Acids are substances producing the hydrogen ion in solution: HCl <=> H˙ + Cl'. Bases are substances producing the hydroxide ion in solution: NaOH <=> Na˙ + OH'.
If an acid and a base in solution are mixed, a salt is formed, and the solution becomes neutral. This is usually represented by such equations as: HCl + NaOH = NaCl + H2O. Since the acid, base, and salt are ionised in solution, the reaction really occurs between the ions: (H˙ + Cl') + (Na˙ + OH') = (Na˙ + Cl') + H2O. It will be seen that the anion of the acid (Cl'), and the cation of the base (Na˙), which together constitute the ions of the salt, take no part in the change: they are free before and after the reaction. The net change in neutralisation is the union of the hydrogen ion of the acid with the hydroxide ion of the base to form practically undissociated water: H˙ + OH' = H2O. This is the sole reaction with strong acids and bases, i.e., those which are practically completely ionised. Salts are nearly always largely ionised in solution.
The hydrogen ions and hydroxide ions are those which possess the greatest mobility. After neutralisation, therefore, when these ions have been withdrawn, the conductivity will be appreciably diminished.
Expt. 7. - Fit a rectangular glass trough with two electrodes of sheet copper.  | Fig: Apparatus to demonstrate diminution in conductivity on neutralisation |
Connect these through an ammeter with two accumulators in series. Pour into the cell N-caustic soda solution, containing urea to increase its density, and coloured with phenolphthalein. Float a slice of cork on this solution, and by means of a burette introduce an equal volume of N-hydrochloric acid as a definite stratum above the alkali. Switch on the current and observe the deflection of the ammeter. This is a measure of the current carried by all the ions, Na˙, H˙, OH', Cl'. Now stir the two liquids with a glass rod, and notice the reduced reading of the ammeter. The ions Na˙ and Cl' alone now carry the current.
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