RBSE Solutions Class 7 Science Chapter 2 Exploring Substances Acidic Basic and Neutral

Class 7 Science Curiosity Chapter 2 Question Answer (Exercise)

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Question 1. A solution turns the red litmus paper to blue. Excess addition of which of the following solution would reverse the change? (iii) Vinegar
Answer:

🎯 Exam Tip: When a substance turns red litmus blue, it is basic. To reverse this, an acidic substance must be added to neutralize it, causing the color change to revert.

Question 2. 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
Answer: (iv) Basic, basic, and basic
In simple words: Solution A turns red litmus blue, meaning it is basic. Solution B turns turmeric red, showing it is basic. Solution C turns red rose extract green, which also indicates it is basic. All three solutions are basic.

🎯 Exam Tip: Remember how different indicators (litmus, turmeric, red rose) change color in acidic and basic solutions to correctly identify their nature.

Question 3. 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.
Answer: The red rose extract paper strips help us find out if a solution is acidic, basic, or neutral. For Fig. 2.13, the color changed to green, which means the solution is basic. For Fig. 2.14, there was no change in color, indicating a neutral solution. For Fig. 2.15, the color changed to red, showing the solution is acidic. Natural indicators like red rose extract are useful because they are safe to use.
In simple words: Red rose extract helps test solutions. If it turns green (Fig. 2.13), the solution is basic. If no color change (Fig. 2.14), it's neutral. If it turns red (Fig. 2.15), it's acidic.

🎯 Exam Tip: Clearly state the observation (color change) and then the conclusion (acidic, basic, or neutral) for each figure.

Question 5. Manya is blind folded. 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?
Answer: Since Manya cannot see, she should use olfactory indicators to test the solutions. These indicators change their smell when they are mixed with an acidic or a basic substance. This way, she can figure out if the solutions are acidic or basic just by smelling them. Onions and vanilla extract are good examples of such indicators.
In simple words: Manya is blindfolded, so she needs indicators that change smell. These are called olfactory indicators, and they help tell if a solution is acid or base by how their smell changes.

🎯 Exam Tip: For blind individuals, olfactory indicators are essential because they rely on the sense of smell rather than color change.

Question 6. Could you suggest various materials which can be used for writing the message on the white sheet of paper (given at the beginning of the chapter) and what could be in the spray bottle? Make a table of various possible combinations and the colour of the writing obtained.
Answer: To write a secret message on white paper, you can use a solution that changes color when it mixes with another substance. For example, you could write with soap solution and spray turmeric solution over it. Soap solution is basic, and turmeric solution is yellow, but it turns red when it touches a basic solution. This way, the secret message appears. You can also use other combinations. The interaction of the chemicals creates the visible message.
In simple words: We can use a liquid that changes color when sprayed with another liquid to write a secret message. Like writing with soap (basic) and spraying turmeric (yellow), which will turn red/brown.

🎯 Exam Tip: When making invisible ink, remember to pair a substance with an indicator that shows a clear color change upon reaction.

Question 7. 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.
Answer: Grape juice is an acidic substance, which makes the red rose extract turn red. If baking soda, which is basic, is added to this mixture, it will neutralize the acid in the grape juice. As a result, the mixture will become basic. The red rose extract will then change its color from red to green, because red rose extract shows a green color in basic solutions. This color change helps identify the presence of a base.
In simple words: Grape juice is acidic and turns red rose extract red. When baking soda (a base) is added, it neutralizes the acid, making the mixture basic. The red rose extract will then change from red to green.

🎯 Exam Tip: Always remember that red rose extract indicates acidic solutions with red color and basic solutions with green color.

Question 8. 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?
Answer: Orange juice is an acidic substance. To reveal a secret message written with an acidic solution like orange juice, an indicator that changes color in acidic conditions is needed. Red rose extract is a good choice because it turns red in acidic solutions and green in basic solutions. Spraying red rose extract on the secret message will make it turn red, thus making the message visible. This is a common method for creating invisible ink.
In simple words: Orange juice is acidic. To see the message, Keerthi's grandmother should spray red rose extract on it. The red rose extract will turn red where the orange juice is, making the message appear.

🎯 Exam Tip: To reveal messages written with acidic invisible ink, choose an indicator that reacts with acids and produces a clear color change.

Question 9. How can natural indicators be prepared? Explain by giving an example.
Answer: Natural indicators are substances that show different colors when they come into contact with acidic or basic solutions. These indicators can be made from various colorful parts of plants, such as flowers or leaves. For example, red rose extract can be prepared by soaking rose petals in water and then grinding them. This extract will show a red color in an acidic liquid and a green color in a basic liquid. Another example is turmeric paper, made by dipping paper in turmeric paste and letting it dry. Turmeric paper turns red or pink in acidic conditions and green or yellow in basic conditions. These plant-based indicators are safe and easy to use.
In simple words: Natural indicators are made from colorful plant parts like flowers or leaves. They change color in acids or bases. For example, red rose extract turns red in acid and green in base. Turmeric paper turns red/pink in acid and green/yellow in base.

🎯 Exam Tip: When explaining natural indicators, always describe how they are prepared, what colors they show in acidic and basic mediums, and provide clear examples.

