ZingPath: Acids and Bases

Why do some compounds change color when their pH levels are measured?

~ Compounds change color based on how acidic or basic they are. The chemical dyes are called indicators. When the indicator reaches a specific pH value, it changes color.

A scientist is testing the pH of dishwashing soap. Typically, soap is known to have a pH between 7 and 9. Which indicator should the scientist use? Why?

~ The scientist should use two indicators to get an accurate answer. Bromothymol Blue has a pH range of about 6 to 7.6, and Phenolphthalein has a pH range of 8.3 to 10. Since soap has a known pH between 7 and 9, using both indicators will give the scientist the most accurate results.

How should a scientist decide which indicator to use to determine a specific pH value?

~ Each indicator has a specific end point that determines a specific pH value of a solution. Each indicator determines only one pH value because each indicator's end point value is stable.

a) The chemical dyes that change color depending on the pH of a solution are called indicators.

b) Indicators change color when they reach the end point, or a specific pH value.

c) Each indicator determines only one pH value because its end point value is stable.

c) Each indicator determines only one pH value because its end point value is stable.

A. Hand lens

B. Microscope slides

C. pH probe

D. Stereoscope

~ The correct answer is C. Measuring the pH level of the soil will determine whether the soil is acidic or alkaline. By using a pH probe, the gardener can determine the color of his/her hydrangea blooms by measuring the pH of the soil.