Question Video: Estimating pH Values of Solutions with Added Universal Indicator Chemistry • 7th Grade

Video Transcript

A few drops of universal indicator were added to four flasks containing unknown solutions. Which of the following values provide the best estimate for the pH of each solution? (A) Where 𝐴 equals five, 𝐵 equals seven, 𝐶 equals 11, and 𝐷 equals one. (B) Where 𝐴 equals seven, 𝐵 equals 11, 𝐶 equals five, and 𝐷 equals one. (C) Where 𝐴 equals 11, 𝐵 equals one, 𝐶 equals five, and 𝐷 equals seven. (D) Where 𝐴 equals one, 𝐵 equals 11, 𝐶 equals seven, and 𝐷 equals five. Or (E) where 𝐴 equals five, 𝐵 equals one, 𝐶 equals 11, and 𝐷 equals seven.

The pH scale allows us to measure the acidity or basicity of that solution. Most solutions have a pH value between zero and 14. Solutions with a pH below seven are acidic, while solutions with a pH above seven are basic, and solutions with a pH of seven are neutral, such as pure water. The question tells us that universal indicator has been added to the unknown solutions. An indicator is a chemical whose color is affected by the pH of the solution to which it is added. Universal indicator is a mixture of different indicators.

And to answer this question, we need to know the corresponding colors of universal indicator when affected by different pHs. We can do this by remembering the order of the colors in the rainbow. We can use the acronym ROYGBIV to recall the order of the colors. The rainbow spectrum begins with ROY, or reds, oranges, and yellows, which correspond to low pHs and therefore acidic solutions. In the middle of the rainbow at G, or greens, the pH of the solution is approximately seven. And at the end of the rainbow at BIV, or blues, indigos, and violets, the pH of the solution is above seven and therefore basic.

Let’s look at solution 𝐴. We can see that the solution has turned orange and must therefore have a pH below seven. If we look at solution 𝐵, solution 𝐵 has turned green and likely has a pH of approximately seven, while solution 𝐶 has turned violet and must have a pH higher than seven. Finally, looking at solution 𝐷, we see that it has turned to red and must have the lowest pH of the four solutions.

Now, we must identify the answer where 𝐷 has the lowest estimated pH and where 𝐴 has a pH estimate lower than seven but higher than the pH of 𝐷, 𝐵 is estimated to have a pH of approximately seven, and where 𝐶 has the highest estimated pH of the four solutions. Looking at the answer choices, we can see that the answer choice that corresponds to the conditions we have established is answer (A), where 𝐴 equals five, 𝐵 equals seven, 𝐶 equals 11, and 𝐷 equals one.