Question 10. 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.
Answer: No, you cannot identify all three liquids (vinegar, baking soda solution, and sugar solution) using only turmeric paper. Turmeric paper is originally yellow and turns red/brown when it comes into contact with a basic solution, like baking soda solution. However, it shows no color change in an acidic medium like vinegar, and also no color change in a neutral solution like sugar solution. Since both acidic (vinegar) and neutral (sugar solution) liquids give the same "no change" reaction with turmeric paper, it is impossible to tell them apart using only this indicator. A different indicator, such as litmus paper, would be needed to distinguish between acidic and neutral solutions.
In simple words: No, we cannot tell them all apart with only turmeric paper. Turmeric paper turns red/brown for baking soda (basic). But it stays yellow for both vinegar (acidic) and sugar solution (neutral). So, we can't tell vinegar from sugar water.

🎯 Exam Tip: When asked to distinguish between substances using an indicator, consider if the indicator provides unique reactions for *all* substances involved. If two substances give the same result, that indicator is insufficient.

Question 11. 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?
Answer: The red rose extract turns green in a basic medium. So, if the red rose extract turns liquid X green, it means liquid X is basic in nature. When an excess of amla juice, which is acidic, is added to the basic liquid X, a neutralization reaction will occur. The amla juice will react with liquid X, making the overall solution less basic or even acidic if enough amla juice is added. The color will change as the pH shifts.
In simple words: Liquid X is basic because red rose extract turns green in basic liquids. If you add a lot of amla juice (which is acidic) to liquid X, the acid and base will mix and cancel each other out, making the solution less basic.

🎯 Exam Tip: Understand that indicators show a specific color for acids and another for bases. Neutralization occurs when an acid and a base are mixed, reducing their individual effects.

Question 12. 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? The acidic soil can be treated with______. The basic soil can be treated with______.
Answer: The flowchart discusses indicators for soil and treatments for different soil types.
(i) Acidic
(ii) Basic
(ii) Red/blue litmus, natural indicators like Red cabbage, Red rose extract
(iv) Lime water/calcium hydroxide
(v) Organic substance/compost
Maintaining a balanced soil pH is crucial for healthy plant growth.
In simple words: To test soil, you can use litmus paper or natural things like red cabbage. Acidic soil needs basic things like lime water to fix it. Basic soil needs organic matter or compost.

🎯 Exam Tip: When completing flowcharts or diagrams, make sure your answers directly fill the blanks and use appropriate scientific terms for both testing and treatment methods.

Dive Deeper

Question. Aman accidentally split vinegar on some pieces of an eggshell or marble and noticed bubbling. He then poured a soap solution on another piece of eggshell or marble, but no bubbles appeared. Why did bubbles occur with vinegar but not with soap solution?
Answer: Eggshells and marble both contain calcium carbonate. Vinegar is an acidic substance. When vinegar comes into contact with calcium carbonate, a chemical reaction occurs, which produces carbon dioxide gas (CO₂). This gas is released as bubbles, causing the bubbling observed. In contrast, soap solution is basic, not acidic. Basic solutions do not react with calcium carbonate to produce carbon dioxide gas, so no bubbles are formed. This reaction helps identify the presence of carbonates.
In simple words: Eggshells and marble have calcium carbonate. Vinegar is acid, so it reacts and makes gas bubbles. Soap solution is a base, and it does not react with calcium carbonate, so no bubbles appear.

🎯 Exam Tip: Remember that acids react with carbonates to produce carbon dioxide gas, leading to effervescence (bubbling).

Exploring Substances Acidic Basic and Neutral Class 7 Questions and Answers (Intext)

(Page No. 7)

Question 1. In science fair entrance, after receiving white paper sheet, why the message appeared after spraying liquid?
Answer:

🎯 Exam Tip: To answer questions about invisible ink, explain the principle of using an indicator solution to reveal messages written with an acidic or basic substance.

(Page No. 11)

Question 2. What are these red and blue litmus paper strips made of? Why do they change colour when drops of some samples are put on them?
Answer: Red and blue litmus paper strips are made from litmus, which is a natural dye extracted from lichens. Litmus acts as a pH indicator. These papers change color because litmus reacts differently in acidic and basic solutions. Blue litmus turns red when it comes into contact with an acidic solution, while red litmus turns blue when it comes into contact with a basic solution. This color change helps to quickly identify if a substance is an acid or a base.
In simple words: Litmus paper is made from lichens. It changes color because it reacts with acids and bases. Blue litmus turns red in acid, and red litmus turns blue in base.

🎯 Exam Tip: Clearly state the source of litmus (lichens) and the specific color changes for both red and blue litmus in acidic and basic mediums.

(Page No. 16)

Question 3. If litmus is not available, are there some other natural substances that can serve as acid- base indicators?
Answer: Yes, if litmus is not available, several other natural substances can be used as acid-base indicators. Many colorful plants can serve this purpose. Common examples include turmeric, which changes color from yellow to red-brown in basic solutions; red rose petals, which can be extracted to show different colors in acids and bases; and hibiscus flowers, whose extract also changes color based on pH. These natural alternatives are easily accessible and safe to use.
In simple words: Yes, many natural things can be indicators if litmus is not there. Examples are turmeric, red rose petals, and hibiscus flowers, which all change color in acids or bases.

🎯 Exam Tip: When listing alternative natural indicators, try to remember at least two examples and briefly mention how they indicate acidity or basicity.

Question 4. Are there any substances whose odours change on adding acidic or basic substances?
Answer: Yes, there are indeed some substances whose smells change when they are mixed with acidic or basic substances. These special substances are known as olfactory indicators. They help us identify acids and bases not by color change, but by a noticeable alteration in their odor. For example, onion and vanilla extract are good examples of olfactory indicators.
In simple words: Yes, some substances change their smell when acids or bases are added. These are called olfactory indicators.

🎯 Exam Tip: Define olfactory indicators clearly and state that they change smell, not color, to distinguish acids and bases.