Question

Phenolphthalein is used as an indicator in the reaction between

• A. acid and base
• B. acid and metal
• C. base and metal oxide
• D. acid and non-metallic oxide

Hint:

Remember the colour change of phenolphthalein: "Pink in base." This helps in recalling its use in reactions that lead to a basic pH.

Answer 1

The Correct Option is A

Step 1: Understand the function of an indicator.
An indicator is a substance that shows a visible change, usually a colour change, when the conditions in its environment change, particularly the pH. It is often used to determine the endpoint of a titration reaction.
Step 2: Recall the colour changes of phenolphthalein in different pH ranges.
Phenolphthalein is a pH indicator that is:

• Colourless in acidic solutions (pH \(<\) 8.2)
• Pink to magenta in basic solutions (pH \(>\) 10.0)
• Colour changes occur in the pH range of 8.2 to 10.0

Step 3: Analyze the given reaction types.
Acid and base: The reaction between an acid and a base is a neutralization reaction, which results in a change in pH. Phenolphthalein's colour change in the pH range of 8.2 to 10.0 makes it suitable for indicating the endpoint of titrations involving strong acids and strong bases, or strong acids and weak bases where the equivalence point is in the basic range.
Acid and metal: The reaction between an acid and a metal typically produces a salt and hydrogen gas. This reaction does not necessarily involve a significant change in pH that would be effectively indicated by phenolphthalein.
Base and metal oxide: The reaction between a base and a metal oxide usually does not occur, except for amphoteric metal oxides. Even if a reaction occurs, it may not involve a clear pH change suitable for phenolphthalein.
Acid and non-metallic oxide: The reaction between an acid and a non-metallic oxide generally does not occur. Non-metallic oxides are typically acidic or neutral and react with bases. 
Step 4: Identify the reaction type where phenolphthalein is commonly used as an indicator.
Phenolphthalein is most commonly used as an indicator in acid-base titrations where the pH at the equivalence point is in the basic range, allowing for a clear colour change from colourless to pink.