Universal indicator solution is composed of a blend of molecules that elicit a color alteration upon encountering H+ and OH– ions. This solution is employed by scientists to estimate the pH of an unfamiliar substance. Analyze the chart provided to observe the array of pH levels identified by universal indicator solution and the colors they associate with.

Question

Red: ph: 1
Pink: ph: 2
Orange: ph: 3
Beige: ph: 4
Yellow: ph: 5
Lime-green: ph: 6
Green: ph: 7
Dark green: ph: 8
Turquoise: ph: 9
Pale blue: ph: 10
Blue: ph: 11
Dark blue: ph: 12
Violet: ph: 13
Purple: ph: 14
A nutritionist aims to investigate the influence of food combinations on stomach pH. She introduces ten drops of freshly extracted potato juice into a beaker containing a universal indicator solution. This action results in the solution turning yellow in color. Subsequently, she introduces ten drops of white vinegar, a solution of acetic acid.
Using the color chart for the universal indicator provided above, and considering a comprehension of acid-base chemistry, select the response that accurately predicts the PROBABLE color the universal indicator will exhibit when vinegar is introduced to potato juice.
A. The solution would remain yellow
B. The solution would turn blue
C. The solution would turn red
D. The solution would turn green

Answer 1

To analyze the question, we start by looking at the pH levels indicated by the colors provided for the universal indicator.

When the potato juice was added, the solution turned yellow, which corresponds to a pH of around 5. This indicates that the potato juice is somewhat acidic but not as acidic as vinegar.

Now, when white vinegar (which has a low pH, typically around 2-3 because it is acetic acid) is introduced into the yellow solution, we need to consider what happens. Vinegar is considerably more acidic than the potato juice.

Since the addition of vinegar introduces a significant amount of H+ ions into the solution (lowering the pH further), we can predict the color change. Given that vinegar's pH is much lower than the potato juice's pH, it is expected that the mixture will become more acidic. The universal indicator would therefore reflect this increase in acidity.

The probable color for this lower pH, when combined, will fall around pH 2, which corresponds to a red color on the universal indicator chart.

So, the correct answer, predicting the color exhibited by the universal indicator when vinegar is introduced to potato juice, is:

C. The solution would turn red.