Use the pH scale and a homemade natural indicator to classify common household substances as acids, bases, or neutral.
The pH scale runs from 0 to 14 and measures how acidic or basic a substance is. Substances below 7 are acids, often tasting sour and able to react with metals and with carbonates like baking soda; substances above 7 are bases, often tasting bitter and feeling slippery; exactly 7, like pure water, is neutral. Rather than tasting unknown substances, which is not safe, chemists use indicators: substances that change color at different pH levels.
Red cabbage juice is a natural indicator you can make safely at home, thanks to a pigment called anthocyanin that changes color across the pH range: reddish-pink in a strong acid, purple around neutral, and blue-green to yellow in a base. Common household acids include vinegar, lemon juice, and soda; common household bases include baking soda dissolved in water, soap, and ammonia-based cleaners. Testing a substance with an indicator, rather than guessing from taste or feel alone, is real evidence-based chemistry.
Mastery looks like: They can classify tested household substances as acidic, basic, or neutral based on indicator color and explain why an indicator is safer than direct tasting.
Common stumbles: Assuming every safe-to-eat substance must be neutral; forgetting that indicator color, not personal taste, is the evidence being used to classify a substance.