Start with the key idea
An acid–base indicator changes colour over a particular range of conditions. In the usual school litmus test, an acidic solution turns blue litmus red and a basic solution turns red litmus blue. A neutral solution changes neither strip. Record the starting colour, the final colour and the indicator’s name before drawing a conclusion.
1. What are acidic, basic and neutral solutions?
This guide compares solutions in water. An acidic solution and a basic solution behave differently in suitable chemical tests. A solution that is neither acidic nor basic is called neutral. Familiar food examples can help you remember the words, but tasting is not a test to carry out in Science.
Lemon juice contains acids; an appropriate diluted soap solution is usually basic. Pure water is neutral under the usual classroom conditions. A product name alone is not enough to classify every mixture: its ingredients and concentration matter. An unknown clear liquid could belong to any group.
Alkali means a base that dissolves in water. Not every base is soluble. At this level, concentrate on interpreting the observations for the supplied solutions rather than using appearance as evidence. Colourless does not mean neutral, and natural does not mean harmless.
2. Use both litmus results
Red and blue litmus have different starting colours. Saying ‘litmus became red’ is incomplete unless we know which colour it was initially. Write a change as blue → red, or explicitly write red → red: no change.
| Sample | Red strip | Blue strip |
|---|---|---|
| Acidic | Stays red | Turns red |
| Basic | Turns blue | Stays blue |
| Neutral | Stays red | Stays blue |
A colour change provides useful evidence. No change in only one strip can leave two possibilities. A blue strip staying blue fits a basic solution, but also fits a neutral solution. Use the other strip before choosing between those two categories.
This simple table assumes a suitable sample and functioning indicator. Strongly coloured or bleaching samples can make a visual result hard to interpret. In a real investigation, report a doubtful observation and seek an appropriate test rather than forcing it into a category.
3. Different indicators have different colour rules
Never transfer the litmus colour rule to another indicator. In a typical school turmeric test, the indicator is yellow in acidic and neutral samples and becomes reddish-brown in a basic sample. Yellow alone therefore cannot distinguish an acid from a neutral solution.
Phenolphthalein is colourless in acidic and neutral solutions and pink over its alkaline transition range. A colourless result by itself is not proof of acidity. Universal indicator gives a range of colours that you compare with the chart supplied for that indicator. It estimates pH; it does not identify the exact chemical in the sample.
Plant extracts such as red cabbage can also act as indicators. Their colours depend on preparation and pH, so use the stated reference chart. A chart for one extract should not be substituted for another. Record colour words as well as a coloured sketch so the evidence is understandable in a black-and-white worksheet.
For the questions below, use the exact indicator key supplied. This is an exercise in reading evidence, not an invitation to prepare or test household chemicals.
4. What does pH add?
Under the usual classroom condition of about 25°C, pH below 7 is acidic, pH 7 is neutral and pH above 7 is basic. A pH reading of 5 therefore gives more specific information than a litmus classification of ‘acidic’. The colours of universal indicator give an approximate range rather than unlimited precision.
For example, if a supplied chart labels orange as approximately pH 4 and green as approximately pH 7, an orange sample is acidic under that chart. Do not write pH 4.000 from a rough colour match. Do not assume an orange-coloured liquid has pH 4 before adding or checking the indicator: its original colour may be unrelated.
Acid strength and concentration are different ideas. Strength concerns how an acid ionises in water; concentration concerns how much acid is present in a given amount of solution. ‘Weak’ does not mean safe, and ‘dilute’ does not mean neutral. Litmus alone does not measure either quantity.
5. Neutralisation: explain the reaction and its limits
A common school model is acid + metal hydroxide base → salt + water. For example, hydrochloric acid reacts with sodium hydroxide to form sodium chloride and water. This is a written reaction model here, not a practical activity to try at home.
Neutralisation is not simply dilution. Adding water reduces concentration; reacting an acid with a suitable base changes the reacting substances. Many neutralisation reactions release heat. The amount of heating depends on the substances, quantities and conditions.
