An explanation of the experiment
One of the goals of the ancient alchemists was to convert base metals into gold. Although this goal was never attained by chemical methods, the alchemists were able to perform many color changes to make metals resemble gold. In this experiment you will produce some color changes to a copper token and demonstrate diffusion in the solid state.

In this reaction, zinc dissolves in the hot concentrated sodium hydroxide solution to form sodium zincate, commonly written as Na2ZnO2 or, as obtained in solid form from concentrated solutions, NaZn(OH)3. As an ionic equation this can be written:

Zn + 2 OH− → ZnO2 -2 + H2

When the copper token is added to the solution, an electrochemical couple formed by the copper-zinc contact causes the zincate ion to migrate to the copper surface where it is decomposed and reduced to metallic zinc by hydrogen which forms a coating on the token. The resulting token will be silver in color due to a coating of zinc on its surface. When the token is heated, the zinc diffuses into the copper to form a layer of the alloy brass, which results in the
gold color.

It should be noted that the reduction of the zincate ion to zinc will only take place if the copper metal is in direct contact with zinc metal. Also, no copper dissolves in the solution during the reaction.