When salt water evaporates, only the water turns into a gas (water vapor), leaving the salt behind as a solid.
The Evaporation Process
Here's a breakdown of why this happens:
- Water's Boiling Point: Water has a much lower boiling point (100°C or 212°F) than salt (Sodium Chloride, NaCl), which has a boiling point of 1413°C (2575°F).
- Energy Input: When heat is applied to salt water, the water molecules gain enough energy to overcome the attractive forces holding them together in the liquid state.
- Phase Change: These energized water molecules then transition into a gaseous state (water vapor) and escape into the air. This is evaporation.
- Salt's Stability: The salt molecules require significantly more energy (much higher temperatures) to change into a gas. At the temperatures typically involved in evaporation, the salt remains a solid.
- Residue: As the water evaporates, the salt is left behind as a residue. You can observe this phenomenon when seawater evaporates on the beach, leaving behind salt crystals.
Key Differences
| Feature | Water | Salt (NaCl) |
|---|---|---|
| Boiling Point | 100°C (212°F) | 1413°C (2575°F) |
| State at Room Temp | Liquid | Solid |
| During Evaporation | Turns into water vapor and evaporates | Remains as a solid residue |
In summary, when salt water evaporates, the water molecules gain enough energy to turn into a gas and escape, while the salt's much higher boiling point prevents it from evaporating and it is left behind as a solid.
