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Change of State

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Estimated time: 11 minutes
  • Introduction
  • Definition: Change of State
  • The Six Types of Phase Changes
  • Practical Applications
  • Key Points: Practical Applications of State
Maharashtra State Board: Class 11

Introduction

Matter exists in three common states: solid, liquid, and gas. A transition from one state to another is called a change of state (or phase change). These transitions are driven by heat exchange between a substance and its surroundings.

When heat is added to a substance, its molecules gain kinetic energy and vibrate more vigorously. At certain critical temperatures, the supplied energy goes into breaking intermolecular bonds rather than raising the temperature. This is why temperature pauses during a phase change, even though heat continues to flow in.

Maharashtra State Board: Class 11
CISCE: Class 10

Definition: Change of State

A transition from one state to another is called a change of state (or phase change).

Or

The process of change from one state to another at a constant temperature is called the change of phase.

Maharashtra State Board: Class 6

The Six Types of Phase Changes

All possible transitions between solid, liquid, and gas — and their reverse — are summarised below.

Phase Change Direction Process Name Energy Example
Solid → Liquid Gains heat Melting (Fusion) Endothermic Ice → Water at 0 °C
Liquid → Solid Loses heat Freezing Exothermic Water → Ice at 0 °C
Liquid → Gas Gains heat Vaporisation (Boiling) Endothermic Water → Steam at 100 °C
Gas → Liquid Loses heat Condensation Exothermic Steam → Water droplets
Solid → Gas Gains heat Sublimation Endothermic Dry ice (CO₂), Naphthalene mothballs
Gas → Solid Loses heat Deposition Exothermic Frost forming on cold surfaces
Maharashtra State Board: Class 6, 11

Practical Applications

  • Pressure cooker: Increased pressure raises the boiling point above 100 °C, so food cooks faster at higher temperature.
  • Cooking at hill stations: Lower atmospheric pressure means water boils 100 °C below, making cooking slower.
  • Vacuum distillation: Reducing pressure lowers boiling points, allowing heat-sensitive substances to be distilled without decomposition.
Maharashtra State Board: Class 6, 11

Key Points: Practical Applications of State

  • A change of state occurs when heat exchange causes a substance to transition between solid, liquid, and gas phases.
  • Temperature remains constant during a phase change because heat energy changes molecular arrangement (potential energy), not molecular speed (kinetic energy).
  • The heating curve has flat plateaus at the melting point (0 °C) and boiling point (100 °C) for water, with rising slopes in between.

Test Yourself

Shaalaa.com | Changes in states of matter

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