Your silver turns black because of the presence of sulfur in food. Here’s how the cleaning works: The tarnished spoon has silver sulfide on it, and when you put it in the solution, the silver sulfide combines with the baking soda and salt in the water solution to break apart into sulfur (which gets deposited on the foil) and silver (which goes back onto the spoon). Using the heat from your stove, you’ve just relocated the tarnish from the spoon to the foil. Just rinse clean and wipe dry!
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The reaction between silver sulfide and aluminum takes place when the two are in contact while they are immersed in a baking soda solution. The reaction is faster when the solution is warm. The solution carries the sulfur from the silver to the aluminum. The aluminum sulfide may adhere to the aluminum foil, or it may form tiny, pale yellow flakes in the bottom of the pan. The silver and aluminum must be in contact with each other, because a small electric current flows between them during the reaction. This type of reaction, which involves an electric current, is called an electrochemical reaction. Reactions of this type are used in batteries to produce electricity.
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