Draw a 5-by-1/2 inches rectangle on paper, either as a horizontal or a vertical bar. Use your ruler to divide the rectangle in 14 equal parts.
Use the rainbow colors to paint each grade of the pH scale, starting from the left -- or top -- of the bar with red. On the third block use orange, on the fifth yellow, on the seventh green, on the ninth blue, on the 11th indigo, and on the 13th violet. For the grades in between, mix the color of their adjacent blocks: for example, the sixth block will be a mixture of yellow and green and the 14th a mixture of violet and black.
Add the pH degree above -- or to the left of -- each line separating the graph's blocks. Place the value "0" on left -- or top -- end of the rectangle and continue with "1" on the line separating the red and red-orange blocks, "2" on the line separating the red-orange and orange blocks and so forth.
Write "Neutral" above the seventh degree and add two 1/2-inch arrows pointing to the two ends of the rectangle. Write "Increasing Acidity" next to the arrow pointing towards 0 and "Increased Alkalinity" next to the arrow pointing towards 14.
Add substances under -- or to the right of -- their corresponding pH grade. For example, water goes under grade 7, while vinegar, with a pH of 2.4, goes on grade 2.