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Joined 2026-09-21 Message
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Rock salt structure 2026-10-14
The rock salt structure of ordinary sodium chloride has a coordination number of six for each kind of ion: six nearest neighbours of the opposite sign surround each ion along three perpendicular directions. Its conventional cubic unit cell contains four formula units; sharing corner, face, and edge sites between adjacent cells must be included in that count. Sixfold local coordination is compatible with overall stoichiometry.[ref]
The checkerboard plane in the diagram is a slice through this three-dimensional arrangement. It leaves out the two nearest neighbours above and below the plane. Repeating a complete even-by-even patch gives equal numbers of signs; the displayed odd-by-odd crop has a boundary imbalance and is not a complete neutral ionic crystal or a unit cell. Other salts need not have the rock salt structure or the same coordination number.
Solubility 2026-10-14
Solubility specifies the amount that can remain dissolved at chemical equilibrium under stated conditions; it is different from how quickly a sample dissolves. Separating ions from an ionic crystal costs energy, but solvation establishes new interactions with the solvent. In water, hydration of ions couples charged ions to polar water molecules. The net enthalpy of dissolution can have either sign; entropy and concentration also enter the Gibbs free energy balance. Consequently, being a salt does not guarantee high solubility in water or any other solvent.[ref]
Chemistry 2e. §11.1, “The Formation of Solutions”, Figure 11.4; §11.3, definition of solubility; §16.4, free energy
Low solubility must also be distinguished from weak electrolyte behavior. A small amount of an ionic compound can dissolve essentially completely as ions, giving few charge carriers because little material dissolved. Calling that a weak electrolyte solely because its chemical solution conducts poorly would conflate two different questions.[ref]
Solution (chemistry) Created 2026-10-13 Updated 2026-10-14
A chemical solution is a homogeneous mixture in which designated solutes are distributed in a solvent. Representative equal-volume samples have the same composition after mixing. Individual molecules and ions remain distinguishable. The solute/solvent labels describe their roles in this mixture.[ref]
A dissolved solute is dispersed as molecules, chemical atoms, or ions, rather than as the larger solid particles of a particle suspension. For example, sucrose retains its molecular formula during dissolution in water; a chemical reaction is not required. Dissolved sodium chloride supplies sodium cations and chloride anions, surrounded by water molecules through solvation. Dissolved does not mean chemically destroyed or converted into water.[ref]
11.1 The Dissolution Process. Chemistry 2e, §11.1, opening molecular and ionic examples and defining traits of solutions
Dissolution disperses particles while preserving their chemical elements
. Original schematic with invented particle positions, not a microscopy image. Top: four generic neutral molecules retain their identity. Bottom: four sodium cations and four chloride anions separate from an ionic crystal. Pale dots represent water molecules; sizes, spacing, and counts are explanatory, not measured. Solvation surrounds each dissolved species; the diagram does not specify a hydration shell geometry. Details.
This makes a useful connection between microscopic identity and laboratory sampling. If a chemical solution is well mixed, the amount of substance in a measured portion follows from its concentration and volume. A scoop from a poorly mixed particle suspension need not represent the whole vessel. Visual clarity alone cannot establish composition or exclude a fine dispersion; the quantitative claim needs representative sampling.