Questions: Indicators: Testing for Acids and Bases

Question 1

What happens when you place blue litmus paper into an acidic solution?

• A. It stays blue
• B. It turns red
• C. It turns green
• D. It dissolves

Answer: (B)

Blue litmus paper turns red in acidic solutions. This is one of the classic quick tests for acidity. If the blue litmus stays blue, the solution is either neutral or basic. Litmus is a simple indicator that gives a yes/no answer for acidity.

Question 2

An acid-base indicator changes the pH of the solution it is testing.

• T. True
• F. False

Answer: False

Indicators are used in very small amounts and do not significantly change the pH of the solution being tested. They simply change color in response to the existing H+ or OH- ion concentration. Think of them as observers, not participants.

Question 3

Why is red cabbage juice useful as an acid-base indicator?

Model answer: Red cabbage juice contains natural pigments that change color across a wide range of pH values, turning red/pink in acids, purple in neutral solutions, and green/yellow in bases.

The pigment in red cabbage (called anthocyanin) is sensitive to pH and produces a wide spectrum of color changes. This makes it a more informative indicator than litmus, which only gives a two-color response. Cabbage juice can roughly estimate pH based on the specific color produced.