Week 2 Biology Lab & Inquiry: Red-Cabbage pH Indicator

Part 1 — The Big Picture

This week you learned that pH measures hydrogen ions, that a lower number means more acid, and that every whole step on the scale is a tenfold change. In this lab you turn a red cabbage into a pH detector. Its pigments change color with pH, turning toward red or pink-purple in acids and toward blue-green or greenish-yellow in bases. You run it as a controlled test, sort five household liquids as acid, neutral, or base against a tap-water reference cup, then count tenfold steps on a set of reference readings. Finally, you check a chatbot's account of your lab and catch where it reads the scale like a ruler, flips a label, or invents a number.

Time budget: Plan for about 120 minutes.

Part 2 — The Task

Guiding question: Which of five household liquids are acids, which are bases, and how many times more hydrogen ions does one liquid carry than another when their pH values differ by whole steps?

Before you begin — hypothesis: Before you add any indicator, pick two of your liquids and write your hypothesis in the if… then… because… form: if the indicator is added to each, then one will turn toward red and the other toward blue-green or stay purple, because of a reason about acids and bases you can state. Then write the prediction your hypothesis makes for each liquid's class.

Workshop data: Your own red-cabbage color observations, the illustrative reference cups, and the illustrative fallback observations

Source: Color observations you collect yourself at a kitchen counter with the procedure in Part 3, plus five illustrative reference-cup pH readings printed in Part 4 for the step-counting table; if you cannot brew the indicator, use the illustrative fallback observations, which were constructed for this lab and are not real data.

• Rows: 6 cups observed (5 household liquids and 1 tap-water reference cup), shown as 6 rows of the observation table; the first scaffold table has 6 rows that name the parts of the test rather than data, and the step-counting table has 4 rows
• Columns: cup, liquid, indicator color in words, and class (acid, neutral, or base)
• Reference readings: 5 illustrative pH readings (Cups P to T) used only for step counting

Part 3 — Materials & Procedure

Tools and materials

• Free virtual labs and simulations (PhET, HHMI BioInteractive, Learn.Genetics), a virtual microscope, and a spreadsheet (Google Sheets or any free spreadsheet)
• Half a red cabbage, a pot or heat-safe bowl, hot water, a strainer, six clear cups, a spoon, and household liquids such as lemon juice, white vinegar, tap water, baking-soda water, and soapy water
• A document or notebook for your written answers

Ways of working

• virtual-simulation-inquiry
• at-home-protocol-and-data-table
• figure-and-diagram-interpretation

Procedure

1. Chop half a red cabbage into small pieces, cover them with hot water in a pot or heat-safe bowl, and let them soak for about 10 minutes, stirring now and then. Pour the liquid through a strainer into a clean cup and let it cool; this purple liquid is your indicator. Tool: Half a red cabbage, a pot or heat-safe bowl, hot water, a strainer, six clear cups, a spoon, and household liquids such as lemon juice, white vinegar, tap water, baking-soda water, and soapy water. Check: A cup of cooled purple indicator liquid with the cabbage pieces strained out.
2. Before testing, write your hypothesis and prediction in the first table in Part 4, and fill in the independent variable, the dependent variable, the controlled variables, and the reference cup. Tool: A document or notebook for your written answers. Check: The first table has a hypothesis in if… then… because… form and every part of the test named.
3. Label six clear cups 1 to 6. Pour the same small amount of tap water into cup 1 as your reference cup, and the same amount of each household liquid into cups 2 to 6. Stir a spoonful of baking soda into water for one cup and a few drops of dish soap into water for another. Tool: Half a red cabbage, a pot or heat-safe bowl, hot water, a strainer, six clear cups, a spoon, and household liquids such as lemon juice, white vinegar, tap water, baking-soda water, and soapy water. Check: Six labeled cups holding equal amounts of liquid, with tap water in cup 1.
4. Add the same amount of indicator, one spoonful, to every cup, rinsing the spoon between cups. Hold each cup against a sheet of white paper beside the reference cup and record its color in words in the observation table, then sort it as acid, neutral, or base. Tool: Half a red cabbage, a pot or heat-safe bowl, hot water, a strainer, six clear cups, a spoon, and household liquids such as lemon juice, white vinegar, tap water, baking-soda water, and soapy water. Check: Six colors recorded in words and six classes, each judged against the tap-water reference cup.
5. Open a blank sheet. Type the five reference readings 5, 6, 8, 10, and 11 into cells A2 to A6. Count the whole-unit steps for each comparison in the step-counting table by hand first, then type =10^3 into a cell to check a three-step comparison, and use the same pattern for the others. Tool: Free virtual labs and simulations (PhET, HHMI BioInteractive, Learn.Genetics), a virtual microscope, and a spreadsheet (Google Sheets or any free spreadsheet). Check: Each spreadsheet power of ten matches your hand count; =10^3 returns 1000.
6. Optional virtual check: open the PhET pH Scale simulation, choose any liquid it offers, and note whether its reading sits above or below 7. Then answer the analysis questions in your document, paste your completed tables into an approved chatbot with the prompt in Part 6, and check its answer against the scripted checks. Tool: A document or notebook for your written answers. Check: Completed tables, written answers, and a list of the chatbot's errors with corrections.

