Exploring Substances: Acidic, Basic, and Neutral · Lesson 3 of 5
Turmeric and Indicators You Can Smell
“Investigate turmeric and onion indicators, recognise their limits, and use indicator responses to create hidden messages.”
• Describe how turmeric paper is prepared and used. • Interpret the red response of turmeric to a basic sample. • Explain why unchanged fresh turmeric paper cannot distinguish acidic and neutral samples. • Describe an olfactory indicator investigation with onion-treated strips. • Choose an indicator suited to the evidence needed and explain how indicator messages work.
A kitchen spice becomes a test paper
A yellow curry stain can change colour when soap is applied. The colour change is not simply a new layer of dirt. Turmeric in the stain responds to a basic material in the soap. This familiar observation gives us another indicator to investigate and another clue to the message at the science fair.
Turmeric, or haldi, is a plant material commonly used as a spice and colouring ingredient. A yellow turmeric preparation can turn red or reddish-brown in a basic sample. For the chapter’s tests, we will describe the change as yellow to red. Its response differs from red rose extract, even though both indicators come from plants.
Make and use turmeric paper
Activity 2.5 begins by mixing a small amount of turmeric powder with a little water to make a paste. Fresh turmeric can also be ground for the preparation. Dip filter paper into the paste so that it becomes yellow, remove it, and allow it to dry. With adult guidance, cut the dried paper into small strips.
Dry strips are convenient because each sample can be placed on a separate labelled piece. Place one drop of a teacher-selected sample on a fresh strip using a clean dropper. Record whether its yellow colour changes. Repeat with separate strips for the other samples in the chapter’s sample set.
The meaning of an unchanged result is especially important. Both acidic and neutral samples leave fresh turmeric paper yellow in this introductory test. Basic samples give the red response. Therefore, a yellow result does not establish that the sample is acidic, and it does not establish that the sample is neutral.
| Sample | Expected response of fresh turmeric paper | What the result tells us |
|---|---|---|
| Lemon juice | Remains yellow | Consistent with an acidic sample, but fresh turmeric alone does not prove acidity. |
| Amla juice | Remains yellow | Needs another indicator to distinguish from neutral. |
| Tamarind water | Remains yellow | Needs another indicator to distinguish from neutral. |
| Vinegar | Remains yellow | Needs another indicator to distinguish from neutral. |
| Soap solution | Turns red | Basic response. |
| Baking soda solution | Turns red | Basic response. |
| Lime water | Turns red | Basic response. |
| Washing powder solution | Turns red | Basic response. |
| Sugar solution | Remains yellow | Consistent with neutral, but fresh turmeric alone does not prove neutrality. |
| Common salt solution | Remains yellow | Needs another indicator to distinguish from acidic. |
| The neutral tap-water sample | Remains yellow | Needs another indicator to distinguish from acidic. |
This is the reasoning behind Table 2.4. When you group the unchanged samples, you combine the acidic Group A and neutral Group C from the litmus investigation. You have not discovered a new single category of substances. You have found that this indicator gives the same visible response for two categories.
Problem
A drop of solution S makes fresh yellow turmeric paper red. What classification does that support?
- 1.The starting paper was yellow and changed to red.
- 2.This is the characteristic basic response of turmeric in the investigation.
- 3.Classify S as basic under the test conditions.
- 4.The result does not identify the exact base and does not establish that S is safe to handle or drink.
Problem
Fresh turmeric paper stays yellow with vinegar and sugar solution. Must the two liquids have the same nature?
- 1.The indicator gives the same visible result for both samples.
- 2.Fresh turmeric paper remains yellow with acidic and neutral samples, so that result can fit either group.
- 3.Use blue and red litmus, or another suitable indicator, to distinguish the two.
- 4.In the chapter’s sample set, vinegar is acidic and sugar solution is neutral. Their same turmeric result does not make their natures the same.
An unchanged fresh turmeric strip rules out the characteristic red response in this test, but leaves acidic and neutral possibilities. Use an additional suitable test when your question requires choosing between those possibilities.
Explain the stain and create a message
Now return to the curry stain. If soap solution is basic, turmeric in the stain can change from yellow to red. The visible change follows the same pattern as the test strip. A student who knows the pattern can explain the change without assuming that a new red substance was added to the shirt.
The chapter also describes a greeting card with a message written in Odia. Ashwin coats paper with turmeric paste and lets it dry. A teacher-selected basic solution can then be used to write on the yellow surface. The written parts turn red, while the untouched background stays yellow. The writing becomes visible because different regions of the paper have different indicator responses.
Problem
How could a plain-looking sheet reveal writing when a turmeric liquid is sprayed on it?
- 1.Write with a suitable dilute basic solution, such as the chapter’s soap solution, and allow the writing to dry. Test a small sample first because some preparations leave visible traces.
- 2.The dried writing can be difficult to see before the indicator is applied.
- 3.When a turmeric preparation reaches the writing, the basic regions turn red.
- 4.Untouched regions keep the yellow turmeric colour, giving a contrasting background.
- 5.This is one possible explanation of the fair demonstration, rather than proof of the actual materials used there. Spraying or painting is done with adult guidance and care to avoid the eyes.