The final mixture is not automatically neutral just because both an acid and a base were added. If acid remains in excess, the mixture can still be acidic. If base remains in excess, it can still be basic. Equal liquid volumes do not guarantee chemically matching amounts.
‘Salt’ is a class of compounds, not another name for neutral water. Sodium chloride is one example. Different salts can give acidic, basic or neutral solutions. This is why the final result needs evidence, not a conclusion based only on the word neutralisation.
6. Make an observation record you can defend
Divide your record into four columns: sample label; indicator and starting colour; final colour; conclusion with reason. Keep observations separate from interpretations. ‘The blue strip became red’ is an observation. ‘The sample is acidic under the litmus test’ is the interpretation.
For a fair comparison, a teacher would use suitable labelled samples, fresh indicator portions, consistent quantities and a known reference. Reusing a strip or dropper can transfer material and confuse the result. We use supplied data instead of a practical procedure in this guide.
If two observations contradict the simple key, do not silently delete one. Write ‘inconsistent results; repeat with fresh material and check labels’. Recognising that the evidence is insufficient is a stronger scientific answer than inventing certainty.
Everyday applications include controlled treatment of acidic soil or waste streams, but the treatment depends on measurements. A classroom colour rule is not advice to treat a sting, take a medicine or add chemicals to soil.
5 worked examples with reasoning
1. Sample R turns blue litmus red; red litmus stays red. What does the evidence support?
- Name the changing strip: blue litmus became red.
- Match this change to the litmus key: acidic.
- The unchanged red strip is consistent with the same conclusion. It does not tell us which acid or its concentration.
2. Sample S leaves blue litmus blue. Can you label it basic?
- Record no change, rather than treating the final colour as a change.
- Both basic and neutral solutions can leave blue litmus blue.
- A red-litmus result is needed to distinguish those possibilities under the simple key.
3. Turmeric stays yellow in sample T. Red litmus stays red and blue litmus turns red. Classify T.
- Turmeric alone leaves acidic and neutral possibilities.
- Blue litmus turning red supports an acidic classification.
- Use the more discriminating observation. The yellow turmeric result does not contradict acidity.
4. A model reaction mixture is tested after acid and base were added. Blue litmus still turns red. Must the result be wrong?
- The test indicates an acidic mixture under the given key.
- Acid may remain in excess; merely adding a base does not prove complete neutralisation.
- Do not infer matching reacting amounts from equal volumes or from the names of the ingredients.
5. A chart assigns orange to approximately pH 4 and green to approximately pH 7. Sample U matches orange. What should the report say?
- State the chart and approximate match: about pH 4.
- At the stated classroom temperature this is acidic.
- Do not report extra decimal places or claim to identify the chemical from this colour alone.
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These are paper-based observation problems. Do not taste, touch or mix unknown substances. Use the results supplied in each question; an indicator result does not establish that a material is safe.
16 practice questions with explanations
These are paper-based observation problems. Do not taste, touch or mix unknown substances. Use the results supplied in each question; an indicator result does not establish that a material is safe.
Write an answer and your reason before opening the explanation. Check the reasoning as well as the final answer.
- Litmus
Blue litmus becomes red in sample A. Classify it using the usual school key.
Answer and reasoning
Acidic.
The blue-to-red change is evidence for acidity. It is not an exact pH measurement.
- Litmus
Red litmus becomes blue in sample B. What classification fits?
Answer and reasoning
Basic.
The change, not just the final blue colour, is the relevant observation.
- Evidence
Red litmus stays red in sample C. List the two possibilities left by this result.
Answer and reasoning
Acidic or neutral.
Both leave red litmus red in the simple key. Blue litmus can help distinguish them.
- Evidence
Both strips keep their original colours in sample D. What conclusion fits the simplified key?
Answer and reasoning
Neutral.
Red stays red and blue stays blue. The conclusion assumes reliable strips and a suitable sample.
- Vocabulary
Is every base an alkali? Explain.
Answer and reasoning
No. An alkali is a base that dissolves in water.
