Class 7 Science · Original EZT practice
Exploring Substances (Class 7): original worked practice
A colour change is evidence, not a dare. Use an indicator, name what changed, and say exactly what the result does and does not show.
6 original worked examples, 6 fresh self-check questions and 4 mixed questions, with explained answers. These are not copied textbook exercises, an official chapter index or a full answer key. No affiliation with NCERT is claimed.
Official sources for topic and edition context, checked 17 October 2026:
Choose, solve, check, then try a fresh question.
Before calculating, read the method prompt and name the condition that makes it useful. Write your own attempt. Open the steps only when you need them, and use the check to catch a wrong sign, unit or interpretation. Finish with the fresh question before reading its answer.
Classify by indicator evidence
Choose the method: Use the observed litmus change, not taste, smell or guesswork, to classify a tested sample.
EZT practice question
In written class records, lemon juice turns blue litmus red, soap solution turns red litmus blue, and sugar solution changes neither. Classify the three samples.
Show the worked solution
- Blue litmus turning red indicates an acidic sample.
- Red litmus turning blue indicates a basic sample.
- No change in either tested paper places this sample in the neutral group for this test.
Answer: Lemon juice: acidic; soap solution: basic; sugar solution: neutral in this test.
Check: These classifications match the chapter’s tested sample groups.
Watch for this: Do not taste an unknown substance to decide its group.
Now try it yourself
A written record says vinegar turns blue litmus red. What does that show?
Check your answer
The tested vinegar is acidic.
The indicator change is the evidence.
State what litmus proves and what it does not
Choose the method: A litmus result supports a broad group; it does not measure an exact pH.
EZT practice question
A solution turns blue litmus red. What can you conclude, and what can you not conclude from that single result?
Show the worked solution
- The tested solution behaves as acidic toward blue litmus.
- The result does not give a numerical pH or identify the acid.
Answer: It is acidic in this test, but its exact pH and identity remain unknown.
Check: Litmus is an acid-base indicator, not a pH meter.
Watch for this: Do not call every red or blue colour change proof of a particular chemical.
Now try it yourself
A sample changes neither red nor blue litmus. Is that alone proof that its pH is exactly 7?
Check your answer
No.
It supports a neutral result in that test, but exact pH needs appropriate measurement and conditions.
Use turmeric only for the group it can detect
Choose the method: Turmeric is useful for basic samples but does not separate acidic from neutral samples.
EZT practice question
A turmeric-paper patch turns reddish-brown after a soap-solution drop. What does that show, and what would no change fail to distinguish?
Show the worked solution
- Soap solution is basic in the chapter’s tested example.
- Turmeric turns reddish-brown in a basic medium but does not distinguish acidic from neutral samples.
Answer: The soap solution is basic; unchanged turmeric could be acidic or neutral.
Check: The chapter explicitly states this limit for turmeric paper.
Watch for this: Do not call no colour change proof of acidity.
Now try it yourself
Why can turmeric alone not identify both vinegar and sugar solution?
Check your answer
Both can leave turmeric without the basic reddish-brown change.
A second indicator is needed to separate them.
Match each natural indicator to its colour pattern
Choose the method: Different indicators use different colour systems; do not copy litmus colours onto every extract.
EZT practice question
In the chapter’s written red-rose extract results, what colours suggest acidic and basic samples?
Show the worked solution
- The extract appears to turn a shade of red in an acidic solution.
- It appears to turn a shade of green in a basic solution.
Answer: Red suggests acidic and green suggests basic for the tested red-rose extract.
Check: The textbook uses careful wording because observations can vary with the extract.
Watch for this: Do not say blue or red litmus uses the same green signal.
Now try it yourself
Why is a result with red-rose extract still evidence about the tested sample rather than every sample with the same name?
Check your answer
Concentration, preparation and sample differences can affect observations.
The test supports a classification for that sample; it is not a universal identity check.
Track the acid-base model precisely
Choose the method: In the chapter model, an acid and a base in suitable amounts neutralise, forming salt and water with heat.
EZT practice question
A written model mixes an acidic solution with lime water until the mixture is no longer acidic. What is this reaction called, and what products does the chapter model give?
Show the worked solution
- Lime water is basic and neutralises the acidic solution.
- The chapter model gives salt and water, with heat released.
Answer: Neutralisation; salt and water, with heat.
Check: The model equation is acid + base → salt + water + heat.
Watch for this: Do not say every reaction of an acid with any substance is neutralisation.
Now try it yourself
Why does an acid reacting with a metal not fit the simple salt-plus-water neutralisation model?
Check your answer
That different reaction can produce hydrogen rather than following the acid-plus-hydroxide model.
Neutralisation has its own stated reactant types and products.
Apply neutralisation without giving treatment instructions
Choose the method: A chemistry explanation is not automatically a safe personal or farm treatment plan.
EZT practice question
Why should this page explain antacids, ant bites and acidic soil only as examples, not tell a reader what to use?
Show the worked solution
- The chapter uses these contexts to show neutralisation in daily life.
- Real treatment or soil amendment depends on the substance, dose, conditions and qualified advice.
Answer: Neutralisation explains the idea, but safe action needs product directions and appropriate expert guidance.
Check: Too much of a base or the wrong material can create a new problem.
Watch for this: Do not infer that every bite, stomach problem or field needs baking soda or lime.
Now try it yourself
A soil report says a field is too acidic. What is the safe next step in this written-practice context?
Check your answer
Use the report and qualified agricultural guidance before choosing any amendment.
The page can name the neutralisation idea without prescribing a dose or material.
Try a mixed set
Choose a method or explanation before opening each answer.
1. Does every bitter substance prove that it is basic?
Show answer and reason
No.
Taste is not a safe or reliable classification test, and some bitter foods are not bases.
2. Is every salt solution neutral?
Show answer and reason
No.
A tested common salt solution can be neutral, but salt solutions can also be acidic or basic.
3. Can turmeric distinguish an acidic solution from a neutral one?
Show answer and reason
No.
Turmeric changes in a basic solution but does not separate acidic from neutral in this chapter test.
4. Does this page recommend treating an ant bite with baking soda?
Show answer and reason
No.
It discusses the textbook’s neutralisation example without giving bite-treatment advice.
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Questions about this practice page
- What is an acid-base indicator?
- A substance that changes colour or odour differently in acidic and basic samples. Litmus, red-rose extract, turmeric and some odour-based substances are examples.
- What does litmus show?
- Blue litmus turning red indicates an acidic sample. Red litmus turning blue indicates a basic sample. No change supports a neutral result for that tested sample, not an exact pH.
- What is the limit of turmeric as an indicator?
- Turmeric turns reddish-brown in a basic sample, but it does not distinguish an acidic sample from a neutral one by itself.
- What is neutralisation?
- In the chapter model, a suitable acid and base react to form salt and water with heat released. It is not a universal description of every acid reaction.
- Are these the Curiosity Chapter 2 textbook questions?
- No. These are original EZT written practice questions on the chapter ideas, not a lab procedure or textbook answer key.