Phase Changes and Phase Diagrams - Chemistry

1. Fundamental Concepts

Phase: Distinct matter form with uniform composition and state (solid/liquid/gas).
Phase Change: Physical transition between phases (no chemical bond breakage).
Phase Diagram: Graph showing stable phases of a substance at different temperature (T) and pressure (P).
Key Processes: Melting/freezing (solid↔liquid), vaporization/condensation (liquid↔gas), sublimation/deposition (solid↔gas).
Key Points: Triple point (all 3 phases coexist), critical point (gas cannot be liquefied above this T/P).

2. Key Concepts

Energy of Phase Changes: Endothermic (requires energy: melting, vaporization, sublimation); exothermic (releases energy: freezing, condensation, deposition). Temperature remains constant during phase change (latent heat).

Phase Diagram Structure: X-axis = T, Y-axis = P; lines = phase boundaries (equilibrium between two phases); points = triple/critical points.

Boundary Behavior: Crossing a boundary = phase change; along a boundary = two phases coexist.

Triple/Critical Point Significance: Triple point is unique for each substance; critical point marks the limit of liquid-gas distinction.

3. Examples

Easy

Question: Identify the phase change when solid dry ice turns directly into carbon dioxide gas.
Analysis:

Phase Transition: Solid → Gas
Matching Process: The direct transition from solid to gas is defined as sublimation.

Answer: Sublimation

Medium

Question: On a phase diagram, what does the line separating the solid and liquid regions represent?
Analysis:

Phase Diagram Basics: Lines on a phase diagram are phase boundaries, representing conditions where two phases are in dynamic equilibrium.
Solid-Liquid Boundary: At any T/P along this line, the rate of melting (solid→liquid) equals the rate of freezing (liquid→solid).

Answer: The set of temperature and pressure conditions where the solid and liquid phases of a substance coexist in equilibrium (melting/freezing boundary).

Hard

Question: A substance has a triple point at -10 °C and 0.5 atm. Its solid-liquid boundary has a positive slope (slants to the right). At 1 atm (standard pressure), its melting point is 5 °C. Predict the phase of the substance at -5 °C and 1 atm, and explain the phase change that occurs if the temperature is increased to 10 °C at constant pressure.
Analysis:

Step 1: Locate Initial Conditions (T=-5 °C, P=1 atm)
- The melting point at 1 atm is 5 °C, meaning below 5 °C at 1 atm, the substance is in the solid phase (since melting occurs when T rises above the melting point).
- -5 °C < 5 °C, so initial phase = solid.
Step 2: Analyze Temperature Increase to 10 °C (Constant P=1 atm)
- As T increases from -5 °C to 10 °C, it crosses the solid-liquid boundary at 5 °C (the melting point at 1 atm).
- Crossing this boundary from lower to higher T corresponds to the phase change of melting (solid→liquid).
  
  Answer:

Phase at -5 °C and 1 atm: Solid
Phase change when T increases to 10 °C: Melting (solid → liquid)

4. Problem-Solving Techniques

Phase Change Identification:

Map the starting and ending phases to the 6 core phase processes (e.g., liquid→gas = vaporization).

Remember: Direct solid→gas = sublimation; direct gas→solid = deposition.

Phase Diagram Interpretation:

Locate Phase: Plot the given T/P on the diagram; identify the region it falls into.

Boundary Crossings: Determine which boundary is crossed when T/P changes; the crossing corresponds to a phase change between the two adjacent regions.

Point Analysis: If T/P matches the triple point, all 3 phases coexist; if above critical point, the substance is a supercritical fluid.

Energy Calculation for Phase Changes:

Use the latent heat equation: \(Q = m \times L\) (m = mass, L = latent heat of fusion/vaporization/sublimation).

Key Note: Only calculate phase change energy when temperature is constant (ignore temperature change energy during the phase transition itself).

Slope of Solid-Liquid Boundary:

Positive slope: Increasing pressure raises the melting point (most substances, e.g., water is an exception with negative slope).

Use slope to predict how pressure affects the solid-liquid equilibrium.