Spooky Halloween Science: Apple Skull Volcanos

Part 2 of 4 in our Spooky Halloween Science series (find part 1 here)

By Nicole Rhodes

Apples are often in abundance this time of year, and if you walk around any apple tree you are sure to find several on the ground. Grab a few and do this experiment! This is really two experiments in one–if you peel and carve the apple several days in advance, you will be tracking decay, plus you will have an extra spooky-looking volcano with a STEM chemical reaction!

PART 1: Creepy Apple Skulls

Materials

• 2 apples
• Lemon juice
• Carving tools (with grownup help)

Instructions

Peel and carve out two apples as you would a pumpkin, by taking out the core, but leave the bottom intact to hold the “lava” in Part 2. Cut out eyes and a mouth. You can peel the apples to look like a skull, or keep the skin on to watch it wrinkle. Treat ONE apple with lemon juice by painting it on with a pastry brush, or slicing a lemon and rubbing the juice all over the exposed apple flesh. Leave the apples out and expose to air overnight. Observe how they changed. The apple that was treated with lemon juice should be much less brown. 

Why do apples turn brown after you cut them? How does lemon juice stop apples from browning? 

You may have heard that they “oxidize,” but what does that mean? When you cut apples, you open up the plant cells and expose them to oxygen. Oxygen is a catalyst for the enzymes present in the apple cells, causing colorless compounds to start breaking down into brown-colored compounds. At the natural pH of an apple, this happens quickly. If you change the pH of the cut apple by adding an acid, like lemon juice, the enzyme cannot work as well. Therefore, there will be less enzyme activity and less browning of the apple.  The enzymatic process can be broken down as follows:  Polyphenol Oxidase (enzyme in the apple) + Oxygen (in the air) → Melanin (brown color)

PART 2: Volcano Skulls

Materials

• Carved apples from Part 1
• Baking Soda
• Vinegar
• Dish soap (optional)
• Food coloring (optional)

Instructions

Fill each apple with 1 tablespoon of dish soap and 2 tablespoons of baking soda. Add a few drops of food coloring for a colorful foamy volcanic explosion (kids can explore color mixing too!). Slowly pour vinegar over the volcano and watch it “erupt” in a chemical reaction! Kids can continue adding baking soda, vinegar, and food coloring to continue the foamy fun. 

How is this a chemical reaction? Why do bubbles form when baking soda and vinegar combine? Chemical reactions always start and end with chemically different substances. We know a chemical reaction is likely happening when we see a color change, gas produced, heat or light produced, or a new substance forming. This chemical reaction produces carbon dioxide bubbles. This reaction occurs because baking soda is a base, with a pH of 9, and vinegar is an acid, with a pH around 2-3. When a base and acid are mixed together, the negative ions of the base (sometimes called OH- ions) and the positive ions of the acid (H+ ions) are released. These charged particles are reactive, meaning they can break up or change other molecules. In this case, acetic acid (vinegar) and sodium bicarbonate (baking soda) react and form sodium acetate (solid), water (liquid), and carbon dioxide (gas). The chemical reaction is as follows: 

C2H4O2 (vinegar) + NaHCO3 (baking soda) → NaC2H3O2 (sodium acetate) + H2O (water) + CO2 (carbon dioxide)

Want more? This is part 2 of 4. You can also find:

• Part 1 (Gooey Osmosis)
• Part 3 (Spider Lava Lamps)
• And view the live Mass Appeal television segment here.