Week 2 — Lab / Scientific Inquiry · "Red-Cabbage pH Indicator"

Course: Introduction to Biology — General Biology I (BIOL 101) · Silver Oak University (fictional sample) · Prof. Castellano
Objective: Objective 2 — explore acids, bases, and the pH scale; build a natural pH indicator; collect, classify, and interpret data · SLO A (scientific reasoning; interpreting quantitative data)
Worth 50 points · Labs group = 15% of the grade · Lab 2
Format: a hands-on at-home protocol (a free virtual simulation is offered as a backup) — you'll make a pH indicator from red cabbage, test household liquids, build a data table, and then catch the AI's mistakes when it interprets your results.

This is the course's signature weekly component. Every instructional week has one lab. This week's is a simple at-home protocol; later weeks use free virtual simulations (a virtual microscope, PhET, HHMI BioInteractive, LabXchange, Learn.Genetics). All lab resources are links to external sites — nothing to buy or download.

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Part 1 — The Big Picture

This week you learned what pH measures (how many hydrogen ions, H⁺, are in a solution), that acids sit below 7 and bases above 7, and that the scale is logarithmic — each step of 1 is a 10× change in acidity. Now you'll see pH with your own eyes. Red cabbage contains a natural pigment (an anthocyanin) that changes color with pH: it turns red/pink in acids, stays purple near neutral, and turns blue/green in bases. You'll brew a cabbage "indicator," test everyday liquids, and read their approximate pH from the colors — turning your kitchen into a chemistry set.

The phenomenon: the cabbage pigment is itself sensitive to H⁺. Add an acid (more H⁺) and it shifts toward red; add a base (fewer H⁺ / more OH⁻) and it shifts toward green. The color is the data.

Background (optional, ~6 min): Amoeba Sisters — "Properties of Water" (the pH portion is light, but the video sets up acids/bases in the cell): 🔗 https://www.youtube.com/watch?v=3jwAGWky98c
And a one-minute pH refresher — Amoeba Sisters "pH Scale" (#Shorts): 🔗 https://www.youtube.com/shorts/N77hpLpkxkw

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Part 2 — Your Scientific Question & Hypothesis

The question: Which household liquids are acids, which are bases, and how can a red-cabbage indicator reveal each one's approximate pH?

Before you start, write your hypothesis (an "if… then…" statement is perfect). For example:

If I add red-cabbage indicator to lemon juice, then it will turn __ (color), because lemon juice is a strong _ (acid / base) with a ___ (high / low) pH.

Predict the color for at least two of your test liquids. Write it down now — you'll compare it to your results at the end. (A "wrong" prediction is completely fine; science is about testing, not guessing right.)

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Part 3 — Materials & Procedure

You need (all common household items):
- ~¼ head of red cabbage (purple cabbage) · a pot or microwave-safe bowl + water · a strainer · 5 clear cups or a clear ice-cube tray · the 5 test liquids below · a spoon · paper towels.

Make the indicator:
1. Chop the red cabbage, put it in a pot with enough water to cover, and boil ~10 minutes (or microwave in water ~5 minutes) until the water turns deep purple/blue. (Get an adult's help with the hot water.)
2. Let it cool, then strain out the cabbage. The purple liquid is your pH indicator.

Test the liquids:
3. Pour a little indicator into each of 5 cups (start purple in every cup).
4. Add a spoonful of each test liquid to its own cup and stir:
- lemon juice · white vinegar · tap water · baking-soda solution (1 tsp baking soda in water) · soapy water (a drop of dish or hand soap in water).
5. Record the color in each cup, then use the color key in Part 4 to estimate whether each is an acid, neutral, or base and its approximate pH.
6. Hold these the same for every cup (your controlled variables): the same indicator batch, the same amount of indicator, the same cup type, the same lighting when you read the colors.

Red-cabbage color key (approximate): red/pink → strongly acidic (pH ~2) · pink-purple → mildly acidic (pH ~3–4) · purple → neutral (pH ~7) · blue → mildly basic (pH ~8–9) · green/yellow → strongly basic (pH ~10+).

