Acids, bases and indicators
Acids taste sour and turn blue litmus red. Bases taste bitter, feel soapy and turn red litmus blue. Indicators tell them apart by colour or smell.
Indicator colours
| Indicator | In acid | In base |
|---|---|---|
| Blue litmus | Turns red | No change |
| Red litmus | No change | Turns blue |
| Phenolphthalein (synthetic) | Colourless | Pink |
| Methyl orange (synthetic) | Red | Yellow |
| Turmeric (natural) | No change (stays yellow) | Reddish-brown |
Kinds of indicators
- Natural indicators come from plants: litmus (from lichen), turmeric, red cabbage leaves, beetroot, and the coloured petals of flowers such as Hydrangea, Petunia and Geranium. Each shows one colour in an acid and a different colour in a base.
- Synthetic indicators are made in laboratories: phenolphthalein and methyl orange.
- Olfactory indicators change their smell in an acid or a base. Onion, vanilla essence and clove oil lose their smell in a base.
- Olfactory indicators let students who cannot see colours, including visually impaired students, test acids and bases.
- Never taste a substance to test it. Use an indicator.
Activity: onion as an olfactory indicator
- 1Put finely chopped onion and some strips of clean cloth in a plastic bag. Tie it and leave it overnight in the fridge.
- 2Smell two of the cloth strips.
- 3Put a few drops of dilute HCl on one strip and a few drops of dilute NaOH on the other. Smell them again.
- 4★Result: the onion smell stays with HCl, but is lost with NaOH.Onion is an olfactory indicator for bases
★ key step
A turmeric stain test
A sambar or curry stain on a white cloth is yellow because of turmeric. When you scrub it with soap, which is basic, the stain turns reddish-brown. Rinse it with plenty of water and it turns yellow again. Turmeric is acting as a natural indicator.
A haldi (turmeric) stain from sambar turns reddish-brown when you scrub it with soap, because soap is basic. Tamarind, lemon and curd taste sour because they contain acids.
1 marks: “A drop of soap solution turns red litmus paper blue. What does this tell you about soap?” Soap is basic, because a base turns red litmus blue.
2 marks: “What are olfactory indicators? Name two.” Substances whose smell changes in an acidic or basic medium. Examples: onion, vanilla essence, clove oil.
3 marks: “Three unlabelled bottles hold lemon juice, soap solution and distilled water. You have only blue litmus paper. How will you identify each?” Dip blue litmus in each. Lemon juice (an acid) turns it red. Dip this red paper in the other two: soap solution (a base) turns it blue again, and distilled water causes no change.
A-R-B: an Acid turns blue litmus Red; a Base turns red litmus Blue. Phenolphthalein turns Pink in bases.