Physical change vs. Chemical change

A physical change is a change in a substance’s form or appearance without changing its chemical identity. Chemical change, on the other hand, involves a transformation in which the fundamental identity of a substance changes due to the rearrangement of atoms, leading to the formation of one or more new substances.

Example #1: Physical Change - Bending or Melting a Metal

• Initial State:

  • Metal atoms are organized in a highly structured, regular lattice configuration.

  • The atoms are held together by metallic bonding, where electrons are shared among the lattice of atoms.

• Final State (After):

  • The fundamental identity of the atoms remains unchanged; they are the same type as in the initial state.

  • The bonding mechanism remains metallic, meaning the shared electron system persists.

• Observed Changes:

  • There is a slight adjustment in the spacing between individual atoms.

  • The macro-level shape of the substance is altered (either through bending the solid or moving into a liquid phase via melting).

Example #2: Chemical Change - Rusting of Iron

• Environmental Factors: The chemical transformation requires the specific presence of water and oxygen

• Site-Specific Electrochemical Reactions:

  • Chemical Equation: Fe(s) → Fe²⁺(aq) + 2e⁻

    • Cathodic Site (Reduction): Oxygen gas gains electrons in the presence of hydrogen ions to form water. Reduction is the gain of electrons.

  • Chemical Equation: O₂(g) + 4H⁺(aq) + 4e⁻ → 2H₂O(l)

    • Anodic Site (Oxidation): Iron atoms ($Fe_{(s)}$) release electrons to form iron ions. Oxidation is the loss of electron

• Characteristic Outcomes: These reactions result in the formation of a brand-new substance called rust.

Example #3: Physical Change - Dissolution

• Example Case:

  • This model tracks the process of Sodium Chloride ($NaCl$) dissolving in water ($H_2O$).

• Structural Transformation:

  • The process begins with an $NaCl$ crystal structure, which is a rigid lattice of sodium and chlorine.

  • When introduced to water, the ionic compound breaks apart into its constituent ions: sodium ($Na^+$) and chlorine ($Cl^-$).

  • These ions become solvated, meaning they are surrounded by water molecules ($H_2O$).

• Nature of the Change:

  • No new chemical substances are formed during dissolution; the sodium and chlorine ions maintain their identity in the solution.

  • The particles are rearranged spatially, but not chemically changed.

• Reversibility:This is a physical change because the original salt crystal can be fully recovered by removing the water through the process of evaporation.