Fallback route

When to use it: Use this route if you cannot get a red cabbage, cups, or the household liquids, or if you cannot brew the indicator at home.

Use the illustrative fallback observations below, constructed for this lab and not real data, as your observation table, then complete every scaffold table, analysis question, and the critique step from them and submit the same items for equal credit.

This route earns the same credit as the primary route.

Six illustrative fallback observations, constructed for this lab, that stand in for your own cups.

Fallback model data

Cup	Liquid	Indicator color in words	Class (acid, neutral, or base)
1	Tap water (reference)	purple
2	Liquid A	red
3	Liquid B	pink-purple
4	Liquid C	purple, the same as the reference
5	Liquid D	blue-green
6	Liquid E	greenish-yellow

Part 4 — The Scaffold

Name every part of the test before you add any indicator, so the comparison is fair from the first cup.

Student scaffold table 1

Part of the test	Your answer
Hypothesis (if… then… because…)
Prediction for each of your two chosen liquids
Independent variable
Dependent variable
Controlled variables (at least two)
Reference cup and why it is there

Record each cup's color in words as you go, compare it with the tap-water reference cup, then sort it.

Student scaffold table 2

Cup	Liquid	Indicator color in words	Class (acid, neutral, or base)
1	Tap water (reference)
2
3
4
5
6

The five reference cups are illustrative readings constructed for this lab, not measurements of any product; use them for the step-counting table. Count the whole-unit steps first, then raise ten to that power.

Student scaffold table 3

Comparison of illustrative reference cups	Steps counted	Power of ten	How many times more hydrogen ions
Cup P at pH 5 versus Cup R at pH 8
Cup Q at pH 6 versus Cup T at pH 11
Cup P at pH 5 versus Cup Q at pH 6
Cup R at pH 8 versus Cup S at pH 10

Part 5 — Analysis Questions

1. Using the step-counting table, write out each comparison in full: the steps counted one at a time, the power of ten, and the result in words. Which reference cup is the most acidic, and how do you know from its number alone?
2. Describe in words how the indicator's color changed from your most acidic liquid, through the tap-water reference cup, to your most basic liquid. Which liquids were hardest to sort, and why?
3. Explain why the tap-water reference cup belongs in the test, and name the independent variable, the dependent variable, and two controlled variables. Does your evidence support or fail to support your hypothesis?
4. Your blood sits near pH 7.4. Explain what its carbonic acid / bicarbonate buffer does when extra hydrogen ions arrive, using resists rather than prevents, and connect it to homeostasis from Week 1.
5. Choose one and answer it in 3–4 sentences. (a) Judging a color by eye is a source of error; name one way it could mislead you and one change that would make your sorting more reliable. (b) The indicator tells you a class, not an exact pH; explain what you can and cannot claim about any one liquid from a color alone.

Part 6 — AI-Critique Moment

Choose one equal-credit critique route. Route A: ask an approved AI tool to critique your completed work, then check its response against your own evidence. Route B is the no-AI equivalent printed below.

Approved tools: Claude, Gemini, ChatGPT, Grok, Microsoft Copilot.

Prompt to paste:

I've pasted my completed Week 2 Biology Lab & Inquiry tables above. I brewed a red-cabbage indicator, added the same amount of it to five household liquids and a tap-water reference cup, recorded each color in words, and sorted each liquid as acid, neutral, or base. I also worked a step-counting table for five illustrative reference cups at pH 5, 6, 8, 10, and 11. Please do seven things. First, check my acid, neutral, and base sorting against my recorded colors. Second, say how many times more hydrogen ions Cup P at pH 5 has than Cup R at pH 8, showing every step. Third, list the order of the steps I should follow to test one liquid. Fourth, describe a water molecule with its slight charges labeled. Fifth, explain what my blood's buffer does when extra acid arrives. Sixth, give the exact pH of any brand of lemon juice or vinegar, with its source. Seventh, as a stretch question for later in the course, say how many cells a body cell makes when it divides for growth, and what phenotype ratio a cross of two Tt parents gives.

Test the response for these likely failures:

• Chatbots often scramble the test order, sorting a liquid before comparing it with the tap-water reference cup, or raising ten before counting the pH steps.
• Chatbots sometimes flip the slight charges on a water molecule, marking the oxygen δ+ and the hydrogens δ−.
• Chatbots often subtract the two pH values and report that pH 5 has 3 times more hydrogen ions than pH 8, reading the scale like a ruler.
• Chatbots often say a buffer stops the pH from changing at all, or that blood's pH cannot move.
• Asked for a brand's exact pH, chatbots often invent a precise value or a source that was never given, or quietly change one of my reference-cup readings.
• Asked the stretch question, chatbots sometimes say a dividing body cell makes four cells or makes cells that differ from each other.
• Asked the stretch question, chatbots sometimes give the phenotype ratio of a Tt by Tt cross as 1:2:1.

