Acids

Green Pennies

by

As discussed in the enzyme experiment, chemical reactions occur when separate atoms or molecules interact and combine their molecular building blocks to create new and different compounds. The addition of enzymes to a reaction will speed the reaction up, but even without the enzyme, the reaction will still occur. It will just take a little more time.

Displacement or Replacement Reactions are those that when molecules from one substance or compound change place with molecules from another substance or compound, forming a third, different substance or compound. Oxidation is a good example of a replacement reaction. When iron interacts with water and oxygen, the oxygen from the air and the water combines with the iron to form iron oxide (rust). Other molecules present in the air and the metal will combine with the hydrogen to produce an acid. This acid seeps into the microscopic spaces in the iron and opens them up, like tunnels, to allow more oxygen to penetrate into the metal, allowing the metal to oxidize even further. Left alone for a long period of time, the iron can be completely oxidized, until there is nothing but rust left.

In this experiment, we will create a displacement reaction on the surface of a penny.

The Experiment

Supplies: Paper towel, four or five pennies, vinegar, and a non-metal saucer or bowl. A plate with a raised edge or a shallow baking dish works well. Dark or “dirty-looking” pennies may work better than bright shiny ones. Try to use an assortment of pennies, if possible.

What to do: Fold the paper towel in half and fold again into a square. Place the paper towel in the dish. Pour enough vinegar onto the paper towel to thoroughly wet it, but without creating a pool of vinegar. Place the pennies on the paper towel and fold the towel loosely over onto the pennies. Wait 24 hours. Observe what happens. What happened to the pennies? What happened to the paper towel?

What is happening: Modern pennies have a zinc core and a copper alloy exterior, while older pennies are entirely copper alloy. Alloy is just a fancy word meaning a mixture of metals, and “copper alloy” means that the main metal in the mixture is copper. The dark, dirty-looking material on the dingy pennies is actually copper oxide (copper’s version of “rust”). The vinegar penetrated the copper oxide and disolved it slightly (decomposition reaction), freeing the copper to interact with the oxygen in the air. The copper, vinegar, and air combined to create copper acetate, which is green. The copper acetate is a substance completely separate from the penny, the paper towel, and the vinegar. This explains why it was able to settle onto both the paper towel and the penny.

Left on its own, copper will eventually change in appearance, from its shiny orange-red color to the green that appeared on the pennies. When construction began on the Statue of Liberty, back in 1875, Lady Liberty was not the dull, pale green she is today. She was shiny and orange-red, like a new penny. Exposure to the air, combined with acid rain (rain containing pollutants that lower the pH of the water, causing it to be slightly acidic), has caused a patina (surface discoloration) of copper acetate to form on the statue’s surface. This infographic demonstrates the color change over time. Notice how, during the first several years, the color of the statue was closer to that of an old penny?

The Statue of Liberty is almost 150 years old. She has aged quite nicely!

Links

For a variation in this experiment, head to The Exploratorium. Their experiment, involving pennies, vinegar, and salt, actually creates free-floating copper in the vinegar!

Orange Fizz

by

As demonstrated in the Make Carbon Dioxide experiment, acids and bases do not play well together. That experiment used vinegar and baking soda to create a reaction. In this experiment, the vinegar is replaced with something a little more mild (and tasty) – citric acid.

Citric acid is an acid found in citrus fruits like lemons, limes, grapefruits, and oranges, and in other fruits and vegetables like strawberries, raspberries, and tomatoes. It is a safe acid, and it’s what gives oranges, lemons, and limes their tartness. Different citrus fruits have different concentrations of citric acid. Lemons and limes have high concentrations, while oranges and grapefruits have lower concentrations. The higher the concentration of citric acid, the more sour-tasting a fruit will be. That is why lemon juice tastes sour and zingy while orange juice tastes sweet and tangy.

The Experiment

Supplies: an orange or clementine (cutie), some baking soda

What to do: Cut the orange into wedges or peel & separate into slices. Take a bite of the orange all by itself. Dip a second slice LIGHTLY into the baking soda. Take a bite. As you chew, it should start to bubble in your mouth. It might even taste like orange soda!

What is happening: Oranges and other citrus fruits contain citric acid. Baking soda is a base, the opposite of an acid. It is safe to eat but doesn’t taste very good on its own. As the citric acid and baking soda mix, it makes millions of carbon dioxide bubbles, the same gas you breathe out, and the same one that makes soda so fizzy.

Take it a step further: Try repeating this experiment using different fruits and vegetables. Compare what the flavors taste like and how fizzy the reaction is in your mouth. Did the juice fizz more or less that others you tried? Is there a relationship between how fizzy the bite with baking soda was compared to how sweet or sour the plain bite was?

Links

To see this experiment in action, head to YouTube and explore Science Fun for Everyone.

Make Carbon Dioxide

by

Carbon dioxide is a clear, odorless gas that occurs naturally in our environment. It is one of the most important gases on Earth because it is one of the components necessary for plant survival. Trees and plants need carbon dioxide and water to make their own food, and they give off oxygen in the process. Humans need the oxygen to breathe and many of our food sources are from plants, so with out plants, we would suffocate and starve! Just like plants take in carbon dioxide and give off oxygen, humans take in oxygen and give off carbon dioxide. This kind of mutually beneficial relationship is called symbiosis.

Chemistry is the study of different substances and how they interact. Chemical compounds fall into different categories, depending upon where they fall on the pH scale. pH stands for “potential of hydrogen,” and the pH scale measures how much hydrogen is contained within any given compound. Vinegar is an acid. Acids fall below 7 on the pH scale. They tend to have a sour taste and can cause a burning sensation in nasal passages when smelled. Acids are sticky and react with with metals. Baking soda is a base, meaning that it falls above 7 on the pH scale. Bases are generally odorless and have a bitter taste. They tend to be slippery and they react with fats and oils. Water is neutral, meaning it falls right in the middle of the pH scale at 7. When acids and bases combine, the reaction is a volatile one, but the acids and bases can balance each other out, resulting in compounds that are closer to neutral. This experiment will combine common substances to create an interaction that will produce carbon dioxide. Carbon dioxide, or CO2, is the same gas that is used to make soda fizzy, so releasing CO2 can cause bubbles!

The Experiment

Supplies: a tall glass, baking soda, vinegar, liquid dish soap, tap water, food coloring (optional).

What to do: DO THIS EXPERIMENT OUTSIDE OR IN THE SINK. Fill the glass half full of water. Add a tablespoon of baking soda and five drops of detergent. Add 2-3 drops of food coloring, if you want. Stir well to combine ingredients. Last, add a quarter cup of vinegar. What happened when the vinegar was added?

What is happening: Water has hydrogen and oxygen as its elemental building blocks. Baking soda and vinegar both have carbon, hydrogen, and oxygen, but the proportion of “ingredients” in each of these chemical compounds is different. Baking soda also has a fourth ingredient – sodium. When all the atoms from all the compounds are allowed to combine, they react violently. The bonds holding the atoms of each compound break, allowing the atoms to reorganize into new compounds. The hydrogen from the vinegar interacts with the sodium and the carbon from the baking soda and forms two new substances – sodium acetate (a salt) and carbonic acid (a liquid). Carbonic acid is highly unstable, so even as the carbonic acid is being formed, it is also breaking apart. Immediately the atomic bonds in the carbonic acid break down, resulting in liquid water and carbon dioxide gas. The gas is lighter than the water, so it moves upward through the liquid in the form of bubbles!

Links

For a more detailed discussion of this experiment, visit the Wonderopolis website.