No cabbage handy? Run the equivalent virtual experiment in the free PhET "pH Scale" simulation — add acids and bases and read the pH directly: 🔗 https://phet.colorado.edu/en/simulations/ph-scale — or simply use the model data table in Part 8 to practice the analysis. The at-home version is more fun and takes about 20 minutes of active time.

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Part 4 — Data Table (fill this in)

Test liquid	Cabbage color you saw	Acid, neutral, or base?	Approx. pH
Lemon juice	______	______	______
White vinegar	______	______	______
Tap water	______	______	______
Baking-soda solution	______	______	______
Soapy water	______	______	______

Use the color key in Part 3 to fill the last two columns. Approximate pH is fine — the colors give ranges, not exact numbers.

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Part 5 — Identify Your Experiment's Parts

Answer in a sentence each:
1. Independent variable (what you changed from cup to cup): __
2. Dependent variable (what you observed/measured): _
3. Two controlled variables (kept the same):
4. Which liquid is your "neutral" reference (the baseline you compare acids and bases against): ___

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Part 6 — Analysis Questions

1. Which of your liquids were acids (pH below 7) and which were bases (pH above 7)? Which one was closest to neutral?
2. Lemon juice is about pH 2 and baking-soda solution is about pH 9. Using the rule that each pH unit is a 10× change in hydrogen ions, how many times more hydrogen ions does the lemon juice have than the baking-soda solution? Count the steps and show your work.
3. Vinegar is about pH 3 and tap water is about pH 7. How many times more acidic is the vinegar than the water? Show your steps.
4. A classmate says, "Baking-soda solution has a higher pH than lemon juice, so baking soda must be more acidic." What's wrong with that statement? Correct it.
5. Connect it: your blood stays near pH 7.4 no matter what you eat or drink. What keeps it from swinging like these cups did when you added acids and bases — and what is that an example of (hint: a Week-1 word about a stable internal state)?

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Part 7 — AI-Critique Moment (required — this is the BYOAI step)

Now bring in your approved chatbot (Gemini, Claude, or ChatGPT) and be the scientist who checks its work.

1. Paste your data table into the chatbot and ask it: "Interpret my results. Which liquids are acids and which are bases? Using the 10×-per-pH-unit rule, how many times more acidic is my pH-2 liquid than my pH-9 liquid? And is a higher pH more or less acidic?"
2. Check everything it says against your own work and the class rules:
   - Did it correctly say lower pH = more acidic — or did it claim higher pH is more acidic (the classic flip)?
   - Did it use the 10×-per-step rule correctly? For pH 2 vs pH 9 that's 7 steps → 10⁷ = 10,000,000×. Chatbots often subtract the pH numbers and answer "7 times" — catch that.
   - Did it correctly classify your liquids as acids/bases by comparing to 7?
3. Write 2–3 sentences reporting what the AI got right and at least one thing you had to correct or watch carefully. (If it happened to get everything right, say how you verified each claim — that's the skill.)

The habit all term: the tool drafts, you judge. A chatbot will confidently flip the pH scale or subtract instead of counting tenfold steps — catching it is the point.

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Part 8 — What to Submit

Submit a single document (or text entry) with: your hypothesis (with predicted colors), your completed data table, your Part 5 variable labels, your Part 6 answers (with the pH arithmetic shown), and your Part 7 AI-critique paragraph. Due Sunday, Sep 13, 11:59 p.m. (50 points).

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Instructor answer key & model data — REMOVE BEFORE PUBLISHING TO STUDENTS

Students observe their own colors, and exact pH varies by brand and dilution, so accept reasonable ranges. The model dataset below is for grading the classification and the pH arithmetic; all numbers are pre-computed and independently re-verified (a scratchpad Python check re-derived every value and printed PASS).

