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Lesson plan Grade 11 Science

Merrow Vale Stream pH: The Log Scale and Buffering

A 50-minute grade 11 lesson on the logarithmic pH scale and how carbonate alkalinity from limestone buffers a fictional stream against acid inputs.

Objectives

  1. Students will calculate pH from hydrogen-ion concentration and interpret the scale as logarithmic.
  2. Students will explain how carbonate alkalinity buffers a stream against acid inputs.
  3. Students will connect a stream's bedrock to its vulnerability to pH change.

Vocabulary

pH
pH is a number that tells whether water is acidic, like lemon juice, or basic, like baking soda mixed in water. Pure water sits in the middle of the scale at a pH of 7.
Alkalinity
Alkalinity is water's ability to resist becoming acidic, acting like a buffer that keeps pH from swinging wildly when acids are added. Water with good alkalinity helps protect fish and other aquatic life from sudden pH changes.

Also: logarithmic scale, hydrogen ion, buffering, chemical weathering

Teacher overview

Two streams can receive the same acid rain and end up at very different pH values, because the rock and soil their water passes through decide how much acid the water can absorb before its pH shifts. This lesson uses the fictional Merrow Vale stream, which flows first across granite and later across limestone, to teach two ideas together: that pH is a logarithmic measure of hydrogen-ion concentration, and that carbonate alkalinity acts as a buffer. A common misconception is that pH is a linear scale, so students assume a stream at pH 5 is only "a little" more acidic than one at pH 6; the lesson corrects this by showing that each whole pH unit is a tenfold change in hydrogen-ion concentration. The secondary standard for drinking-water pH, 6.5 to 8.5, gives a reference band for the discussion.

Procedure

Hook (5 min): Project two clear water samples labeled granite reach and limestone reach from the fictional Merrow Vale stream, and ask students to predict which is more likely to harm sensitive aquatic life after a spring storm. Record predictions without comment.

Direct instruction (15 min): Define pH as the negative base-ten logarithm of the hydrogen-ion concentration, pH = -log[H+], and work one example on the board, converting [H+] = 1 x 10^-6 mol/L to pH 6. Stress that dropping one pH unit means ten times more hydrogen ions. Then introduce alkalinity as a water's capacity to neutralize acid, carried mostly by bicarbonate and carbonate ions. Explain that where the stream crosses limestone, the mineral calcium carbonate slowly dissolves through chemical weathering, adding carbonate alkalinity that buffers the water; over granite, which weathers to release almost no carbonate, the water stays poorly buffered.

Guided practice (10 min): With the class, compute the pH of a sample with [H+] = 1 x 10^-7 mol/L (pH 7), then ask how many times more acidic a pH 4 sample is than a pH 7 sample (1,000 times). Have students explain the answer to a partner before moving on.

Independent practice (15 min): Students complete the Student Handout individually or in pairs, calculating pH values, comparing acidity across the two reaches, and reasoning about which reach resists pH change and why.

Closure and exit ticket (5 min): On an index card, students answer: "A stream's pH falls from 6 to 5 after a storm. By what factor did its hydrogen-ion concentration change, and what does that tell you about the stream's alkalinity?" Collect and scan for the tenfold factor and a link to low buffering before dismissal.

Merrow Vale Stream pH: The Log Scale and Buffering — Grade 11 Science
Name: ____________________________ Date: ______________

Student Handout

Water chemists sample the fictional Merrow Vale stream at two points. Use pH = -log[H+], and treat each whole pH unit as a factor of ten in hydrogen-ion concentration.

  1. The granite reach sample has a hydrogen-ion concentration of 1 x 10^-5 mol/L. Calculate its pH.
  2. The limestone reach sample has a hydrogen-ion concentration of 1 x 10^-8 mol/L. Calculate its pH.
  3. How many times more acidic (how many times higher in hydrogen-ion concentration) is the granite reach than the limestone reach?
  4. The secondary drinking-water standard sets pH between 6.5 and 8.5. Which reach falls outside that band, and on which side?
  5. A titration gives the limestone reach an alkalinity of 90 mg/L as calcium carbonate and the granite reach only 8 mg/L as calcium carbonate. Which reach can neutralize more acid before its pH drops?
  6. A spring storm delivers the same amount of acid to both reaches. Using alkalinity, explain why the granite reach's pH is expected to fall much more than the limestone reach's.
  7. Explain how dissolving limestone (calcium carbonate) raises a stream's alkalinity and buffers its pH.
  8. If the granite reach's hydrogen-ion concentration increased tenfold after the storm, what would its new pH be?

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key
  1. Answer: 5.0. pH = -log(1 x 10^-5) = 5.0.
  2. Answer: 8.0. pH = -log(1 x 10^-8) = 8.0.
  3. Answer: 1,000 times.
    The difference is 8 minus 5 = 3 pH units, and each unit is a factor of ten, so 10^3 = 1,000 times more acidic.
  4. Answer: The granite reach, below the low end.
    Its pH of 5.0 is below 6.5, so the granite reach is more acidic than the secondary band allows; the limestone reach at 8.0 is inside the band.
  5. Answer: The limestone reach.
    With 90 mg/L of alkalinity as calcium carbonate versus 8 mg/L, the limestone reach can absorb far more acid before its pH falls.
  6. Answer: The granite reach has little buffering capacity.
    Low alkalinity means few carbonate and bicarbonate ions are available to neutralize incoming acid, so added hydrogen ions stay in solution and the pH drops sharply; the limestone reach's high alkalinity consumes the acid and holds pH nearly steady.
  7. Answer: Dissolving calcium carbonate adds carbonate and bicarbonate ions.
    As acid weathers limestone, calcium carbonate dissolves and releases bicarbonate and carbonate ions into the water, which raises alkalinity; those ions react with and neutralize hydrogen ions, which keeps pH from falling.
  8. Answer: 4.0.
    A tenfold rise in hydrogen-ion concentration lowers pH by exactly one unit, from 5.0 to 4.0.

Differentiation

Support

Give students a filled-in place-value chart linking hydrogen-ion concentrations (1 x 10^-4 through 1 x 10^-9) to their pH values, so items 1 and 2 become a lookup before they compute item 8 independently. Provide a calculator with a log key and model its use.

Extension

Have students research (from teacher-provided material only) why a stream with a pH of 5.0 can be lethal to trout eggs even when adult fish survive, and connect that answer to the tenfold-per-unit nature of the scale.

ELL supports

Pre-teach "acidic," "neutralize," "buffer," and "weathering" with a visual showing acid drops meeting carbonate ions and being canceled out. Provide sentence frames for items 6 and 7, such as "The granite reach has ___ alkalinity, so its pH ___ when acid is added."