Phase Rule

Question 1

Phase Diagram

Answer 1

A graphical representation of the physical states of a substance under different conditions of temperature and pressure.

Question 2

X-axis of a Phase Diagram

Answer 2

Temperature

Question 3

Y-axis of a Phase diagram

Answer 3

Pressure

Question 4

Lines or Curves on a Phase Diagram

Answer 4

Represent the conditions for a phase change to occur. Two states of the substance coexist in equilibrium along these lines.

Question 5

Triple Point

Answer 5

The unique point on a phase diagram where all three states of matter (solid, liquid, and gas) coexist in equilibrium.

Question 6

Critical Point

Answer 6

The point on a phase diagram where the distinction between liquid and gas disappears. The properties become identical.

Question 7

Fusion (Melting) or Freezing Curve

Answer 7

The line on a phase diagram representing the transition between solid and liquid states. Melting occurs moving up the curve, freezing occurs moving down.

Question 8

Vaporization (Condensation) Curve

Answer 8

The line on a phase diagram representing the transition between liquid and gas states. Vaporisation occurs moving up the curve, condensation occurs moving down.

Question 9

Sublimation (Deposition) Curve

Answer 9

The line on a phase diagram representing the transition between solid and gas states. Sublimation occurs moving up the curve, deposition occurs moving down.

Question 10

Monoclinic Sulphur - Stable form below 95.5°C

Answer 10

Description: The most stable form of Sulphur at room temperature and pressure.

Question 11

Rhombic Sulphur - Stable form above 95.5°C

Answer 11

Description: Becomes the stable form of Sulphur above 95.5°C.

Question 12

Transition Line (Monoclinic - Rhombic)

Answer 12

Definition: Represents the temperature (95.5°C) at which monoclinic and rhombic Sulphur can coexist in equilibrium.

Question 13

Solid-Liquid Line (Melting Point) for Sulphur

Answer 13

Represents the conditions of temperature and pressure at which solid Sulphur (either monoclinic or rhombic depending on temperature) and liquid Sulphur are in equilibrium.
Slants upwards with increasing pressure.

Question 14

Liquid-Gas Line (Boiling Point) of Sulphur

Answer 14

Represents the conditions of temperature and pressure at which liquid Sulphur and Sulphur vapor are in equilibrium.
Slants upwards with increasing temperature.

Question 15

Triple Point (if it exists for Sulphur)

Answer 15

The temperature and pressure at which all three phases (solid, liquid, gas) of Sulphur can coexist in equilibrium.

Question 16

Condensed Phase Rule

Answer 16

An equation relating the number of phases (P), components (C), and degrees of freedom (F) in a closed heterogeneous equilibrium system at constant temperature and pressure.
* Formula: F = C - P + 2

Question 17

• Degrees of Freedom (F)

Answer 17

The number of intensive variables (such as temperature, pressure, or concentration) that can be changed independently without affecting the number of phases present in the system.

Question 18

• Components (C)

Answer 18

The number of chemically independent constituents in a system. A system can have multiple phases, but each component must be able to exist in at least one of those phases.

Question 19

Phases (P)

Answer 19

A homogenous region of a system that has distinct physical and chemical properties from other regions. Examples include solids, liquids, and gases.

Question 20

A one-component system (like water) at equilibrium exists with two phases (liquid and gas). How many degrees of freedom does the system have?

Answer 20

• C = 1 (water)
• P = 2 (liquid and gas)
• F = 1 - 2 + 2 = 1
• The system has only one degree of freedom (e.g., temperature can be changed independently without affecting the number of phases).
  Additional Notes:
• The condensed phase rule applies specifically to systems with condensed phases (liquids and solids).
• The rule assumes constant temperature and pressure.
• The rule can be used to predict the number of phases present in a system or the effect of changing temperature or pressure on the number of phases.

Question 21

Eutectic System

Answer 21

A mixture of substances that solidifies at a single, lower temperature than any other combination of the same components.

Question 22

Eutectic Composition

Answer 22

The specific ratio of components in a eutectic system that solidifies at the eutectic temperature.

Question 23

Eutectic Point

Answer 23

The point on a phase diagram where the eutectic temperature and eutectic composition intersect. Represents the specific conditions for a eutectic system to solidify.

Question 24

Non-Eutectic Mixtures

Answer 24

Mixtures with a composition other than the eutectic composition. These mixtures solidify over a temperature range, with individual components solidifying at different temperatures.

Question 25

• A mixture of Lead (Pb) and Tin (Sn) is being cooled. The composition is not the eutectic composition. What will happen during solidification?

Answer 25

One component (Pb or Sn) will begin to solidify first at its individual melting point. The remaining liquid will become richer in the other component until the eutectic composition is reached. At the eutectic temperature, the remaining liquid will solidify as a eutectic mixture.

Question 26

Phase Rule Merits

Answer 26

Applicable to both physical & chemical equilibria
Macroscopic, no need for molecular details
Classifies equilibrium states
Predicts system behavior
Similar behavior for same degrees of freedom
Ignores component nature and quantity

Question 27

Phase Rule Applications

Answer 27

Helps decide behavior under given conditions:
(a) Equilibrium coexistence
(b) Interconversion of substances
(c) Elimination of substances

Question 28

Limitation of Phase Rule

Answer 28

Considers number of phases, not their amounts. Even trace amounts count.

Applies to single equilibrium state. Doesn’t predict other possible equilibria.

All phases must coexist at the same temperature and pressure.

Finely divided solids and liquids can cause deviations from the phase rule.