01.02 Changes in States

1. States of Matter

Matter exists in three primary states:

• Solid: Definite shape and volume.
• Liquid: Definite volume but takes the shape of its container.
• Gas: Neither definite shape nor volume; expands to fill its container.

Figure 1.4 Overview:

• Melting and Freezing: Transition between solid and liquid.
• Evaporation and Condensation: Transition between liquid and gas.
• Sublimation: Direct transition between solid and gas.

2. Melting and Freezing

Melting

• Definition: The change from a solid to a liquid.
• Melting Point (m.p.): The specific temperature at which a pure solid turns into a liquid.
  • Example: Pure ice melts at 0°C.

Freezing

• Definition: The change from a liquid to a solid.
• Freezing Point (f.p.): The specific temperature at which a liquid becomes a solid.
  • Note: For pure substances, the melting point and freezing point are identical.
  • Example: Pure water freezes at 0°C.

Key Points:

• Both processes occur sharply at the same temperature for pure substances.
• Figure 1.5 illustrates the relationship between melting and boiling points.

3. Sublimation

Sublimation

• Definition: Direct transition from a solid to a gas or vice versa without passing through the liquid phase.
• Example: Solid carbon dioxide (dry ice) sublimates at atmospheric pressure, turning directly into gas without becoming liquid.
  • Observation: Dry ice does not leave a liquid film, unlike regular ice.

Key Points:

• Sublimation occurs at a specific temperature for each pure solid.
• Note: Sublimation is not a required topic for all IGCSE Chemistry courses but is important for certain applications.

4. Evaporation and Boiling

Evaporation

• Definition: The process by which molecules at the surface of a liquid gain enough energy to enter the gas phase.
• Characteristics:
  • Occurs at temperatures below the boiling point.
  • Faster with increased surface area and higher temperatures.
• Example: Water evaporates more quickly in a large, shallow dish than in a small, deep one.

Boiling

• Definition: The rapid vaporization of a liquid, occurring when vapor pressure equals atmospheric pressure.
• Boiling Point (b.p.): The specific temperature at which a liquid boils.
  • Example: Water boils at 100°C at standard atmospheric pressure.

Key Differences Between Evaporation and Boiling:

• Evaporation: Occurs at the surface and below boiling point.
• Boiling: Occurs throughout the liquid at the boiling point.

Figure 1.8a Illustration:

• Shows bubbles forming throughout a boiling liquid, indicating gas formation within the liquid.

5. Condensation

Condensation

• Definition: The change of a gas or vapor into a liquid.
• Process: Typically occurs when gas is cooled or when pressure is increased.
• Example: Water vapor condenses into liquid droplets on the outside of a cold glass.

Key Points:

• Reverse of Evaporation: While evaporation absorbs heat, condensation releases heat.
• Figure 1.8b Illustration: Shows water vapor from boiling water condensing into visible steam.

6. Volatility

Volatile Substances

• Definition: Liquids that evaporate easily due to low boiling points and weak intermolecular forces.
• Example: Ethanol (b.p. 78°C) is more volatile than water (b.p. 100°C).

Key Terms:

• Volatility: The tendency of a substance to vaporize.
• Intermolecular Forces: Weaker forces result in higher volatility.

7. Factors Affecting Boiling Point

Pressure Influence

• Increased Pressure: Raises the boiling point.
  • Example: Pressure cookers use increased pressure to boil water above 100°C, cooking food faster.
• Decreased Pressure: Lowers the boiling point.
  • Example: At high altitudes, water boils at temperatures below 100°C, affecting cooking times.

Key Points:

• Boiling points are typically measured at standard atmospheric pressure (sea level).
• Figure 1.5 shows how boiling points can change with pressure variations.

8. Key Vocabulary

• Sublimation: Direct transition from solid to gas or vice versa.
• Melting Point (m.p.): Temperature at which a solid becomes a liquid.
• Freezing Point (f.p.): Temperature at which a liquid becomes a solid.
• Evaporation: Surface transition from liquid to gas.
• Boiling: Rapid transition from liquid to gas throughout the liquid.
• Boiling Point (b.p.): Temperature at which a liquid boils.
• Volatile: Easily evaporates; low boiling point.
• Volatility: Measure of how easily a substance evaporates.
• Condensation: Transition from gas to liquid.

9. Practical Examples and Applications

• Dry Ice in Stage Effects: Used in theater productions to create fog through sublimation.
• Cooking Adjustments: Adjusting cooking times and temperatures based on altitude due to changes in boiling points.
• Industrial Distillation: Utilizing differences in boiling points to separate mixtures.