6.1 Physical and Chemical Changes

1. Overview

Physical and chemical changes describe how matter behaves when energy is added or removed, or when different substances interact. Understanding these differences is fundamental to chemistry as it allows scientists to distinguish between simple changes in state and the complex reactions that create entirely new materials.

Key Definitions

• Physical Change: A change in which no new chemical substances are formed; it usually involves a change in state or appearance and is often easily reversible.
• Chemical Change: A process involving a chemical reaction that results in the formation of one or more new substances with different properties.
• Reactants: The starting substances in a chemical reaction.
• Products: The new substances formed as a result of a chemical reaction.
• Exothermic: A process that releases thermal energy to the surroundings.
• Endothermic: A process that absorbs thermal energy from the surroundings.

Core Content

Characteristics of Physical Changes

• No new substances are created.
• The change is usually reversible (e.g., ice can be melted into water and then refrozen).
• The chemical properties of the substance remain the same.
• Energy changes are typically small.
• Examples: Changes of state (melting, boiling, freezing), dissolving a solute (like sugar) in a solvent, or mixing two substances without them reacting.

Example: Melting Ice

• Word Equation: Ice → Water
• Symbol Equation: H₂O (s) → H₂O (l)

Characteristics of Chemical Changes

• New chemical substances are formed.
• The change is usually irreversible (or very difficult to reverse).
• The products have different properties from the reactants.
• Often accompanied by significant energy changes (heat or light).
• Signs of a chemical change:
  • Color changes.
  • Effervescence (bubbles of gas produced).
  • Formation of a precipitate (a solid forming from two liquids).
  • Temperature change (getting hotter or colder).

Example: Reaction between Magnesium and Hydrochloric Acid

• Word Equation: Magnesium + Hydrochloric acid → Magnesium chloride + Hydrogen
• Symbol Equation: Mg (s) + 2HCl (aq) → MgCl₂ (aq) + H₂ (g)

Summary Comparison Table

Feature	Physical Change	Chemical Change
New substance formed?	No	Yes
Reversibility	Usually easy to reverse	Usually hard to reverse
Energy change	Small	Often large
Mass	Total mass is conserved	Total mass is conserved

Extended Content (Extended Only)

There are no specific Supplement objectives for Topic 6.1 in the current syllabus.

Key Equations

1. Combustion of Methane (Chemical Change)

• Methane + Oxygen → Carbon dioxide + Water
• CH₄ (g) + 2O₂ (g) → CO₂ (g) + 2H₂O (l)

2. Thermal Decomposition of Calcium Carbonate (Chemical Change)

• Calcium carbonate → Calcium oxide + Carbon dioxide
• CaCO₃ (s) → CaO (s) + CO₂ (g)

3. Condensation of Steam (Physical Change)

• Water (gas) → Water (liquid)
• H₂O (g) → H₂O (l)

Common Mistakes to Avoid

• ❌ Wrong: Boiling water is a chemical change because bubbles are formed.
• ✓ Right: Boiling is a physical change (change of state). The bubbles are steam (H₂O gas), not a new chemical substance like Hydrogen gas.
• ❌ Wrong: Dissolving salt in water is a chemical change because the salt "disappears."
• ✓ Right: Dissolving is a physical change. The salt and water can be separated by evaporation, and no new substance is formed.
• ❌ Wrong: Forgetting to balance the atoms on both sides of a symbol equation.
• ✓ Right: Ensure the number of atoms for each element is equal on the reactant and product sides (e.g., 2HCl is needed to balance the two Cl atoms in MgCl₂).

Exam Tips

• Identify the Context: If a question mentions "heating a substance and then cooling it back to its original form," it is describing a physical change.
• Command Words: If asked to "describe the differences," ensure you compare specific features like reversibility and the formation of new substances.
• Observation Questions: If you are asked for evidence of a chemical reaction, mention specific observations like "fizzing/effervescence" or "the test tube feels hot," rather than just saying "a gas was made."
• State Symbols: Always check if the question asks for state symbols. Even if it doesn't, including correct state symbols (s, l, g, aq) shows a high level of chemical understanding.