How does pH of Food help Prevent Tarnish onCopper Pots?

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Author: Janice VanCleave

What Effect Does the pH of Food Have on Preventing Tarnish on Copper Pots?

Category: Chemistry—Chemical Changes

Project Idea by: Saba Javadi and Kate Foley

A chemical is any substance with a definite composition made up of one or more elements (substances that contain only one kind of atom). Compounds are chemicals made up of two or more different elements. Examples of elements include the metal copper (Cu) and the gas oxygen (O), which is found in air. An example of a compound is a chemical made up of copper and oxygen: called copper oxide (CuO).

The formation of copper oxide involves a chemical change, which is the change of one or more substances to something new.

Another name for a chemical change is a chemical reaction. Chemicals that are changed during chemical reactions are called reactants, and chemicals produced in chemical reactions are called products. If a chemical reaction involves the combination of oxygen with another element, the reaction is called oxidation and the product is called an oxide. In the formation of copper oxide, copper and oxygen are the reactants and copper oxide is the product.

The atoms on the surface of metals often combine with oxygen atoms from the air to form metal oxides. Copper surfaces, such as those on some cooking pots and coins, are often coated with different kinds of copper oxides. Any coating on a metal that discolors and/or dulls on the surface is called a tarnish. The copper oxides on coins or other copper surfaces are considered a tarnish.

What Effect Does the pH of Food Have on Preventing Tarnish on Copper Pots?

Tarnish is the result of a chemical change, and it can be removed by a chemical change such as occurs with an acid. An acid is a sour-tasting chemical that forms salt and water when mixed with a base (a bitter-tasting chemical, including metal oxides). Copper oxide can be chemically removed from a copper surface by soaking it in an acid such as vinegar or lemon juice. The special scale for measuring the strength of an acid or base is called the pH scale. The values on the pH scale range from 0 to 14. Water is neutral (neither an acid nor a base) and has a pH value of 7. Acids have a pH of less than 7, and bases have a pH greater than 7. The less the pH, the more concentrated the acid. The greater the pH, the more concentrated the base. A project question might be, "What effect does the pH of food have on preventing tarnish on copper pots?"

Clues for Your Investigation

Design a way to test the effect of acidic foods on the removal of tarnish from copper. One way is to use tarnished pennies and foods with different pH values. You do not have to know the exact value of the testing solution, just that the pH is different. For example, some vinegar has a pH of about 2.8 and distilled water is always 7. So vinegar and water can be mixed in different concentrations to produce solutions of different pH values. Tarnished pennies can be placed in the different acid solutions for a predetermined amount of time. Determine which solution cleans the coins faster. CAUTION: Even though vinegar is a food, it is an acid, so try not to get it on your skin. If you do get some on your skin, rinse it off thoroughly with water. While you perform the experiment, wear goggles to protect your eyes.

Independent Variable: pH of acid solution

Dependent Variable: Removal of tarnish from copper

Controlled Variables: Degree of tarnish on coins, number of coins for each test, temperature of solutions, time in testing solutions

Control: Water

Other Questions to Explore

  1. Would other acidic foods work as well as the vinegar? (critric acid is a weak acid is a weak acid found in citrus fruit such as lemon, lime, and grapefruit.)
  2. What effect does temperature have on the speed of the reaction?

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