Physical vs. Chemical Changes

Physical Changes

• Definition: Appearance or state of a substance is altered, but its chemical composition remains unchanged. 

• Nature: Change in state of matter without chemical decomposition; the change in strength of intermolecular forces. 

• Reversibility: More reversibility compared to chemical changes. 

• Indicators: Size difference, state of matter change, dissolving (technically). 

• Examples of Physical Changes: 

  • Ice (solid H₂O) melting into liquid water. 

  • Dry ice (solid CO₂) sublimating into gaseous CO₂. 

  • Chopping a chunk of wood to split into smaller pieces. 

Chemical Changes

• Definition: Change in chemical composition that result in different substances (products) formed from previous substances (reactants). 

• Nature: The breaking and forming of intermolecular forces, such as molecular bonds. 

• Reversibility: Less reversibility compared to physical changes, as the products are often more chemically stable. 

• Indicators: Changes in color and temperature, emission of light, and the formation of gases and precipitates. 

• Examples of Chemical Changes: 

  • Methane (CH₄) combusting with oxygen gas (O₂) to form CO₂ and H₂O vapor. 

  • Redox reaction of Fe₂O₃ with aluminum metal to form Al₂O₃ and molten iron. 

  • Burning wood in a campfire. 

Endothermic vs. Exothermic Processes

Endothermic reactions absorb energy. 

• Melting of ice (solid H₂O) into liquid water.

• Evaporation of liquid nitrogen (N₂) into gas. 

• Photosynthesis of CO₂ and H₂O by taking in light energy. 

Exothermic reactions release energy. 

• Methane (CH₄) combusting with oxygen gas (O₂), releasing heat and light as a byproduct. 

• Freezing of gallium metal as temperatures drop below 29.76°C. 

• Cellular respiration by breaking down glucose with O₂ into CO₂, H₂O, and adenosine triphosphate. 

To learn more about the difference, check out this tutorial.