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?
- (a)Yellow – pink – blue
- (b)Yellow – reddish brown – yellow
- (c)Yellow – reddish brown – blue
- (d)Yellow – blue – pink
Answer
Why
Correct — B, Yellow – reddish brown – yellow. Turmeric (curcumin) is a natural acid-base indicator: yellow in neutral or acidic conditions and reddish-brown in basic conditions. The stain starts yellow; scrubbing with soap (which is alkaline) turns it reddish-brown; then rinsing with water removes the base and the stain returns to yellow, giving the sequence yellow, reddish brown, yellow.
Why the others are wrong
- (a)Yellow – pink – blue — Turmeric does not turn pink or blue; those belong to other indicators, not curcumin.
- (c)Yellow – reddish brown – blue — The first two steps are right, but on washing off the alkali the stain goes back to yellow, not blue.
- (d)Yellow – blue – pink — Turmeric never turns blue or pink; this is not its indicator behaviour.
Concept
Turmeric is one of several natural acid-base indicators, along with litmus and china rose. The active pigment, curcumin, is yellow in acidic or neutral surroundings and changes to reddish-brown when it meets a base. Because soap solution is alkaline, it triggers the colour change, and the change reverses when the base is rinsed away.
The key facts are that soap is basic and that turmeric's colour change (yellow to reddish-brown) is reversible, so once the alkali is washed out the yellow returns.
Key facts
- Turmeric (curcumin) is a natural indicator: yellow in acid or neutral, reddish-brown in base.
- Soap solution is basic (alkaline), so it turns a turmeric stain reddish-brown.
- Washing out the alkali with water restores the yellow colour, so the change is reversible.
- Other natural indicators include litmus (from lichens) and china rose, which turns green in base and pink in acid.
Study next
Common traps
- Assuming turmeric behaves like litmus (red or blue) — it is yellow and reddish-brown.
- Forgetting the change is reversible once the base is washed away.
Uses turmeric as a natural indicator and asks the colour change on adding an alkali (soap) and then water.
Related PYQs
Litmus, a well-known acid-base indicator, is derived from:
- (a) Fungi
- (b) Lichens
- (c) Bacteria
- (d) Termite
Answer(b) Lichens
Practice
- practice — not a real PYQ
Turmeric turns reddish-brown in the presence of
- (a)an acid
- (b)a base
- (c)distilled water
- (d)common salt
Answer(b) a base — turmeric is yellow in acid or neutral and reddish-brown in base. - practice — not a real PYQ
Litmus, a common natural indicator, is obtained from
- (a)roses
- (b)lichens
- (c)turmeric
- (d)algae
Answer(b) lichens.