Exploring Substances: Acidic, Basic, and Neutral — Mastery

Q1. At the National Science Day fair, why did the words 'Welcome to the Wonderful World of Science' appear on the white paper given to Ashwin and Keerthi when a liquid was sprayed?

• A. The paper had a basic substance written on it and an acid–base indicator was sprayed, making the writing visible
• B. The paper was warmed by the spray
• C. The spray contained fluorescent dye that glows in light
• D. The paper had hidden ink that reacts with water

Q2. The chapter classifies substances into three groups — acidic, basic, and neutral — using litmus. The main purpose of this three-way classification is to

• A. Decide the price of the substance
• B. Group substances by their colour
• C. Decide which substances are edible
• D. Predict how a substance will behave with other substances and indicators

Q3. Activity 2.1 of the chapter does NOT include which of the following samples for testing with litmus paper?

• A. Amla juice
• B. Tamarind water
• C. Washing powder solution
• D. Coconut oil

Q4. The chapter notes that besides natural indicators like litmus, some other indicators are also used. Where are these synthetic indicators made?

• A. In laboratories
• B. In flowering plants
• C. In animal cells
• D. In monsoon rainwater

Q5. Figure 2.4 marks tomato as containing citric acid and

• A. Tartaric acid
• B. Lactic acid
• C. Formic acid
• D. Oxalic acid

Q6. Vinegar, used to add a sour taste to food, contains which common acid?

• A. Citric acid
• B. Ascorbic acid
• C. Tartaric acid
• D. Acetic acid

Q7. Which statement best captures the chapter's view about bases and their bitter taste?

• A. Bases are generally bitter, but everything bitter need not be a base
• B. Every bitter substance is a base
• C. Bases are always sweet
• D. Only edible bases are bitter

Q8. While preparing the red rose indicator, the chapter advises students to

• A. Pluck fresh rose flowers from the garden
• B. Use only synthetic rose petals
• C. Avoid red roses and use only white roses
• D. Use fallen petals or flowers from the ground, not plucked ones

Q9. In Activity 2.3, the liquid obtained after filtering the crushed red rose petals soaked in hot water is called

• A. Residue
• B. Filtrate
• C. Sediment
• D. Mixture

Q10. While preparing turmeric paper in Activity 2.5, a piece of filter paper is dipped in

• A. Turmeric paste
• B. Red rose extract
• C. Plain water
• D. Vinegar

Q11. According to the chapter's Fascinating Facts box, turmeric belongs to which plant family?

• A. Cabbage family
• B. Citrus family
• C. Pea family
• D. Ginger family

Q12. In Activity 2.6, to make olfactory indicator strips, students are asked to

• A. Boil onion in vinegar for ten minutes
• B. Take finely chopped onions in a container along with cotton cloth or filter paper strips and leave overnight
• C. Soak onion slices in red rose extract
• D. Dry onion peels in sunlight and grind them

Q13. Manya is blindfolded and given two unknown solutions to test whether they are acidic or basic. Which indicator is the BEST choice for her, and why?

• A. An olfactory indicator like onion strips — colour change cannot be observed by a blindfolded person, but odour change can be sensed
• B. Litmus paper — she can still see the colour change later
• C. Red rose extract — it changes red or green
• D. Turmeric paper — she will feel it heat up

Q14. When an acid solution is mixed with a basic solution in sufficient quantity, the resulting solution is

• A. Always acidic
• B. Always basic
• C. Neither acidic nor basic
• D. First basic, then acidic

Q15. Why does rubbing moist baking soda over a red ant bite relieve the pain, as per the chapter?

• A. Baking soda cools the skin
• B. Baking soda is basic and neutralises the acidic (formic) liquid injected by the ant
• C. Baking soda kills the ant
• D. Baking soda is acidic and reacts with the skin

Q16. While doing Activity 2.1, Ravi places one drop of lemon juice on a red litmus strip and one drop on a blue litmus strip. According to the chapter's procedure, what should he observe?

• A. Both strips remain unchanged in colour
• B. Red strip stays red; blue strip turns red
• C. Red strip turns blue; blue strip stays blue
• D. Both strips turn green

Q17. The chapter teaches acidic, basic and neutral nature by first asking students to test eleven samples with litmus and only then grouping them into A, B and C. The teaching strategy at work is

• A. Definition first, then examples
• B. Lecture first, then activity
• C. Memorise the equation, then verify
• D. Observation first, then classification by pattern

Q18. A Class 7 student finds a clear liquid in a science-lab bottle whose label has fallen off. To check whether it is acidic or basic, the chapter would advise her to

• A. Taste a small drop on the tongue
• B. Smell the bottle deeply for a long time
• C. Test the liquid with litmus paper or another indicator without tasting it
• D. Pour it onto her palm and rub

Q19. After a Class 7 lesson, some students conclude that 'every bitter food is basic'. As per the chapter, which simple classroom counter-example best corrects this misconception?

