Turmeric is a natural acid-base indicator. Its original colour is yellow. In acidic or neutral solutions, it remains yellow. In basic (alkaline) solutions, it turns reddish-brown. Soap solution is basic. Therefore, when a turmeric stain (yellow) on white clothes is scrubbed with soap, it turns reddish-brown. When the cloth is then washed with plain water, which is usually neutral or slightly acidic due to dissolved CO2, the basic soap is rinsed away, and the turmeric indicator returns to its yellow colour.
Which one of the following is the correct sequence of change in colours when a turmeric stain on white clothes is scrubbed by soap and then washed with water ?
Asked in UPSC NDA 2 2020 · Topic: Acid, Base and Salt (Acid-Base Indicators (Litmus, Phenolphthalein, Methyl Orange, Universal Indicator))
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