PREVIOUSLY ASKED IN:
WBPSC Miscellaneous Preliminary 2019
Answer
H2O molecules
Explanation
In a solid NaCl crystal lattice, every Na+ ion is surrounded by Cl- ions. However, the question specifically asks what happens when NaCl is *dissolved in water*. Water is a highly polar solvent. When dissolved, the water molecules break the ionic bonds between Na+ and Cl-. The partially negative oxygen ends of the water molecules align themselves around the positive Na+ ions, forming a hydration sphere. Thus, the Na+ ions are surrounded by H2O molecules.
Key Points
- > Water is an excellent polar solvent with a high dielectric constant and dipole moment.
- > The process of water molecules surrounding solute ions is called hydration or solvation.
- > In a solid, crystalline state, NaCl forms a face-centered cubic lattice where each Na+ is surrounded by 6 Cl- ions.
- > For the Cl- ions in water, the partially positive hydrogen ends of the water molecules surround them.
- > Aqueous NaCl conducts electricity because the separated ions are free to move in the solution.
Additional Information
Structure of NaCl based on State
| State | Surrounded by (around Na+) | Electrical Conductivity |
|---|---|---|
| Solid Crystal | 6 Cl- ions | Insulator (Non-conductor) |
| Molten State | Free moving ions | Good Conductor |
| Aqueous Solution | H2O Molecules (Hydration) | Good Conductor |
Memory Tips
- Trick Question Alert: Always check if the question says "solid crystal" or "dissolved in water". In water, hydration spheres separate the ions, meaning water molecules wrap around the individual ions.