Tears and Transformations: The Chemistry of Onions
Students will be able to analyze and explain the chemical changes that occur when onions are cut, boiled, and caramelized by conducting guided experiments, interpreting pH and sensory data, and using scientific models and vocabulary (e.g., enzymatic reactions, thermal decomposition, Maillard reaction) to distinguish between physical and chemical changes.
Overview
Students investigate the chemical changes that occur when onions are cut, boiled, and caramelized. They begin with a video on why onions make people cry and record observations of pre-cut onions, deciding whether the change is chemical or physical. In the lab, they observe fogging on a mirror, test acidity with pH strips, and rotate through boiled and caramelized onion stations. They then read an article, answer claim-evidence-reasoning questions on the enzyme allinase and thermal decomposition, and complete a molecule-mapping flowchart. Students choose a mini-poster, an onion and garlic lab proposal, or a podcast or video, followed by a summative assessment. Engagement totals 19 minutes, exploration is two 20-25 minute labs, explanation 45-50 minutes, and elaboration is one 60-90 minute activity.
Lesson Objectives
- Analyze and explain the chemical changes that occur when onions are cut, boiled, and caramelized by conducting guided experiments, interpreting pH and sensory data, and using scientific models and vocabulary (e.g., enzymatic reactions, thermal decomposition, Maillard reaction) to distinguish between physical and chemical changes.
Standards
NGSS · 3
- HS-PS1-2Construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties. ➤ Relates to identifying chemical changes during cutting, boiling, and caramelizing onions.
- HS-PS1-4Develop a model to illustrate that the release or absorption of energy from a chemical reaction system depends on the changes in total bond energy. ➤ Connects to energy changes in caramelization and boiling reactions.
- HS-PS1-5Apply scientific principles and evidence to provide an explanation about the effects of changing the temperature or concentration of the reacting particles on the rate at which a reaction occurs. ➤ Students compare the rate of reaction at different heat levels (caramelization vs. boiling).
CCSS Math · 1
- HSN-Q.A.1Use units to understand problems and guide the solution of multi-step problems; define appropriate quantities for the purpose of descriptive modeling; choose a level of accuracy appropriate to limitations of measurement. ➤ Used when measuring pH, time of heating, temperature.
CCSS ELA · 2
- RST.9-10.1 / RST.11-12.1Cite specific textual evidence to support analysis of science and technical texts. → Students analyze diagrams or readings on sulfur compounds and cooking chemistry.
- RST.9-10.4 / RST.11-12.4Determine the meaning of symbols, key terms, and domain-specific words and phrases. → Interpreting scientific vocabulary like “sulfoxide,” “volatile,” “Maillard reaction.”