When chocolate is heated, thermal energy disrupts its ordered crystalline structure, causing it to melt into a liquid. Because chocolate can contain different types of crystals that melt at unique temperatures, a careful melting process called “tempering” is used to melt all but the most desirable crystal type, known as a beta-crystal. As the liquid chocolate cools after tempering, it recrystallizes using the remaining beta-crystals as a template. The resulting structure is almost entirely made up of beta-crystals, which are what make chocolate bars hard and shiny. Chocolatiers will sometimes add pre-tempered chocolate bits to cooling liquid chocolate to help guide beta-crystal formation in a method called “seeding.” Extreme temperature changes and the presence of other contaminants can interfere with the recrystallization process by preventing the chocolate crystals from organizing properly into a stable crystalline structure, resulting in mushy chocolate.Watch the Science Friday video “Choc Full of Science” to learn about how chocolate is made.
- 4 - 6 small microwave-safe glass bowls or coffee mugs, cleaned and thoroughly dried
- 4 large bars of semi-sweet or dark chocolate—about 60-70% cacao is recommended (no nuts or other add-ins). Set aside a couple of squares, then chop the remaining chocolate into little bits and separate them evenly among the bowls
- A microwave OR a pot of water just smaller than the glass bowls so that one bowl can be set inside the pot without touching the bottom
- 4 - 6 clean and dry spoons or spatulas (non-wooden)
- Cookie sheet lined with parchment paper or wax paper
- Thermometer (optional, but helpful)
- Glass of water
- Pencil and ruler
Safety Considerations: Make sure an adult cuts up the chocolate using a sharp knife and cutting board. The bowls and the melted chocolate inside them might get pretty hot, so experimenters should use hot pads and wear aprons when handling heated bowls of melted chocolate. Restrict this experiment to the kitchen or laboratory to avoid chocolate stains on any fabrics or carpeting.
Melting & Sampling Procedure
Use your pencil and ruler to draw a 2x3-inch grid of three-inch squares on the paper lining your cookie sheet. This is where you will deposit chocolate samples to cool for observation.
Microwave method - Place one of the bowls of chocolate in the microwave, and microwave it on half-power in 30-second bursts, stirring each time, until the chocolate is mostly melted. Remove the bowl from the microwave and stir with the spatula until all the little clumps are gone. The liquid chocolate should be warm, verging on hot, to the touch (about 110oF if using a thermometer).
Double-boiler method - If you are using a double-boiler, melt the chocolate over simmering (not boiling) water, stirring constantly with a clean spatula until the chocolate is mostly melted. Remove from heat and stir with the spatula until all the little clumps are gone. The liquid chocolate should be warm, verging on hot, to the touch (about 110oF if using a thermometer).
- Using the tip of your spatula, scoop out and dribble a small amount of chocolate into one of the squares that you drew on the parchment/wax paper, and use your pencil to label the square with the preparation method. Note: One of these squares should be filled with a control sample of chocolate immediately following Step 2.
Use one of the bowls of crushed chocolate for each of the following modifications (for some, you may be able to reuse the chocolate). After each experiment, label the chocolate sample on the parchment paper with a brief description of how it was prepared.
Seeding - After step #2, let the chocolate cool slightly (to about 89oF), and add a chunk of solid chocolate from your original bar directly into the melted chocolate. Mix well to cool the chocolate. The chunk acts as a “seed” crystal to provide a scaffold for new crystals to grow on and does not need to melt completely. Remove any leftover chunks before putting a sample onto the cookie sheet.
Scorching - In step #2, continue heating the chocolate beyond the point where it has just melted, until it begins to steam or bubble slightly. This will not only melt all crystals in the chocolate but willl begin to break down other molecules in the chocolate as well, causing them to separate and, in some cases, evaporate from the molten chocolate.
Seizing - After step #2, sprinkle some droplets of cold water into the melted chocolate, and keep stirring as it cools. Some of the molecules in chocolate are water-soluble and will dissolve into the added water, while others are hydrophobic (repelled by water) and will clump together.
Shocking - After step #2, rest the bottom of one of the bowls in ice water while stirring the chocolate continuously until it becomes difficult to stir, then continue with step #3.
Marble tempering - After step #2, pour the liquid chocolate out onto a marble slab. Using a clean spatula or metal scraping tool, smooth the chocolate out to help it cool, and then “work” it continuously around the slab with the spatula until it becomes thick and viscous.
DIY method - Think creatively about another way that you could treat the chocolate before it cools. What could you add or do differently that might change the shine, hardness, and texture of the chocolate? If you try your own method, be sure to place a sample on the cookie sheet and label it.
MS-PS1-4: Develop a model that predicts and describes changes in particle motion, temperature, and state of a pure substance when thermal energy is added or removed.
CCSS.ELA-Literacy.RST.6-8.3: Follow precisely a multistep procedure when carrying out experiments, taking measurements, or performing technical tasks.