Section 50. _chemical Change Caused by Electricity._

How are storage batteries charged?

How is silver plating done by electricity?

You have already done an experiment showing that electricity can start chemical change, for you changed water into hydrogen and oxygen by passing a current of electricity through the water.

The plating of metals is made possible by the fact that electricity helps chemical change. You can nickel plate a piece of copper in the following manner:

EXPERIMENT 103. Dissolve a few green crystals of "double nickel salts" in water, until the water is a clear green. The water should be about 2 or 3 inches deep in a glass or china bowl that is not less than 5 inches across.

Lay two bare copper wires across the bowl, about 3 inches apart, as shown in Figure 177. Connect the positive wire from a storage battery, or the wire from the carbon of a battery of three or four cells, to an end of one bare wire. Connect the negative wire from the storage or the negative wire from the zinc of the other battery to an end of the second bare wire.

Now fasten a fine bare wire 5 or 6 inches long around a small piece of copper, and another like it around a piece of nickel, as shown in Figure 176. Then put the piece of copper in the bottom of an evaporating dish, with the wire hanging out, as in Figure 177.

Pour over the piece of copper enough of the cleansing solution to cover it. The cleansing solution contains strong acids. If you get any on your skin or clothes, wash it off immediately with ammonia or soda. As soon as the copper is bright and clean, take it out of the cleansing solution and suspend it by the negative wire in the green nickel solution. You can tell if you have it on the negative wire, for in that case bubbles will rise from it during the experiment. The copper should be entirely covered by the nickel solution, but should not touch the bottom or sides of the bowl. Pour the cleansing solution from the evaporating dish back into the bottle. Suspend the nickel, in the same way as the copper, from the positive wire crossing the bowl. When set up, the apparatus should appear as shown in Figure 178.

1 cup water.

1 cup concentrated sulfuric acid.

1 cup concentrated nitric acid.

1 teaspoonful concentrated hydrochloric acid.

The sulfuric and nitric acids must be measured in glass or china cups, and the hydrochloric acid must be measured in a silver-plated spoon or in glass--not in tin.]

Turn on the electricity. If the copper becomes black instead of silvery, clean it again in the cleansing solution, and move the two bare wires much farther apart,--practically the full width of the bowl. If the copper still turns black, it means that too much electricity is flowing. In that case use fewer batteries.

The electricity has started two chemical changes. It has made part of the piece of nickel combine with part of the solution of nickel salt to form more nickel salt, and it has made some of the nickel salt around the copper change into metallic nickel. Then the negative electricity in the copper has attracted the positive bits of nickel metal made from the nickel salt, and made them cling to the copper. If there is no dirt or grease on the copper, the particles of nickel get so close to it that they stick by adhesion, even after the electric attraction has ceased. This leaves the copper nickel-plated, but to make it shiny the nickel plating must be polished.

Silver plating and gold plating are done substantially in the way that you have done the nickel plating, only gold salt or silver salt is used instead of nickel salt.

Just as electricity helps chemical changes in plating, it helps changes in a storage battery. But in the storage battery the new compounds formed by "charging" the battery change back again and generate electricity when the poles of the battery are connected with each other by a good conductor.

APPLICATION 75. Explain how spoons can be silver plated; how water can be changed into hydrogen and oxygen.

INFERENCE EXERCISE

Explain the following:

471. Clothes dry best in the sun and wind.

472. Proofs of photographs that have not been thoroughly "fixed" fade if left out of their envelope.

473. Blowing a match puts it out, yet a good draft is necessary for a hot fire.

474. A cup does not naturally fall apart, yet after it is broken it falls apart even if you fit the pieces together again.

475. Crayon leaves marks on a blackboard.

476. A baked potato tastes very different from a raw one.

477. An air-filled automobile tire is harder at noon than in the early morning.

478. When a live trolley wire breaks and falls to the street, it becomes so hot that it burns.

479. Glass jars of fruit should be kept in a fairly dark place.

480. You wash dishes in hot water.