressure equals the external (atmospheric) pressure.
At the summit of Mt. Fuji, atmospheric pressure is only 0.63 atm, so the vapor pressure reaches 0.63 atm at a lower temperature (~87°C), and boiling begins there.
In other words, when external pressure is lower, a lower temperature is enough for vapor pressure to match it, resulting in a lower boiling point.