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Science lesson plan · 12th grade (Year 13 in England) · 45 minutes

pH and hydrogen ion concentration

Chemical reactions, acids and bases

CCSS NGSS.HS-PS1-6England: GCSE Chemistry: Chemical changes (strong and weak acids, Higher)

Objective and success criteria

Students will: pH and hydrogen ion concentration.

  • I can answer questions on today's skill on my own, without help.
  • I can explain each step and say why it works.
  • I can check my answer and spot a mistake.

Before and after

Needs first: Read indicator colours to find pH

Leads to: the next skill in this area.

Materials: Teacher demo or video, Indicator color chart, Mini whiteboards, A diagram or short video clip of the idea, Printed student sheet (below).

Warm-up · 5 min

  • Quick retrieval from earlier learning (Read indicator colours to find pH). Students answer on mini whiteboards.
  • Show answers together and note who needs support in the main task.
  1. A solution has a pH of 14. What colour will universal indicator turn? (orange or yellow / purple / red / blue) → purplePrompt: Universal indicator runs like a rainbow from strong acid to strong alkali.
  2. Phenolphthalein is added to sodium hydroxide solution. What colour does it go? (blue / pink / colourless / yellow) → pinkPrompt: Sodium hydroxide is an alkali.
  3. Universal indicator results: milk pH 6, pure water pH 7, lemon juice pH 2, vinegar pH 3. Which is the most acidic? (lemon juice / milk / pure water / vinegar) → lemon juicePrompt: The lower the pH, the more acidic.

Hook · 5 min

  • Drop an antacid tablet in water (or show a clip) and ask whether a new substance is being made.
  • Share the objective: pH and hydrogen ion concentration. Read the success criteria together.

I do: model it · 10 min

  • Work through the example out loud, saying why you do each step, not just what you do.
  • Example: Solution X has pH 5 and solution Y has pH 7. How many times greater is the hydrogen ion concentration in X than in Y?
  • Step 1: 10^2 = 100
  • Step 2: So the answer is: 100
  • Answer: 100
  • Pause after each step and ask a student to say what comes next.

We do: guided practice · 10 min

  • Do these together. Students try each step on whiteboards before you reveal it.
  • Use the hints as prompts rather than giving the answer.
  1. Solution X has pH 3 and solution Y has pH 4. How many times greater is the hydrogen ion concentration in X than in Y? → 10Prompt: Each drop of 1 pH unit means 10 times more H⁺ ions.
  2. A strong acid solution has a hydrogen ion concentration of 0.00001 mol/dm³. What is its pH? (pH = −log₁₀[H⁺]) → 5Prompt: Write the concentration as a power of ten.

You do: independent practice · 10 min

  • Students work on the student sheet. Circulate and check the first two answers of every student early.
  • Students who finish: write a question of their own for a partner, with the answer.

The 8 questions are on the student sheet below, with answers in the key.

Exit ticket · 5 min

  • Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.
  1. A strong acid solution has a hydrogen ion concentration of 0.00001 mol/dm³. What is its pH? (pH = −log₁₀[H⁺]) → 5Prompt: Write the concentration as a power of ten.
  2. Solution X has pH 5 and solution Y has pH 6. How many times greater is the hydrogen ion concentration in X than in Y? → 10Prompt: Each drop of 1 pH unit means 10 times more H⁺ ions.

Watch out for

  • Everyday meanings of words getting in the way of the science meaning: define key words explicitly.
  • Remembering facts without the reason: ask "why?" and "what is the evidence?"

Support and stretch

Support

  • Review first: Read indicator colours to find pH.
  • Use concrete materials: teacher demo or video.
  • Give the first step of each question, or a partly worked example to finish.
  • Pair with a confident partner for the guided practice.

Stretch

  • Explain a common mistake in writing and how to fix it.
  • Write a word problem that uses today's skill, then solve a partner's.

Homework and more practice

Free online practice with hints and printable worksheets for this skill: eurekami.com/science/grade-12.

Free lesson plan from eurekami.com/lesson-plan-builder

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pH and hydrogen ion concentration

Name

Date

Score/ 8

  1. Question 1

    Solution X has pH 2 and solution Y has pH 4. How many times greater is the hydrogen ion concentration in X than in Y?

    Answer

  2. Question 2

    A strong acid is diluted so its H⁺ concentration becomes 10 times smaller. What happens to its pH?

    • A. it rises by 1
    • B. it does not change
    • C. it rises by 10
    • D. it falls by 1

  3. Question 3

    A strong acid solution has a hydrogen ion concentration of 0.1 mol/dm³. What is its pH? (pH = −log₁₀[H⁺])

    Answer

  4. Question 4

    Solution X has pH 5 and solution Y has pH 6. How many times greater is the hydrogen ion concentration in X than in Y?

    Answer

  5. Question 5

    Ocean water's pH has fallen from about 8.2 to about 8.1 since the 1800s. Roughly how has the H⁺ concentration changed?

    • A. it has not changed
    • B. it has increased 10 times
    • C. it has fallen by half
    • D. it has increased by about 25–30 %

  6. Question 6

    A strong acid solution has a hydrogen ion concentration of 0.01 mol/dm³. What is its pH? (pH = −log₁₀[H⁺])

    Answer

  7. Question 7

    A strong acid solution has a hydrogen ion concentration of 0.00001 mol/dm³. What is its pH? (pH = −log₁₀[H⁺])

    Answer

  8. Question 8

    Solution X has pH 1 and solution Y has pH 2. How many times greater is the hydrogen ion concentration in X than in Y?

    Answer

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Answer key: pH and hydrogen ion concentration

  1. 1. 100

  2. 2. A. it rises by 1

  3. 3. 1

  4. 4. 10

  5. 5. D. it has increased by about 25–30 %

  6. 6. 2

  7. 7. 5

  8. 8. 10