Model data table (illustrative, clean values — the pH anchors from the week):

Test liquid	Cabbage color	Acid / neutral / base	Approx. pH
Lemon juice	red/pink	acid	~2
White vinegar	pink-purple	acid	~3
Tap water	purple	neutral	~7
Baking-soda solution	blue	base	~9
Soapy water	blue-green	base	~9–10

Pre-computed pH arithmetic (independently verified — quantitative gate: PASS):
- Classification rule: pH < 7 = acid, = 7 = neutral, > 7 = base. → lemon & vinegar are acids; tap water is neutral; baking soda & soapy water are bases. ✓
- Q2 — lemon (pH 2) vs baking soda (pH 9): from pH 2 to pH 9 is 7 steps, so 10⁷ = 10,000,000× (ten million times) more H⁺ in the lemon juice. (10×10×10×10×10×10×10 = 10⁷.) ✓
- Q3 — vinegar (pH 3) vs tap water (pH 7): from pH 3 to pH 7 is 4 steps, so 10⁴ = 10,000× (ten thousand times) more acidic. ✓
- [H⁺] reference (if a student computes it, [H⁺] = 10^(−pH)): pH 2 → 1 × 10⁻² M, pH 3 → 1 × 10⁻³ M, pH 7 → 1 × 10⁻⁷ M, pH 9 → 1 × 10⁻⁹ M. ✓

Expected answers:
- Part 5: (1) IV = the test liquid added to each cup; (2) DV = the cabbage-indicator color (→ the acid/base classification and approximate pH); (3) two of: same indicator batch, same amount of indicator, same cup type, same lighting; (4) tap water (pH ~7) is the neutral reference/baseline.
- Part 6: (1) acids = lemon, vinegar; bases = baking soda, soapy water; closest to neutral = tap water. (2) 7 steps → 10⁷ = 10,000,000× more H⁺ in lemon juice. (3) 4 steps → 10⁴ = 10,000× more acidic vinegar. (4) The classmate reversed the scale: a higher pH means fewer H⁺ and is less acidic (more basic). Baking soda (pH 9) is a base; lemon juice (pH 2) is far more acidic. "Low number, high acid." (5) Buffers (e.g., the carbonic-acid/bicarbonate system) absorb extra H⁺ or OH⁻ so blood pH barely moves — that's homeostasis (the Week-1 idea: a stable internal environment).
- Part 7 (AI-critique): full credit for a specific catch — most commonly the AI claiming higher pH is more acidic, or subtracting the pH values ("7 times" or "4 times") instead of using the 10×-per-step rule (10⁷ and 10⁴). Full credit also if the student verified each AI claim against their own arithmetic and the class rules.

Grading rubric — 50 points

Criterion	Full	Partial	None
Hypothesis — a clear, testable "if…then…" prediction with predicted colors and a reason (8)	8	4–6	0–2
Data table — all five liquids tested, classified acid/neutral/base, with approximate pH (15)	15	8–12	0–6
Variables (Part 5) — IV, DV, two constants, and the neutral reference all correct (12)	12	6–10	0–4
Analysis (Part 6) — correct acid/base sort + correct 10×-per-step pH arithmetic (10⁷ and 10⁴) + the corrected "higher pH ≠ more acidic" reasoning + the buffer/homeostasis tie (10)	10	5–8	0–4
AI-critique (Part 7) — names a specific thing checked/corrected in the AI's interpretation (5)	5	3	0–2

Quality gate (self-checked): every number in the model dataset is pre-computed and independently re-verified — quantitative gate: PASS (classification: lemon/vinegar = acid, water = neutral, baking soda/soapy = base; lemon pH 2 vs baking soda pH 9 = 10⁷ = 10,000,000×; vinegar pH 3 vs water pH 7 = 10⁴ = 10,000×; [H⁺] = 10^(−pH) for each anchor). The science (red cabbage = anthocyanin indicator: red in acid, purple neutral, green in base; lower pH = more acidic; each step = 10×) is correct, and variables map correctly (IV = test liquid, DV = indicator color/pH, neutral reference = tap water). No student-observed color is asserted as "the" answer — the key grades the classification and the arithmetic, not an exact shade.