Final answer:
The correct answer is that amniotic fluid, being a basic substance, will turn blue at a pH greater than 7.0. Red litmus paper and phenolphthalein are pH indicators that turn blue and pink respectively in basic conditions, confirming that option 4: pH 8.0 is the correct threshold.
Step-by-step explanation:
The pH scale is a measure of how acidic or basic a solution is. A solution with a pH lower than 7 is considered acidic, a solution with a pH of 7 is neutral like pure water, and a solution with a pH higher than 7 is termed basic or alkaline. When referring to amniotic fluid, the question implies that the fluid will turn blue if it is basic, as many common pH indicators, such as red litmus paper or phenolphthalein, will shift to blue or pink respectively, in the presence of a basic substance.
Knowing that anything above pH 7 is basic, we can determine that the correct answer must be a number above 7. The options given are pH 5.0, pH 6.0, pH 7.0, and pH 8.0. Considering that the amniotic fluid is basic and the pH scale in biology typically references normal physiological temperature, which is around 36-40°C for humans, we would expect a basic solution to have a pH > 7.0. Therefore, the correct answer is that amniotic fluid will turn blue on the pH scale at a value greater than 7.0. This means that Option 3: pH 7.0 is the threshold at which amniotic fluid could start to turn blue.