Extra Questions for Class 7 Science Chapter 2: Exploring Substances – Acidic, Basic and Neutral

Original extra practice questions for Class 7 Science Chapter 2 (Exploring Substances: Acidic, Basic and Neutral), going beyond the textbook exercise. Full explanations included. These Class 7 Science Chapter 2 important questions are handy for last-minute exam practice.

Q1. A gardener notices that a particular flower bed’s soil turns blue litmus red. Is the soil acidic, basic, or neutral, and what could the gardener add to correct highly acidic soil?
Solution: Turning blue litmus red indicates acidic soil. To correct (neutralise) overly acidic soil, gardeners commonly add lime (calcium compounds), which is basic, to raise the soil’s pH toward neutral.

Q2 (Applied reasoning). Ant bites often cause a stinging sensation because ant venom contains formic acid. Why might rubbing baking soda paste on the bite help reduce the sting?
Solution: Baking soda is basic (alkaline). Applying it to a bite caused by an acidic venom (formic acid) allows a neutralisation reaction to occur, reducing the acidity at the site and helping relieve the stinging sensation.

Q3 (Assertion–Reason). Assertion (A): All sour-tasting substances are acidic. Reason (R): Acids are responsible for the sour taste in foods like lemon, tamarind and vinegar.
Solution: R is true (as demonstrated by this chapter’s own testing). A is generally consistent with what’s taught in this chapter (sourness is associated with acids), though as a strict universal claim it should be treated as the working rule for this level rather than an absolute law of all substances. For Class 7 purposes: both are considered true, with R explaining A.

Q4 (Design an experiment). You’re given an unlabelled liquid and only turmeric paper (no litmus). Design a simple test to determine if it’s basic, and explain the limitation of using turmeric paper alone.
Solution: Dip turmeric paper into the liquid. If it turns red, the liquid is basic. If it stays yellow (no change), the liquid could be either acidic or neutral — turmeric paper alone can’t distinguish between these two cases, since both leave it unchanged. A second indicator (like litmus) would be needed to tell acidic and neutral samples apart.

Q5 (Multi-step reasoning). A solution turns red rose extract green. A second, different solution is added to it, and the mixture turns red. What can you conclude about the nature of the substance that was added?
Solution: The original solution was basic (rose extract turned green). For the mixture to turn red (the acidic colour) after adding the second substance, that added substance must have been acidic and strong/plentiful enough to neutralise the base and shift the overall mixture to acidic.

Q6 (Spot the error). A student claims: “Since soap solution is basic and turns red litmus blue, all soaps must taste bitter like other bases.” Evaluate this reasoning.
Solution: While it’s true that bases often have a bitter taste and soapy feel (a general property mentioned for bases), tasting substances to test them is not a safe or recommended practice in chemistry, especially for substances like soap that aren’t meant to be ingested. The student’s chemical reasoning about soap being basic is fine, but the implied testing method (tasting it) should be flagged as unsafe practice, not a valid experimental step.

Q7 (Real-world application). Antacid tablets are commonly used to relieve the discomfort of excess stomach acid. Based on what you’ve learned, what is the likely chemical nature of an antacid, and why does it work?
Solution: An antacid is likely basic (alkaline) in nature. It works by neutralising the excess acid in the stomach, reducing acidity and relieving the associated discomfort — the same neutralisation principle used to treat acidic factory waste or acidic soil.

Q8 (Classification). Classify each as acidic, basic, or neutral based on this chapter’s data: (i) tamarind water (ii) washing powder solution (iii) tap water (iv) baking soda solution
Solution: (i) Tamarind water — acidic (contains tartaric acid). (ii) Washing powder solution — basic. (iii) Tap water — neutral. (iv) Baking soda solution — basic.

Q9 (Predicting an outcome). If you mix equal amounts of a strongly acidic solution and a strongly basic solution, what would you generally expect the resulting mixture’s nature to be, and why?
Solution: You would generally expect the mixture to move toward neutral, since mixing an acid and a base causes a neutralisation reaction where each partially or fully cancels out the other’s acidic/basic character — though the exact result (still slightly acidic, slightly basic, or fully neutral) depends on the relative strengths and amounts of each.

Q10 (Natural indicator preparation, applied). Explain, step by step, how you could prepare a simple natural indicator at home using red rose petals, and how you would use it to test a substance.
Solution: Preparation: (1) Collect and wash red rose petals. (2) Place them in a container and add hot water. (3) Let the mixture sit for several minutes so the colour extracts into the water. (4) Filter out the petals, keeping the coloured liquid (the indicator). Testing: Add a few drops of the extract to the substance being tested. If it turns red, the substance is acidic; if it turns green, the substance is basic; if the colour doesn’t change, the substance is likely neutral.

More on This Chapter

See also: NCERT Solutions for Class 7 Science Chapter 2, Class 7 Science NCERT Book and the Class 7 Science Formulas Handbook.

Revision Notes: Revision Notes for Class 7 Science Chapter 2

Written by Satish

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