Final answer:
Determining the pH of colored substances like grape juice and catsup can be tough with pH test strips because their colors interfere with the color changes of the indicators. A pH meter, which uses voltage changes to measure pH, can offer a more accurate measurement. Substances are classified as acidic, basic, or neutral based on their pH: human blood (7.4) is slightly basic, household ammonia (11.0) is basic, and cherries (3.6) are acidic.
Step-by-step explanation:
Determining the pH of substances such as purple grape juice and catsup can be difficult when using test strips because these substances have strong colors that can interfere with the color change indicators on the pH strips. In particular, the color of the indicator dyes embedded within the pH paper will change depending on the pH level of the substance it comes into contact with. However, highly pigmented substances like grape juice and catsup can mask the indicator's color change, making it hard to accurately compare the strip to a color chart.
For a more precise measurement of pH, a pH meter might be used, which can read the voltage change due to the hydronium ion concentration, providing a digital readout of the pH without the potential for color interference.
As for classifying substances by their pH values, substance 1, human blood, with a pH of 7.4, is slightly basic; substance 2, household ammonia, with a pH of 11.0, is basic; and substance 3, cherries, with a pH of 3.6, is acidic.