Solubility is part of the meaning of alkali, not a property of every base.
- Indicator key
Turmeric stays yellow in sample E. Does this prove E is neutral?
Answer and reasoning
No; an acidic sample can also keep turmeric yellow.
A result shared by two categories cannot distinguish them.
- Combined evidence
Turmeric is reddish-brown and red litmus becomes blue. Do the results agree?
Answer and reasoning
Yes. Both support a basic classification under the stated keys.
Two compatible observations make the interpretation easier to check, without identifying the exact chemical.
- Precision
An approximate indicator chart gives pH 6. Is the sample acidic or basic at about 25°C?
Answer and reasoning
Acidic.
Six is below seven. The chart does not justify extra decimal places.
- Precision
A sample is colourless before any test. What can you conclude about its acidity?
Answer and reasoning
Nothing definite from that observation alone.
Colourless solutions can be acidic, basic or neutral.
- Reasoning
A learner writes ‘the sample is acid because the paper is red’. What detail is missing?
Answer and reasoning
The indicator name and starting colour, including whether a change occurred.
Red litmus staying red is less decisive than blue litmus turning red.
- Reaction
A student adds equal volumes of two unknown acid and base solutions and predicts a neutral mixture. Is that prediction guaranteed?
Answer and reasoning
No.
Equal volumes need not contain matching reacting amounts; concentrations and chemistry matter.
- Reaction
What are the products in the school model hydrochloric acid + sodium hydroxide?
Answer and reasoning
Sodium chloride and water.
This specific acid–hydroxide neutralisation produces a salt and water. It is a written example, not a mixing instruction.
- Vocabulary
Does ‘weak acid’ mean ‘safe to touch’?
Answer and reasoning
No.
Strength describes ionisation behaviour, not an overall safety judgement.
- Investigation
Two supposedly identical fresh samples give conflicting strip results. What should you record?
Answer and reasoning
Record both results and flag the inconsistency for checking or repeating.
Possible causes include mislabelling, contamination or a difficult colour reading. Do not choose a preferred result without evidence.
- Transfer
Write an observation and a separate interpretation for blue litmus becoming red.
Answer and reasoning
Observation: blue litmus became red. Interpretation: the sample is acidic under the litmus key.
The first statement describes what was seen; the second uses a rule to explain it.
- Review
A teacher gives you only a yellow turmeric result. Which next supplied observation would distinguish acid from neutral?
Answer and reasoning
The result of blue litmus: red supports acidic; unchanged blue is consistent with neutral when combined with the other evidence.
Use a test whose expected results differ between the remaining possibilities. Real tests still need suitable samples and reliable indicators.
Use the ideas without looking at the notes
Indicator → starting colour → final colour → rule → conclusion. Keep this sequence beside your practice. If one result fits more than one group, name the uncertainty and explain which additional observation would help.
Try making a three-row observation table for one acidic, one basic and one neutral sample. Supply both red- and blue-litmus results. Swap the table with someone and ask them to justify each classification without relying on the labels.
For parents: if your child remembers the colours but guesses on unknown samples, ask which possibilities the observation rules out. If they confuse the starting and final colours, use arrow notation before introducing another indicator.
Common questions
Does red litmus staying red prove acidity?
No. A neutral sample also leaves red litmus unchanged in the usual school key. Read the blue-litmus result too.
Is a neutral solution always harmless?
No. Acid–base classification is only one property. It does not establish whether a substance is safe to drink, touch or mix.
Are all salts neutral?
No. Different salts can give acidic, basic or neutral solutions. Table salt is one example, not the definition of the whole class.
Should I memorise every indicator colour?
Start with the key supplied in your lesson or question. Explain what an observation establishes and what it leaves uncertain. Different indicators have different rules.
Is this an official NCERT chapter solution?
No. It is an original Grade 7-level learning guide. Chapter numbers and textbook coverage depend on the edition used by your school.
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About this guide
Original explanations, examples, diagrams and practice by BananaLearn. The grade label indicates the intended learning level; this is not an official textbook extract or exam paper.