= Conductivity experiment on salt solutions
How can a clear <chemical solution> reveal otherwise invisible mobile <ions>? The documented <Conductivity of Solutions by Harvard Natural Sciences Lecture Demonstrations> demonstration uses metal <electrodes> enclosed in tubing, an electrical supply, and a lamp in the circuit. The protocol specifies about $150\,\mathrm{mL}$ samples in labeled $250\,\mathrm{mL}$ beakers and a separate deionized-<water> rinse station. The recorded readout is lamp brightness after the <electrodes> enter each sample; the simplified variant tests <water> before and after adding <sodium chloride>. These are apparatus and procedure specifications, not new measurements reported here.<Conductivity of Solutions by Harvard Natural Sciences Lecture Demonstrations>{ref}
\Image[/salt-solution-conductivity-controls.png]
{title=<Electrical conductivity> comparisons with a lamp tester, and the opposite motions of <cations> and <anions> in an <electric field>}
{id=image-salt-solution-conductivity-controls}
{description=Original circuit schematic and qualitative model outcomes, not photographs or recorded brightness data. A dark lamp means a signal below this tester's visible threshold, not identically zero <electrical conductivity>. Arrows show <drift velocities> during one half-cycle of an alternating supply; both signs contribute to the same <conventional current>. <Water> <molecules> and <hydration of ions> are omitted from the microscopic panel.}
The useful comparison is <water>, a <sucrose> <chemical solution>, and a <sodium chloride> <chemical solution> with the same probe geometry. A documented university account reports no visible light with <water>, a glowing lamp with dissolved <sodium chloride>, and a weak filament glow with its <sucrose> sample. The diagram summarizes the latter as little or no visible light; the precise detection threshold depends on the tester. <Sucrose> is a control for the idea that any dissolved material must conduct: its dispersed neutral <molecules> do not provide the abundant mobile <ions> supplied by the <salt>. This contrast is a documented teaching observation, not a numerical calibration of lamp brightness.<Chemistry 2e>[§11.1, sucrose dissolves as molecules; §11.2, Figure 11.6 and ionic versus nonionic charge carriers]{ref}<Conductivity of solutions by the migration of ions>{ref}
Rinsing the <electrodes> between samples controls carryover of dissolved <ions>, and the <water> blank checks the solvent background. Repeating the blank after the <salt> sample would test whether that rinse was effective. Additional methodological controls are fixed immersed area, electrode separation, supply setting, <temperature>, and comparison at stated <concentrations>. These additional controls are proposed checks, not undocumented claims about the source demonstration.
The inference is that mobile <electric charge> carriers complete the circuit. In the <chemical solution>, an <electric field> drives positive <ions> along the field and negative <ions> against it; multiplying each <drift velocity> by its <electric charge> makes both contributions to <conventional current> point the same way. With an alternating supply the direction reverses each half-cycle. The external wires instead carry <electric current> through <electrons>. The lamp alone identifies neither the species nor an exact <electrical conductivity>.
Important limits remain. <Water> has a small intrinsic population of <hydronium> and <hydroxide>, so a dark lamp does not prove absence of <ions>. Changes in probe geometry affect measured <electrical conductance>; <electrolysis>, electrode interfaces, contamination, and heating can also affect a real circuit. The comparison supports a transport model but does not independently measure a <coordination number>, a partitioned <partial atomic charge>, or a <lattice energy>.
Nor does poor conduction by ordinary dry <sodium chloride> establish that all <ionic crystals> are electrical insulators. <Crystallographic defects> can allow transport, and <solid electrolytes> can have substantial ionic <electrical conductivity> without <melting>. The authored abstract of <A lithium superionic conductor by Noriaki Kamaya and others> reports room-<temperature> ionic conduction in a <solid electrolyte>; the full experimental article was not accessible here.<A lithium superionic conductor by Noriaki Kamaya and others>{ref}
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