Final answer:
When an acid is correctly titrated to the end point, the indicator color changes to signify the completion of the reaction, such as from colorless to pink when using phenolphthalein, indicating the equivalence point in an acid-base titration.
Step-by-step explanation:
If the acid is titrated to the correct end point, the indicator color is lost, giving the solution a specific color. During an acid-base titration, a standard solution with a known concentration is slowly added to a solution with an unknown concentration. Indicators such as phenolphthalein or methyl orange are used to signal the end point through a distinct color change.
For instance, when titrating a strong acid with a base and using methyl orange as the indicator, the solution changes from red to orange and finally to yellow at the equivalence point. With phenolphthalein, the change would be from colorless to a faint pink color. The volume of titrant at which this color change occurs allows us to determine the end point accurately, which is crucial for the calculation of the analyte's concentration.
Titration curves help in choosing the right indicator, as they show the pH range over which a specific indicator changes color. The steepness of the curve around the equivalence point is a good indication of whether an indicator will provide a sharp end point or not.
For example, in the titration of acetic acid (a weak acid) with sodium hydroxide (NaOH), the use of methyl orange would not be as appropriate as phenolphthalein, since methyl orange changes color before the equivalence point is reached, while phenolphthalein's color change of colorless to pink closely aligns with the equivalence point.