Chapter 5 · Molecules and Compounds
5.1Sugar and Salt
8 min · two checks
Predict
Sugar and salt are both white crystals. What kind of particles make up each?
The idea
- Contrast ionic and molecular compounds using salt and sugar.
- Predict which is more likely to conduct when dissolved.
Sodium chloride does not contain NaCl molecules. It contains a repeating lattice in which each sodium ion is surrounded by chloride ions and each chloride by sodium ions. The formula NaCl is a ratio: one Na⁺ for every Cl⁻. Sucrose is different. Each grain is a pile of molecules, and each molecule is a fixed cluster of 12 carbon, 22 hydrogen, and 11 oxygen atoms. The formula C₁₂H₂₂O₁₁ counts atoms inside one molecule.
That structural difference shows up in properties. Molten salt and aqueous salt conduct electricity because ions can move. Sugar dissolved in water does not produce ions, so the solution does not conduct the way salt water does. Melting an ionic lattice takes more energy than separating sugar molecules, so ionic compounds often have much higher melting points. Appearance will not tell you which is which. The formula type and the properties will.
Keep these
- Ionic formulas state a ratio of ions, not a molecule.
- Molecular formulas count atoms in one molecule.
- Mobile ions conduct; intact sugar molecules do not.