* Physical Change: The process only alters the state of matter (liquid to gas, gas to liquid), not the chemical composition of the water molecules themselves. Water molecules (H₂O) remain the same throughout the process.
* Equilibrium: The process can be driven in either direction depending on the conditions:
* Boiling: Adding heat energy to liquid water breaks the hydrogen bonds holding the molecules together, allowing them to escape as vapor.
* Condensation: Removing heat energy from water vapor allows the molecules to slow down, form new hydrogen bonds, and return to the liquid state.
* Dynamic Equilibrium: At a specific temperature (the boiling point of water), the rate of evaporation and condensation become equal. This means that the amount of water vapor in the air remains constant, even though there's a constant exchange between liquid and gas states.
In summary: The boiling of water and cooling vapor is a reversible physical change because the water molecules themselves don't change, and the process can be driven in either direction depending on the temperature.