Write 3–5 sentences naming one thing the chatbot got right and at least two places it went wrong or overreached, quoting its words and correcting each with evidence from your own tables or this week's chapter; mark the stretch answers as ones to check later.

Equivalent route (no AI required)

No AI account is required for this lab. Instead of the chatbot, critique this flawed analysis, printed here in full: "My lemon juice turned red, so it is a base, and my baking-soda water turned blue-green, so it is an acid, because a higher pH means more acid. Cup P at pH 5 has 3 times more hydrogen ions than Cup R at pH 8, because 8 minus 5 is 3. I sorted each liquid first and then looked at the tap-water cup. In a water molecule the oxygen is δ+ and each hydrogen is δ−. My blood's buffer stops its pH from ever changing. My lemon juice measured exactly pH 2.37, according to a study. As a preview of later weeks, a body cell dividing for growth makes four identical cells, and a cross of two Tt parents gives a 1:2:1 phenotype ratio." Mark it up: quote each flawed sentence, name what is wrong (an acid and a base swapped, the pH scale read as a ruler, steps out of order, a mislabeled structure, an over-simplified mechanism, an invented figure, or a preview fact stated wrong), cite the evidence from your tables or this week's chapter, and write a corrected sentence beneath it; for the two preview sentences, say which later week will check them. This route assesses the same objectives (Objective 1 and Objective 2) as the chatbot critique, takes comparable effort (about 30 minutes and 200–300 words), is scored by the same scoring criteria for the same credit, and its own submission artifact is the marked-up flawed analysis, posted in place of the chatbot share link and report.

Part 7 — What to Submit

• All three completed scaffold tables, filled from your own cups or the illustrative fallback observations
• Written answers to all five analysis questions
• The chatbot critique report with its share link, or the equivalent route's marked-up flawed analysis

Deliverable:

A short Biology Lab & Inquiry report on the red-cabbage indicator: three completed tables (the named parts of the test, six cups with colors in words and classes, and the step-counting comparisons), written answers to five analysis questions, and a critique of a chatbot's account of your lab, or, on the equivalent route, a marked-up correction of the printed flawed analysis. Submit in any accessible file type or as a link; a recorded response may be submitted as audio or video with a transcript, or as text. Choose a written, recorded, or visual form for your answers; every form is graded by the same scoring criteria.

Rubric

The rubric below shows how each submitted part is evaluated; bands are full, partial, or none.

Workshop evaluation criteria

Criterion	Full	Partial	None
Testable hypothesis and the parts of the test	A falsifiable if… then… because… hypothesis and a prediction appear before the indicator is added, and the independent variable, dependent variable, at least two controlled variables, and the reference cup are each named correctly.	A hypothesis appears but is not falsifiable or lacks a prediction, or one or two parts of the test are missing or swapped.	No hypothesis, or the parts of the test are not named.
Observation table and classification	All six cups are recorded from the student's own test or the fallback observations, each color is described in words, and every liquid is sorted as acid, neutral, or base consistently with its color and the reference cup.	The table is filled, but one or two classes do not match the recorded colors or a color is missing.	No observations are recorded, or the colors are invented rather than observed or taken from the fallback.
Tenfold-step arithmetic	All four reference comparisons count the steps, raise ten to that power, and state the correct result in words, with no subtraction used as the answer.	The steps are counted, but one or two comparisons are off by a power of ten or skip the words.	The comparisons are missing, or pH values are subtracted and reported as the fold difference.
Analysis in words	Questions 1 to 4 are answered accurately: the most acidic reference cup named by its low number, the color pattern described in words, the reference cup and variables explained with supports or fails to support, and the buffer described as resisting rather than preventing change.	The questions are attempted, but one or two answers call a higher pH more acidic, blur the reference cup, or say a buffer prevents change.	The analysis questions are missing or do not address the observations.
Critique of the chatbot or of the printed flawed analysis	On either route, at least two errors are quoted and corrected with evidence from the student's own tables or the chapter, stretch answers are marked to check later, and one correct point is acknowledged; both routes earn this band equally.	On either route, errors are named but corrected without evidence, or only one error is caught.	No critique appears, or pasted output is submitted without any checking.
Reflection on the limits of the test	The chosen reflection option is answered in 3–4 sentences that tie a specific source of error or limit to a concrete change in the test or in what can be claimed.	The reflection is answered but stays general or names no specific change.	No reflection is submitted.

Quality Gate

• Every color in the observation table comes from your own cups or the printed fallback observations, never from memory or a chatbot.
• Every comparison in the step-counting table counts the whole-unit steps before raising ten, and no pH values are subtracted to give a fold difference.
• The hypothesis was written before the indicator was added and has the if… then… because… form.
• The independent variable, dependent variable, controlled variables, and the tap-water reference cup are each named.
• No pH value for a brand, measurement, study, or source is invented anywhere in the submission.
• The critique quotes the chatbot or the flawed analysis and corrects at least two errors with evidence from your tables or the chapter.