• A. Showing that karela (bitter gourd) tastes bitter but is not basic in nature
• B. Showing that sugar tastes sweet, so it is acidic
• C. Showing that water is tasteless, so it is basic
• D. Showing that lemon tastes sour, so it must be bitter too

Q20. In Activity 2.6, a few drops of tamarind water are placed on an onion strip and the strip is smelled after some time. According to the chapter, the most likely observation is

• A. The strip smells exactly like a raw onion as before
• B. The strip smells strongly of vinegar
• C. The strip turns red but the smell is unchanged
• D. The smell of onion fades or changes noticeably

Q21. When a basic substance like baking soda solution is placed on an onion-strip olfactory indicator, the chapter reports that

• A. The onion strip turns blue
• B. Heat is released and the strip burns
• C. No effect is observed at all
• D. The smell of onion changes

Q22. Lemon juice (acid) is added drop by drop to lime water (base). Apart from the colour change of an indicator, what other change is also expected as per the chapter's equation Acid + Base → Salt + Water + Heat?

• A. The mixture becomes lighter in mass
• B. A bright light is produced
• C. An audible sound is heard
• D. The mixture becomes warmer than before

Q23. In Activity 2.7, a few drops of blue litmus solution and one drop of lemon juice are put in a test tube; the contents turn red. Lime water is then added until the colour becomes faintly blue. At the faintly-blue stage, the contents of the test tube are best described as

• A. Strongly acidic
• B. Just neutralised, with a slight excess of base making it weakly basic
• C. Pure salt water with no acid or base left
• D. Strongly acidic again because lemon juice was added first

Q24. The chapter notes that the fish population near Gurbir's village lake has been declining because acidic factory waste is released into it. The most likely chain of cause and effect is

• A. Acidic waste makes the lake water acidic; many fish cannot survive in too-acidic water, so their numbers fall
• B. Acidic waste makes the lake water basic; fish dislike basic water and migrate away
• C. Acidic waste cools the water sharply, freezing the fish
• D. Acidic waste produces sweet water that confuses the fish

Q25. A Class 7 teacher in a rural school has no laboratory and no synthetic indicator. Which of the following is the BEST first activity to introduce 'acidic vs basic' to her students using only easily available items?

• A. Wait for school grant funds to buy a litmus paper pack
• B. Read aloud the definitions of acid and base from the textbook for the whole period
• C. Prepare red rose extract or turmeric paper in the classroom and test lemon juice, baking soda and tap water
• D. Ask students to memorise Figure 2.4 of edible acids overnight

Q26. The 'Know A Scientist' box of the chapter mentions that Acharya P.C. Ray established India's first pharmaceutical company. The year of this establishment, as given in the chapter, is

• A. 1857
• B. 1901
• C. 1947
• D. 1962

Q27. The 'Know A Scientist' box notes that P.C. Ray strongly advocated mother-tongue as the medium of instruction. Which classroom decision, in the spirit of this advocacy, is most consistent with the chapter?

• A. Explaining Activity 2.1 in students' mother tongue while introducing technical terms like 'acidic', 'basic' and 'neutral' in English alongside
• B. Conducting Activity 2.1 only in English so that students 'learn the language of science'
• C. Skipping discussion of activities and asking students to copy English notes from the board
• D. Telling students that mother-tongue learning is unscientific

Q28. Vinegar, baking soda solution and sugar solution are given as three unlabelled bottles. Only turmeric paper is available. As per the chapter, which pair CANNOT be distinguished from each other using turmeric paper alone?

• A. Vinegar and baking soda solution
• B. Baking soda solution and sugar solution
• C. Vinegar and a fourth bottle of soap solution
• D. Vinegar and sugar solution

Q29. A farmer's plants are growing poorly. Soil test shows the soil is acidic. Following the chapter's logic, the correct next step is to add

• A. Lime, a base, to neutralise the acidic soil
• B. Vinegar, to make the soil more acidic
• C. Compost only, because it works for all soil types
• D. Common salt, to balance the pH

Q30. While filling Table 2.3 of Activity 2.3 for red rose extract, a student records that 'lime water turned the extract green'. In the Nature column of the table, she should write

• A. Acidic
• B. Neutral
• C. Cannot be decided
• D. Basic