**LAB ACTIVITY: Rainbow Chemistry — Exploring Acids and Bases with Cabbage Indicator** **Grade:** 7th Grade **Duration:** 55–65 minutes **Standard Alignment:** NGSS MS-PS1-2 — Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occurred. --- **LEARNING OBJECTIVE:** Students will be able to use red cabbage juice as a pH indicator to classify household substances as acidic, basic, or neutral based on observable color changes, and explain their classifications using evidence from collected data. --- ### ⚠️ SAFETY CHECKLIST ⚠️ Do not taste, drink, or smell any substance directly — waft only ⚠️ Wipe up spills immediately — wet floors are a slip hazard ⚠️ Handle cabbage juice carefully — it stains clothing and skin ⚠️ Keep baking soda and vinegar mixing controlled — add slowly to avoid overflow ⚠️ Wash hands thoroughly with soap and water before and after the lab **Required PPE:** Safety goggles (required); lab aprons recommended if available --- ### MATERIALS (per group of 3–4 students) - Red cabbage — ½ cup shredded or chopped - Hot water (pre-prepared by teacher) — 1 cup - Vinegar (white or apple cider) — 3 tablespoons - Baking soda — 2 teaspoons - Lemon juice — 3 tablespoons - Dish soap (clear or light-colored) — 3 tablespoons - Clear plastic cups — 6 - Plastic spoons — 6 (one per substance + one for indicator) - Permanent marker — 1 per group - Strainer or paper towels (for filtering cabbage) — 1 per group - Lab recording sheet — 1 per student - Paper towels — small stack *Teacher prep: Soak shredded cabbage in hot water for 10–15 minutes before class, then strain. Pre-pour into small pitchers or cups for easy distribution. This can be done the period before.* --- ### PRE-LAB QUESTIONS *(Students answer individually before touching materials)* 1. **Prior knowledge:** What do you already know about acids and bases? Give one example of each from everyday life and explain how you know which category it belongs to. 2. **Prediction:** You are about to dip a purple cabbage juice into five different liquids. What do you think will happen to the color? Will every liquid cause the same change? Explain your reasoning. 3. **Variables:** In this experiment, identify the independent variable (what you are changing), the dependent variable (what you are measuring or observing), and at least two controlled variables (what you are keeping the same). --- ### PROCEDURE 1. Put on your safety goggles before handling any materials. Keep them on for the entire lab. 2. Use a permanent marker to label each of your 6 clear cups: **Indicator**, **Vinegar**, **Baking Soda**, **Lemon Juice**, **Dish Soap**, **Water**. 3. Pour approximately 3 tablespoons of red cabbage indicator juice into the cup labeled **Indicator**. Observe and record its starting color in your data table. 4. Pour 3 tablespoons of vinegar into the cup labeled **Vinegar**. 5. Add 1 teaspoon of baking soda to the cup labeled **Baking Soda**, then add 3 tablespoons of water and stir until mostly dissolved. 6. Pour 3 tablespoons of lemon juice into the cup labeled **Lemon Juice**. 7. Pour 3 tablespoons of dish soap into the cup labeled **Dish Soap**. 8. Pour 3 tablespoons of plain water into the cup labeled **Water**. This is your control. 9. Using a clean spoon, add 1 tablespoon of cabbage indicator juice to the **Vinegar** cup. Stir gently for 5 seconds. Record the color you observe immediately in your data table. 10. Repeat Step 9 for each remaining cup — **Baking Soda**, **Lemon Juice**, **Dish Soap**, and **Water** — using a clean spoon each time. 11. After recording all colors, compare your cups side by side. Arrange them in order from the one that changed the least to the most dramatic color change. 12. Using the color key below, classify each substance as **Acid**, **Base**, or **Neutral**: - **Pink/Red** = Acid - **Purple** = Neutral - **Green/Yellow** = Base 13. Record your classifications in the final column of your data table. 14. Do not pour anything down the drain until your teacher gives the signal. Clean up by rinsing cups with water and wiping your workspace with paper towels. --- ### DATA COLLECTION TABLE | Substance | Starting Color of Indicator | Color After Adding Indicator | Classification (Acid / Base / Neutral) | |-----------|----------------------------|------------------------------|----------------------------------------| | Cabbage juice alone | | (reference only) | Neutral reference | | Vinegar | Purple | | | | Baking soda solution | Purple | | | | Lemon juice | Purple | | | | Dish soap | Purple | | | | Plain water | Purple | | | **Observation log — record any other changes you notice (bubbling, smell, texture):** | Substance | Additional Observations | |-----------|------------------------| | Vinegar | | | Baking soda solution | | | Lemon juice | | | Dish soap | | | Plain water | | --- ### POST-LAB ANALYSIS 1. **Recall — What happened?** List each substance and the color the indicator turned. Which substances were acids? Which were bases? Which were neutral? Use your data table as evidence. 2. **Analysis — Why did it happen?** The cabbage juice contains a pigment called anthocyanin that changes shape when it contacts acids or bases. Based on what you observed, what do you think acids do to this pigment compared to bases? Use at least two pieces of evidence from your data to support your answer. 3. **Real-world application — Where does this matter?** Your stomach uses acid to digest food. Your small intestine uses a more basic environment. Why might it be a problem if your stomach became too basic — or your small intestine became too acidic? Connect your lab findings to this biological process. 4. **Extension — What if you changed a variable?** You tested one concentration of each substance. If you doubled the amount of vinegar while keeping the cabbage juice amount the same, predict what would happen to the color and explain your reasoning. What does this suggest about the relationship between concentration and pH? --- ### TEACHER NOTES **Common student mistakes:** - Using the same spoon across multiple cups and contaminating results — remind students before Step 9 that each cup gets its own clean spoon - Adding too much baking soda before dissolving it, causing the color change to appear weaker than expected — pre-dissolving helps - Recording color names inconsistently ("kind of pink" vs. "dark red") — model how to describe color precisely before students begin **Troubleshooting:** - *Indicator isn't changing color:* Cabbage juice may be too diluted. If the starting color looks light lavender instead of deep purple, the soaking time was too short or water ratio too high — brew a new batch with less water or longer soak time - *Baking soda cup doesn't turn green:* Concentration may be too low — add an extra half-teaspoon of baking soda and stir again - *Colors all look similar:* Room lighting affects perception — move groups near a window or hold cups against white paper for comparison **Differentiation:** - *Struggling students:* Provide a completed color key card with photos of expected colors at each pH level so they focus on comparison rather than memorization; reduce post-lab to questions 1 and 2 only - *Advanced students:* Challenge them to create a numbered pH scale (1–14) and estimate where each substance falls based on color intensity; ask them to design a follow-up experiment to test a substance of their choice and write a hypothesis before running it --- **Verification:** - ✅ Safety checklist is specific to cabbage, vinegar, baking soda, and dish soap — not generic - ✅ All materials listed with quantities per group - ✅ Procedure steps are numbered, specific, and substitute-friendly - ✅ Data collection table matches the 5-substance comparison format - ✅ Post-lab includes recall (Q1), analysis (Q2), application (Q3), and higher-order extension (Q4)