. Practical physics. two dissimilar metals immersed in a solution of any acidor salt. For simplicity let ns examine the action of a cell composed ofplates of zinc and copper immersed in a dilute solution of hydrochloricacid. The chemical formula for hydrochloric ^ acid IS IICl. This means that each moleculeof the acid consists of one atom of hydrogencombined with one atom of chlorine. Aswas explained under electrolysis (§ 302), theacid dissociates into positively and negativelycharged ions (Fig. 282). When a zinc plate is placed in such a solu-tion, the acid attacks it and pulls zinc atomsinto
. Practical physics. two dissimilar metals immersed in a solution of any acidor salt. For simplicity let ns examine the action of a cell composed ofplates of zinc and copper immersed in a dilute solution of hydrochloricacid. The chemical formula for hydrochloric ^ acid IS IICl. This means that each moleculeof the acid consists of one atom of hydrogencombined with one atom of chlorine. Aswas explained under electrolysis (§ 302), theacid dissociates into positively and negativelycharged ions (Fig. 282). When a zinc plate is placed in such a solu-tion, the acid attacks it and pulls zinc atomsinto solution. Now, whenever a metal dis-solves in an acid, its atoms, for some unknownreason, go into solution bearing little positive charges. The corresponding negative charges must he left on the zinc platein precisely the same way in which a negative charge is left on silkwhen positive electrification is produced on a glass rod by rubbing it?with the silk. It is in this way, then, that we account for the negative. Fig. 282. Showing disso-ciation of hydrochloric-acid molecules in water 274 ELECTRICITY m MOTION charge which we found upon the zinc plate in the experiment whichwas performed with the galvanic cell and the electroscope (see § 300). The passage of positively charged zinc ions into solution gives a posi-tive charge to the solution about the zinc plate, so that the hydrogenions tend to be repelled away from this plate. When these repelledhydrogen ions reach the copi>er plate, some of them give up their chargesto it and then collect as bubbles of hydrogen gas. It is in this waythat we account for the positive charge which we found on the copperplate in the experiment of § 300. If the zinc and copper plates are not connected by an outside con-ductor, this passage of positively charged zinc ions into solution con-tinues but a very short time, for the zinc soon becomes so strongly chargednegatively that it pulls back on the -f zinc ions with as much force asthe acid
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