Designing a Diverging pH Color Scale for the Stannary Water Report

Hook (5 min): Project a rainbow pH indicator chart beside a grayscale copy of the same chart. Ask the class, "On the gray version, where is neutral?" On most universal-indicator charts the lightest band is the yellow one, which sits on the acidic side of 7, while the green band at 7 prints as a middle gray. Let students notice that the grayscale copy draws the eye to the wrong place. Take two or three observations without resolving them yet.

Direct instruction (12 min): Cover three short points, pausing after each for questions.

First, the chemistry. The pH scale runs from 0 to 14, with 7 as neutral; values below 7 are acidic and values above 7 are basic. Each whole step is a tenfold change. Under EPA's secondary (aesthetic) standard, drinking water pH should fall between 6.5 and 8.5. Many utilities adjust their water to be slightly basic, because acidic water is corrosive and can dissolve metals, including lead and copper, from household plumbing. Explain the indicator-dye point from the Overview here, using red cabbage as the example.

Second, the three kinds of color schemes. A categorical scheme uses clearly different hues for groups that have no order, such as the pipe types on a utility map. A sequential scheme runs from light to dark in one hue for data that goes from low to high, such as rainfall totals. A diverging scheme starts from a pale neutral center and darkens in two different hues toward the two ends; it fits data with a meaningful middle, such as a temperature above or below freezing, or pH above or below 7.

Third, explain why lightness matters. Readers take darker and stronger colors to mean "more," so the order of a scale should be carried by value (lightness and darkness), not by hue alone. A scale that relies on value still reads when the report is photocopied in black and white, and it still reads for customers with a common form of color-vision deficiency that makes red and green hard to tell apart.

Guided practice (8 min): In pairs, students make two value ramps on scrap paper, each with seven small boxes: one ramp in a warm hue such as red-orange and one in a cool hue such as blue-violet. Each ramp runs from nearly white to as dark as colored pencil allows, built up by layering and pressure rather than by switching to a different pencil. Partners then hold the printed gray value strip beside each box and write the matching gray step number (1 for white through 9 for black) under it. Formative check: pairs hold their ramps up at the same time. Any ramp in which two neighboring boxes share the same gray number has to be redone; if more than a quarter of the ramps in the room show a repeated number, stop and model layering pressure again before moving on.

Independent design (15 min): Each student designs the Stannary pH graphic from the Student Handout. Students color the fifteen boxes so that box 7 is the lightest, the acidic boxes darken step by step in one hue toward 0, and the basic boxes darken step by step in the other hue toward 14. Boxes the same distance from 7, such as 5 and 9, should match in darkness so that neither side looks more alarming than the other. Students then add the bracket for the 6.5 to 8.5 range, the pointer for Stannary's reading, the end labels, the tenfold note, and a title in plain words. Circulate and ask each student which box is lightest and why.

Partner gray check (4 min): Partners swap graphics. Each partner compares five boxes (pH 1, 4, 7, 10, and 13) with the gray value strip and records the gray step numbers in the table on the back of the handout. A working diverging scale shows its lowest number at pH 7 and numbers rising in both directions.

Closure and exit ticket (6 min): Students write one paragraph on an index card in answer to the exit-ticket prompt on the handout. Collect the cards. If fewer than two-thirds of the paragraphs cite at least one gray step number as evidence, open the next class by modeling how a test result becomes evidence in an argument.