Other combinations can produce messages too. The important decisions are what nature the writing liquid has, which indicator will respond, and whether the written colour can be distinguished from the background. An acidic writing liquid such as orange juice needs an indicator that gives a useful acidic response; fresh turmeric’s unchanged yellow response will not clearly separate it from a neutral background.
Turmeric belongs to the ginger family and has long been used in cooking and in traditional Indian practices, including Ayurveda. Its cultural use provides context for the familiar material. The colour-change investigation tests its indicator behaviour; it does not establish a medical benefit.
An indicator response need not be a colour
So far, we have used our eyes to observe an indicator. Some substances also give different odour responses in acidic and basic surroundings. “Olfactory” refers to the sense of smell. An olfactory indicator therefore offers a different kind of observable evidence.
A substance whose characteristic odour response differs in acidic and basic surroundings and can be used as an indicator under suitable test conditions.
In Activity 2.6, place clean pieces of cotton cloth or filter paper in a closed container with finely chopped onion and leave them overnight. The strips take on an onion odour. Check the starting odour using the safe method shown by the teacher. Then place the strips on a clean surface and add tamarind water to one and baking soda solution to another.
Let the drops spread, compare the odours with the starting strips, and record what you notice. Keep piece size, treatment time, and the amount of sample similar. The source asks you to obtain these observations rather than providing a table of guaranteed results. Use the known acidic and basic samples as references for the preparation you actually used.
Suppose a classroom comparison gives a retained onion odour after tamarind water but a much less noticeable odour after the basic solution. Those recorded reference responses can help interpret another teacher-selected sample under the same conditions. If your comparison gives an unclear response, record that uncertainty and use another suitable indicator. Smell strength can also depend on the preparation and how the comparison is made.
This activity uses teacher-selected samples and a supervised method for noticing odour. Never put an unknown liquid close to the nose, taste it, or handle it while blindfolded. The blindfolded-student question is a reasoning scenario about a non-visual indicator, not an instruction for unsafe handling.
Problem
In a supervised reasoning task, a student cannot use a colour change to distinguish two samples known to be one acidic and one basic. What kind of indicator could help?
- 1.Litmus, red rose, and turmeric provide visual responses, so a colour-only observation does not meet the stated need.
- 2.An olfactory indicator offers an odour response instead.
- 3.Onion-treated strips can be compared using known acidic and basic reference samples and the safe procedure shown by a teacher.
- 4.The student should use the observed reference pattern, with help handling and labelling the samples, rather than trying to smell unknown chemicals directly.
Choose an indicator for the question
An indicator is a tool, and tools have different strengths. If the question is “Which sample is basic?”, a clear red response on fresh turmeric paper may be useful. If the question is “Is this sample acidic or neutral?”, fresh turmeric alone cannot settle it. A pair of litmus tests gives more suitable evidence for that distinction.
You can also use more than one indicator to check whether the observations agree. A red-to-blue litmus change and a yellow-to-red turmeric change both support a basic classification. If results conflict, check the samples, starting colours, and procedure. Do not select only the result that matches your first guess.
Acharya Prafulla Chandra Ray, often called the Father of Modern Indian Chemistry, earned a doctorate in chemistry in the United Kingdom before returning to India. He helped advance chemistry research in India and established the country’s first pharmaceutical company in 1901. He also wrote about the history of chemistry in India, bringing attention to earlier Indian scientific achievements, and supported education through the mother tongue. His work connects scientific investigation with useful medicines, history, and access to learning.
Check your understanding
Identify what each result can establish and what it leaves uncertain. A useful answer names the indicator and explains how its response fits the question.
Quiz
Fresh turmeric paper remains yellow after testing sample T. Which conclusion is justified?
Which sample gives the chapter’s red response on fresh turmeric paper?
Why does basic writing become visible on turmeric-coated paper?
What kind of observation is used by an olfactory indicator?
A student needs to distinguish an acidic sample from a neutral one. Which plan is most suitable?
Practice Problems
- Describe the stages of making turmeric paper and explain why separate dry pieces are useful.
- Fresh turmeric stays yellow with two unknown liquids. Explain why this does not show that the liquids have the same nature.
- Explain the colour change of a turmeric-containing stain when the chapter’s soap solution is used.
- Choose a teacher-selected writing liquid and indicator for a greeting card. State the expected writing colour and explain how the contrast is produced.
- Describe the onion-strip comparison, including the starting observation, the two known samples, and what should be recorded.
- Explain why a smell indicator may support a non-visual investigation without making it safe to sniff an unknown substance directly.
- A sample turns red litmus blue and turmeric red. Explain whether the observations agree.
- Describe how Prafulla Chandra Ray’s work connected chemistry with education or useful products.
Key Takeaways
• Turmeric paper is made by coating paper with a turmeric preparation, drying it, and cutting suitable pieces. • A basic sample turns fresh yellow turmeric paper red in the chapter’s investigation. • Acidic and neutral samples both leave fresh turmeric yellow, so this test alone cannot separate them. • Indicator colour changes can explain stains and create messages with a contrasting background. • Olfactory indicators provide odour responses; comparisons need safe samples and reliable reference observations. • Choose and interpret an indicator according to the question it can actually answer.