Question
Download Solution PDF'Vanishing colour' may be prepared by mixing phenolphthalein with
This question was previously asked in
WBPSC Miscellaneous Preliminary Exam 2012 Official Paper (Held On 19th May 2013 Shift 1)
Answer (Detailed Solution Below)
Option 3 : dilute solution of caustic soda
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WBPSC Miscellaneous CT: General Studies (Mock Test মক টেস্ট)
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Detailed Solution
Download Solution PDFThe Correct Answer is : 3) Dilute solution of caustic soda
Key Points
- Phenolphthalein is a chemical compound often used as a pH indicator in titrations.
- In a neutral or acidic solution, phenolphthalein is colorless.
- Phenolphthalein's spectrophotometric absorbance value at 546 nanometers is 3.0.
- When it's in a solution that is basic or alkaline (meaning it has a pH greater than 7), phenolphthalein will turn a fuchsia color. Because of this property, it's used as an indicator in acid-base titrations.
- A dilute solution of caustic soda (sodium hydroxide) is a classic example of a basic solution. When phenolphthalein is mixed with this, it will turn pink/fuchsia.
- However, at very high pH values (usually above 10), the indicator becomes colorless again, causing the pink color to "vanish". This is often referred to as the 'vanishing color'. Thus, a dilute solution of caustic soda can be used to prepare "vanishing color".
- Phenolphthalein has been used for a long time in various applications due to its color-changing properties, including in chemistry classrooms for teaching about acids and bases.
- Other than being a pH indicator, phenolphthalein also sees uses in forensic applications for detecting the presence of latent blood stains. It turns pink in response to hemoglobin.
Last updated on Mar 18, 2025
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