Entropy

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Entropy (S) is a measure of disorder.

• More disordered → more entropy
  
• More organized → less entropy
  
  

If a chemical or physical change happens, there might be a change in the amount of disorder (entropy). We call this change ΔS = Sfinal – Sinitial.

• ΔS positive → more entropy → more disorder
  
• ΔS = 0 → no change in entropy → no change in disorder
  
• ΔS negative → less entropy → less disorder (more organized)

Gases are more disordered than liquid, and liquids are more disordered than solids. So it stands to reason that Sgas > Sliquid > Ssolid.

In general, if substances break apart into pieces, entropy increases, since there are more and different types of molecules moving around.

It is also important that I tell you about entropy changes for expansions / compressions...

Vfinal > Vinitial → Vfinal/Vinitial > 1 → ΔS positive → increased entropy (disorder)

• This makes sense, since more space means more freedom for the molecules, and freedom is the opposite of organization
  

Vfinal < Vinitial → Vfinal/Vinitial < 1 → ΔS negative → decreased entropy (more organization)

• This makes sense, since less space means less freedom for the molecules. Remember too, that if you compress a gas enough, it'll turn into a liquid or solid ... both of which have LESS entropy than a gas.