Red Rose, Turmeric and Smell Indicators
Class 7 Science · Chapter 2: Exploring Substances: Acidic, Basic, and Neutral · NCERT Curiosity section 2.1.2–2.1.3
Learn
No litmus? Use flowers! Zoya collects fallen red rose petals, crushes them, soaks them in hot water (with an adult's help) and filters the coloured liquid: red rose extract. With lemon juice it turns red; with soap solution it turns green; with salt solution it does not change.
rose extract: red = acidic · green = basic
Beetroot, purple cabbage, jamun and red hibiscus (gudhal) also make indicators. In the hills, hydrangea flowers are blue in acidic soil and pink or red in basic soil, so a gardener can change their colour by changing the soil!
Turmeric paper: dip filter paper in haldi paste and dry it. A drop of soap or baking soda solution turns it red; acids and neutral substances leave it yellow. So turmeric detects bases but cannot tell acids from neutral things. That is why a curry stain on a white shirt turns red when you scrub it with soap!
turmeric: red = basic · yellow = acidic or neutral
Some indicators work by smell: these are olfactory indicators. Cloth strips kept overnight with chopped onion smell of onion; after a drop of baking soda solution the smell fades, while tamarind water keeps it. Smell indicators help people who cannot see colour. Acharya P.C. Ray, the father of modern Indian chemistry, set up India's first pharmaceutical company in 1901.
Explore: the red rose test.
- Test each sample with red rose extract and sort it.
Remember
- Red rose extract is red in acids and green in bases; turmeric turns red only in bases.
- Olfactory indicators like onion change smell in acids or bases.
Quick check
Turmeric paper stays yellow with a liquid. What can you say?
- it is acidic or neutral, not basic
- it must be acidic
- it must be basic
- it must be neutral
Answer
it is acidic or neutral, not basic. Turmeric turns red only with bases, so yellow rules out basic only.
Think about it
How could Meher write a secret message that appears only when sprayed?
One answer
Coat paper with turmeric and dry it, then write with soap solution: when sprayed with water (or write with lemon juice on rose-extract paper) the writing changes colour. Or write with soap solution and spray turmeric solution: the words turn red.
Practice
Practise with fresh questions at three levels (easy, medium, hard), some answered hands-on, with hints and worked answers. Get 4 of your last 5 right to finish the topic.