A) Two acid-base indicators are:

Here are the answers to Question 3:

a) Two acid-base indicators are:

• Phenolphthalein
• Methyl orange

b) The color change of phenolphthalein is:

• In acidic solutions: Colorless
• In basic solutions: Pink or magenta

c) Methyl orange exhibits a yellow color in a basic solution.

d) Calibration of an analytical balance is the process of adjusting the balance to ensure it provides accurate and precise measurements. This is done by comparing its readings against certified reference weights and making necessary adjustments to correct any deviations.

e) Two characteristics of a primary standard are:

• High purity (typically $\ge 99.9\%$)
• High stability (does not absorb moisture or react with air, and has a high molecular weight to minimize weighing errors)

f) The accuracy of a weighing balance refers to how close a measured mass value is to the true or actual mass of the object being weighed. A highly accurate balance provides readings that are very close to the true value.