Week 2 — Counting pH Steps: A Water Chemistry Report — Traditional alternative (no AI)

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Assignment 2 of the term — count pH steps, explain water's chemistry, and turn the numbers into plain words for someone new to biology.

This week you learned how atoms bond, why water's polarity gives it its remarkable properties, and how to read the pH scale one tenfold step at a time. Week 1 taught you to read a controlled experiment; this week you explain one with chemistry. In this assignment you work four short problems, interpret each one, and explain the pH scale to someone who has never studied biology. This week's lecture tutorial, quiz, discussion, and Biology Lab & Inquiry practise the same ideas from other angles.

The task

This week's shared dataset is the pH ladder. Its household values are approximate and vary by brand: lemon juice about pH 2, white vinegar about pH 3, tap water about pH 7, and baking-soda solution about pH 9. Blood sits near pH 7.4. Each whole step on the pH scale multiplies the hydrogen ions by 10. The illustrative penny experiment from Week 1 returns too: plain water averaged 30 drops and soapy water 14 drops.

Work these four problems, showing your arithmetic:

1. Oxygen makes up about 65%, carbon 18%, hydrogen 10%, and nitrogen 3% of the human body by mass. What percent of body mass do these four elements make up together?
2. A solution at pH 4 is three whole-unit steps below pH 7, and each step multiplies the hydrogen ions by 10. How many times more hydrogen ions does the pH 4 solution have than the pH 7 solution?
3. Lemon juice at about pH 2 is seven whole-unit steps below baking-soda solution at about pH 9, and each step multiplies the hydrogen ions by 10. How many times more hydrogen ions does the lemon juice have?
4. In the illustrative penny experiment, plain water averaged 30 drops and soapy water averaged 14 drops. How many fewer drops, on average, did the penny hold with soap?

Beside each answer, add one brief interpretation:

• Problem 1: say whether water, H₂O, is an atom or a molecule, and name the bond that holds each hydrogen to the oxygen, with the verb that identifies it.
• Problem 2: classify the pH 4 solution as an acid, neutral, or a base, and explain why reading the pH scale like a ruler ("only 3 more") gives the wrong answer.
• Problem 3: classify lemon juice and baking-soda solution, and explain what your blood's carbonic acid / bicarbonate buffer would do if extra hydrogen ions arrived, using "resists" rather than "prevents."
• Problem 4: explain the 16-drop gap with water's polarity, hydrogen bonds, cohesion, and surface tension, and say what the soap changed.

Explain it to a non-expert

Your roommate says, "pH 4 and pH 7 are only three numbers apart, so a drink at pH 4 is barely more acidic than water." Write a 3–5 sentence message that explains what pH measures, why each whole step is a tenfold change, and how big the pH 4 versus pH 7 difference really is, in everyday words. Name one limit, for example that household pH values are approximate and vary by brand, so the class and the direction matter more than one exact number. Explain any technical term in everyday words. More than one good answer is possible.

For a different, ungraded example, imagine a friend who asks why a damp shirt feels cool on a breezy day. Model answer: "Water is made of tiny molecules that cling to each other. The fastest ones break away from the shirt as vapor, and they carry heat away with them, so what stays behind is cooler. That is called evaporative cooling, the same thing sweat does for your skin. It explains the cool feeling, but it does not tell you how much water your body needs on a hot day."

Use that pattern for the pH question, with your own wording and evidence.

Show your arithmetic. A reader should be able to reproduce every number you report from the values given, without guessing which steps you counted or which values you used.

Deliverables

Submit a single report containing:

• the four answers, each with the arithmetic shown and the brief interpretation the task names for that problem;
• your message to your roommate, explaining what pH measures, why each step is tenfold, the size of the pH 4 versus pH 7 difference in everyday words, and one limit; and
• a one-line restatement of the values you used, so your results are reproducible.

Keep it to roughly one page. Clear and correct beats long.

Choose how you show it. Present your report as written text, or as a recorded audio or video explanation of about three minutes with a transcript, or as a labelled sketch of the pH ladder with a short note. Every form is graded by the same scoring criteria. 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.

How you'll be graded

The full analytic rubric is attached to this assignment in Canvas.

Academic integrity and AI use

This version is designed to be completed without AI: do the computations, the interpretations, and the writing yourself, without a chatbot or other AI tool. You may use a calculator or a spreadsheet for the arithmetic. Submitting AI-generated work as your own, or copying another student's work, is an integrity violation under the course policy.

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