Intermolecular forces (IMF) hold molecules together in condensed phases. For water vaporizing:
ΔHvap=energy to break H-bonds between molecules
Experimental value: ΔHvap(H2O)=40.7kJ/mol.
WHY this is small: Hydrogen bonds are ~20 kJ/mol each. Each water molecule makes ~3.5 H-bonds on average in liquid. Breaking them doesn't touch the O–H covalent bonds inside the molecule.
Breaking a covalent bond requires overcoming electron-pair attraction. For water decomposition:
2H2O(l)→2H2(g)+O2(g)
We must break O–H bonds. Bond dissociation energy:
D(O–H)≈463kJ/mol
Each H₂O has two O–H bonds, so breaking both:
2×463=926kJ per mole of H2O
Ratio:
PhysicalChemical=40.7926≈23×
WHY the huge difference: Covalent bonds involve shared electron pairs in overlapping orbitals (strong electrostatic + quantum exchange stabilization). IMFs are just dipole attractions or dispersion—much weaker.
Master these four, and you'll classify 80% of changes correctly. Edge cases (like dissolving ionic compounds—physical by definition, but looks "reactive") require deeper analysis.
Recall Feynman: Explain to a 12-Year-Old
Imagine you have LEGO bricks. A physical change is like taking apart a LEGO house and moving the bricks to a new pile—they're still the same bricks, just rearranged in space or stacked differently. You can always rebuild the house.
A chemical change is like melting the LEGO bricks in a fire. They turn into a pudle of plastic goo—not bricks anymore. You can't un-melt them back into bricks. The plastic molecules broke apart and recombined differently.
In chemistry, the "LEGO bricks" are atoms. Physical change = atoms stay in the same molecule-groups (like H₂O staying H₂O when it freezes). Chemical change = atoms break apart from their molecule-groups and form new ones (like H₂O splitting into H₂ and O₂).
Why care? Chemists need to know if they made a new substance (new medicine, new material) or just changed how it looks. That's why we watch for new colors, gases, or un-do-able changes—signs the "LEGO bricks" got rearranged at the molecule level.
What is a physical change? :: A change in form, state, or appearance of matter without changing its chemical identity (molecular composition). No bonds break or form.
What is a chemical change?
A transformation of matter into one or more new substances with different chemical properties. Bonds break and reform; reactants ≠ products.
Why do chemical changes require more energy than physical changes?
Chemical changes break covalent bonds (400–800 kJ/mol), while physical changes break intermolecular forces (0.1–40 kJ/mol). Covalent bonds are ~10–100× stronger.
Give three indicators of a chemical change.
1) New substance forms (different formula/properties), 2) Irreversible by simple means, 3) Large energy change or observable clues (gas, precipitate, permanent color, light/heat).
Is dissolving salt in water a physical or chemical change? Why?
Physical. NaCl dissociates into Na⁺ and Cl⁻ ions, but they're still Na and Cl (same identity). Evaporate the water → recover NaCl. Ions are just dispersed, not chemically altered.
Is rusting iron a physical or chemical change? Why?
Chemical. Fe reacts with O₂ and H₂O to form Fe₂O₃ (rust), a new substance. Cannot recover shiny Fe by physical means; properties changed (lost magnetism, gained brittleness).
What is the "identity test" for physical vs chemical change?
Check if the molecular formula is the same before and after. Same formula → physical. Different formula → chemical.
True or false: All exothermic processes are chemical changes.
False. Condensation (H₂O(g) → H₂O(l)) releases 40.7 kJ/mol (exothermic) but is physical. Must check if new substances formed.
Why is boiling water a physical change, but electrolyzing water a chemical change?
Dekho, chemistry mein sab kuch change hota hai—lekin sabhi change ek jaisa nahi hota. Physical change matlab sirf shakl badli, lekin chez wahi rahi. Jaise pani ko freeze karo toh ice ban jati hai, phir melt karo toh wapas pani—molecule same hai (H₂O), bas solid se liquid ho gaya. Lekin chemical change mein toh identity hi badal jati hai—jaise lohe pe rust lag gaya, ab woh shiny iron nahi raha, Fe₂O₃ ban gaya, completely new substance. Wapas shiny iron nahi bana sakte aram se.
Pehchaan kaise karein? Agar molecular formula same hai (pehle bhi C₁₂H₂₂O₁₁, bad mein bhi C₁₂H₂₂O₁₁ jaise sugar dissolving mein), toh physical. Agar formula badal gaya (Fe → Fe₂O₃), toh chemical. Aur energy ka bhi difference hota hai—physical change mein chota energy lagta hai (10–40 kJ/mol, sirf molecules ke bech ka weak attraction totta hai), chemical mein bahut zyada (400–800 kJ/mol, kyunki covalent bonds totte hain jo ki atoms ke beech strong connection hote hain). Isliye jab kuch jalata hai (combustion) ya reaction hota hai with gas/precipitate, samajh jao chemical change ho raha hai.
Ye concept important kyun hai? Kyunki lab mein ya industry mein hume pata hona chahiye ki naya substance bana ya nahi. Agar medicine bana rahe ho, toh chemical change chahiye (naya molecule with new properties). Agar sirf purify kar rahe ho (distillation, filtering), toh physical change—same substance, just separated. Exams mein bhi yahi pucha jata hai: rust lagana chemical hai ya physical? Boiling water physical hai ya chemical? Answer dene ke liye yeh core concept solid hona chahiye.
Ek aur baat: mistakes se bachna. Jaise har color change chemical nahi hota—food coloring pani mein mix karo toh colored water, but dye molecule wahi hai, just dispersed (physical). Aur energy release bhi hamesha chemical nahi—steam condensing bhi heat deta hai but still H₂O (physical). Toh test karo: naya substance bana kya? Agar haan, toh chemical. Agar nahi, sirf form change hui, toh physical. Yeh clarity se chemistry ke baki sab topics samajhne mein mad milegi!