Make your coins green!

EQUIPMENTS:

1. Half cup of lemon juice
2. One tablespoon of table salt
3. A drinking glass or a transparent jar
4. Some dirty pennies or coins made of copper or copper coated (Such as American pennies)

INSTRUCTIONS:

1. At first, mix the lemon juice and the table salt inside the drinking glass or a transparent jar.
2. Then add a few dirty copper coins (Such as American pennies) or just pieces of copper inside.
3. After that, let those coins stay in the jar for about five minutes.
4. Then take those coins out and let them dry on a dry paper towel or any clean plain surface.
5. Leave those coins or copper pieces to dry for overnight.
6. Examine those dried coins the next day and record any changes that appear on them.

RESULT: 
Something green and solid should cover the copper coated coin.

EXPLANATIONS:
In the article “Make your coins as shiny as new!” we used lemon juice to clean dirty old coins. But in this experiment, the dirty old coins get covered in green coating.
Now the question is, how can the same chemicals produce completely two different results?
The last step of “Make your coins as shiny as new!” experiment was washing/rinsing the soaked coins in running water. But in this experiment, we won’t wash/rinse those coins. Now that's what creates the main difference in these two experiments. Instead of washing, the mixer of lemon juice and table salt are allowed to dry on the coins. Copper can react with the oxygen to form copper oxide. When the copper coated coin gets old, the copper coating of the coin reacts with the oxygen of the air and produces copper oxide.

CuO (Solid) + H^+ ----------> Cu^2+ + H20
Copper oxide + Hydrogen ion ----------> Copper ions + Water

When lemon juice-salt mixer is allowed to dry on such a coin, the mixer simply dissolves the copper oxide. But without the washing/rinsing, another chemical reaction takes place.

The copper atoms reacts chemically with oxygen from the air again and at the same time also reacts with chlorine from the salt. Reaction with both the oxygen and chlorine creates a blue-green compound. This compound is called Malachite.

MORE EXPLANATIONS:
Malachite is one of the oldest known green pigments mankind has ever known. More than six thousand years ago ancient Egyptians used to use malachite as a makeup material. Around AD 900, the ancient Chinese began to use malachite. They used ground malachite mixed with water to make painting colors for painting.

MORE EXPERIMENTS FOR YOU:

1. Try this experiment by not adding salt into the lemon juice.
2. Try using vinegar to shine the copper coins.
3. Try other kinds of less powerful acids to do the experiment.