Question

a.A stain of a curry on a white cloth becomes reddish brown when soap is scrubbed on it. Why?
b.When this cloth is washed with plenty of water, it again turns yellow. Why?
c.Name any two natural indicators

Answer 1

Hint: The above mentioned changes are because of the reactions between the compounds present in it. Soap contains sodium salts and is chemically a base as it releases OH- ions when dissolved in aqueous solutions. The curry which we eat usually contains spices,oil, vegetables and other ingredients. The turmeric powder present in it is responsible for the stain and the changes as well.

Complete answer:
As I already mentioned the compounds present in it, let us understand how they contribute to these changes. So, when soap is scrubbed over a stain, the turmeric present in the curry reacts with the sodium hydroxide present in the soap and this reaction develops a reddish brown color. Here turmeric acts as an indicator. Indicator is something which changes its color upon changing the concentration i.e. acidic or basic. When the same cloth is washed with plenty of water, it turns yellow because the sodium hydroxide is diluted upon addition of water and the reaction stops turning it back into yellow.. In the labs, litmus paper, phenolphthalein etc are used as indicators whereas some naturally occurring indicators are turmeric, rose petals, grape juice etc.

Note:
In the above reaction turmeric is said to act as an indicator because it shows reddish color in basic conditions and yellow color in neutral or acidic condition. Upon scrubbing with soap, it shows a reddish brown color because soap is basic in nature and when water is added it turns yellow as water is neutral in nature.