This article contains solutions to the new NCERT questions of class 7 science chapter 2 "Acid, Bases and Neutrals"
Q1. A solution turns the red litmus paper to blue. Excess addition of which of the following solution would reverse the change?(i) Lime water
(ii) Baking soda
(iii) Vinegar
(iv) Common salt solution
Ans. (iii) Vinegar (Acidic in Nature)
Q2. You are provided with three unknown solutions labelled A, B, and C, but you do not know which of these are acidic, basic, or neutral. Upon adding a few drops of red litmus solution to solution A, it turns blue. When a few drops of turmeric solution are added to solution B, it turns red. Finally, after adding a few drops of red rose extract to solution C, it turns green. Based on the observations, which of the following is the correct sequence for the nature of solutions A, B, and C?
(i) Acidic, acidic, and acidic
(ii) Neutral, basic, and basic
(iii) Basic, basic, and acidic
(iv) Basic, basic, and basic
Ans.
Solution A: Red litmus turns blue. This indicates Solution A is basic.
Solution B: Turmeric solution turns red. Turmeric turns red in basic solutions.
Solution C: Red rose extract turns green. Red rose extract turns green in basic solutions.
The correct answer is (iv) Basic, basic, and basic
Q3. Observe and analyse Figs. 2.13, 2.14, and 2.15, in which red rose extract paper strips are used. Label the nature of solutions present in each of the containers.
Ans.
- Fig 2.15: Red rose extract paper strip turns red or pink, the solution is acidic.
- Fig 2.14: Red rose extract paper strip shows no significant color change (remains light pink/clear), the solution is neutral.
- Fig 2.13: Red rose extract paper strip turns green, the solution is basic.
Q4. A liquid sample from the laboratory was tested using various indicators:
Based on the tests, identify the acidic or basic nature of the liquid and justify your answer.
Ans. Let's analyse the changes:
- Red litmus: No change. This indicates the liquid is not basic.
- Blue litmus: Turned red. This is a characteristic property of an acidic solution.
- Turmeric: No change in colour. Turmeric remains yellow in acidic and neutral solutions and turns red in basic solutions. The "no change" further confirms it's not basic.
Conclusion: The liquid is acidic in nature.
Q5. Manya is blindfolded. She is given two unknown solutions to test and determine whether they are acidic or basic. Which indicator should Manya use to test the solutions and why?
Ans. Manya should use an olfactory indicator. Reason: Since Manya is blindfolded, she cannot see color changes. Olfactory indicators are substances whose smell changes in acidic or basic medium. Examples of olfactory indicators:
- Onion: Its characteristic smell is lost or diminished in basic solutions but remains in acidic solutions.
- Vanilla extract: Its characteristic smell is lost in basic solutions but persists in acidic solutions.
Q6. Could you suggest various materials which can be used for writing the message on the white sheet of paper and what could be in the spray bottle? Make a table of various possible combinations and the colour of the writing obtained.
Ans. This refers to invisible ink. The writing material is the invisible ink, and the spray bottle contains the indicator to reveal it.
Q7. Grape juice was mixed with red rose extract; the mixture got a tint of red colour. What will happen if baking soda is added to this mixture? Justify your answer.
Ans.
- Initial observation: Grape juice is acidic, so red rose will convert it to dark red.
- Adding baking soda: Baking soda is basic. It will cause neutralization reaction. It will neutralize the acidic solution of grape juice. In basic medium, the red rose will begin to show green color.
Q8. Keerthi wrote a secret message to her grandmother on her birthday using orange juice. Can you assist her grandmother in revealing the message? Which indicator would you use to make it visible?
Ans. Orange juice is an acidic substance. To reveal the message, an indicator that shows a unique color in the presence of an acid can be used.
Red Cabbage Juice: This is an excellent choice. When sprayed or applied to the paper, the areas where orange juice was used (acidic) will turn red or pink, making the message visible against the white paper.
Q9. How can natural indicators be prepared? Explain by giving an example.
Ans. Natural indicators can be prepared by extracting colored pigments from various plant parts (like petals, leaves, fruits, or roots) that exhibit different colors in acidic and basic conditions.
Example: Preparation of Turmeric Paper (Indicator)
Procedure:
1. Prepare Turmeric Paste: Take about a tablespoon of turmeric powder in a small bowl. Add a small amount of water (a few drops at a time) and mix to form a thick, smooth paste.
2. Dilute the Paste: Add a bit more water to the paste to make a slightly runny solution.
3. Soak the Paper: Dip strips of filter paper or white blotting paper into the turmeric solution. Ensure the paper is fully saturated with the turmeric extract.
4. Dry the Paper: Carefully remove the paper strips and lay them flat on a clean surface or hang them to dry completely. It's best to dry them in a shaded area, as direct sunlight can fade the color.
5. Cut into Strips: Once dry, cut the turmeric-stained paper into small strips. This is your turmeric indicator paper.
How it works:
- In acidic solutions: Turmeric paper remains yellow.
- In neutral solutions: Turmeric paper remains yellow.
- In basic solutions: Turmeric paper turns red.
This prepared turmeric paper can then be used to test the nature of unknown solutions.
Q10. Three liquids are given to you. One is vinegar, another is a baking soda solution, and the third is a sugar solution. Can you identify them only using turmeric paper? Explain.
Ans. No, we cannot identify all three liquids using only turmeric paper. Turmeric paper acts as an acid-base indicator with the following color changes:
- Turmeric in acidic solutions (like vinegar): Remains yellow (no change).
- Turmeric in neutral solutions (like sugar solution): Remains yellow (no change).
- Turmeric in basic solutions (like baking soda solution): Turns red.
Q11. The extract of red rose turns the liquid X to green. What will the nature of liquid X be? What will happen when excess of amla juice is added to liquid X?
Ans.
1. Red rose extract acts as an indicator: red/pink in acidic, green in basic, and no significant change/light pink in neutral. Nature of liquid X: If the extract of red rose turns liquid X green, then liquid X is basic.
2. Amla juice is highly acidic (rich in Vitamin C, which is ascorbic acid). When an excess of amla juice (an acid) is added to liquid X (a base), a neutralization reaction will occur. As the acidic amla juice is added, it will first neutralize the basic nature of liquid X. If enough amla juice is added to make the solution overall acidic, the red rose extract indicator will change its color from green (basic) to red or pink (acidic).
Q12. Observe and analyse the information given in the following flowchart. Complete the missing information. Which indicator can be used to test the nature of the soil?
Ans.
The soil can be acidic in nature.
The soil can be basic in nature.
Indicators that can be used: pH paper, Universal indicator.
Acidic soil can be treated with basic substances (lime stone CaCO3, wood ash)
Basic soil can be treated with acidic substances organic matter / acidic fertilizers