NCERT Solutions Class 7 Science Curiosity Chapter 02 Exploring Substances: Acidic, Basic, and Neutral

Question 1. A solution turns the red litmus paper to blue. Excess addition of which of the following solution would reverse the change?
(a) Lime water
(b) Baking soda
(c) Vinegar
(d) Common salt solution
Answer: (c) Vinegar
In simple words: Bases turn red litmus blue. Adding an acid like vinegar cancels the base and changes the color back.

Exam Tip: Remember that acids and bases do opposite things to litmus paper. Acids turn blue to red, and bases turn red to blue.

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?
(a) Acidic, acidic and acidic
(b) Neutral, basic and basic
(c) Basic, basic and acidic
(d) Basic, basic and basic
Answer: (d) Basic, basic and basic
In simple words: All three tests showed that solutions A, B, and C are bases. Litmus, turmeric, and rose paper all changed to their basic colors.

Exam Tip: Memorize the color changes for natural indicators. Turmeric only turns red in bases, and red rose extract turns green in bases.

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 solutions in the containers can be identified as follows:
- Fig. 2.13 represents an Acidic solution, because the red rose strip becomes bright red.
- Fig. 2.14 represents a Basic solution, as the paper strip changes to a green color.
- Fig. 2.15 represents a Neutral solution, since the strip does not show any color change. In simple words: Rose extract turns red in acids, turns green in bases, and stays the same color in neutral liquids like plain water.

Exam Tip: Be sure to write the exact color change associated with each type of substance for the red rose indicator in your exams.

Question 4. 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.
Answer: The laboratory liquid is acidic. This is because it turned the blue litmus paper red, which is what acids do. Also, since there was no change with the red litmus paper or the turmeric, it further proves the liquid is acidic.
In simple words: Only acids turn blue litmus red while keeping turmeric and red litmus unchanged. So, the liquid is acidic.

Exam Tip: A key test for acids is their reaction with blue litmus paper, turning it red. Bases do not change the color of blue litmus.

Question 5. 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?
Answer: Manya should use an olfactory indicator, such as onion strips, to test the liquids. Since she cannot see, she cannot detect color changes on litmus or turmeric paper. Olfactory indicators change their smell depending on whether they are in acidic or basic solutions, which she can easily sniff to tell them apart.
In simple words: Manya cannot see color changes because of the blindfold. She can use onion strips because their smell changes in acids or bases.

Exam Tip: Always define "olfactory indicator" when answering questions like this, mentioning that they rely on the sense of smell rather than sight.

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: This trick works by using chemical reactions between acids, bases, and indicators. Below is a table showing different items you can use to write messages, what to spray them with, and the colors that will appear:

In simple words: You can write with a mild acid or base, then spray it with a natural juice like turmeric or rose extract to reveal the message in a new color.

Exam Tip: Remember that turmeric turns red in bases, while red rose extract turns green in bases and red in acids.

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: The red mixture will turn green. Grape juice is acidic, which is why it turned the red rose extract red. When you add baking soda, which is basic, it neutralizes the acid. Once enough baking soda is mixed in to make the entire solution basic, the red rose indicator shifts to a green color.
In simple words: The acidic grape juice first made the rose extract red. Adding basic baking soda cancels out the acid and turns the liquid green.

Exam Tip: Always explain both parts: why the mixture was red initially (due to acidic grape juice) and why it changes (neutralization and shift to a basic medium).

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: We can help her grandmother by applying an indicator that changes color when it touches an acid. Since orange juice contains citric acid, it is acidic. We can spray blue litmus solution on the paper, which will turn red where the letters are. Alternatively, red rose extract can be used, which will also turn red to reveal the message.
In simple words: Orange juice is an acid. If you brush blue litmus paper or rose juice on it, the secret writing will turn bright red and become visible.

Exam Tip: For secrets written in acidic juices, any indicator that reacts with acids (like blue litmus or red rose extract) is a perfect choice to reveal the message.

Question 9. How can natural indicators be prepared? Explain by giving an example.
Answer: Natural indicators can be made from colorful plant parts like flower petals or beets. For example, to make red rose extract: gather some fallen red rose petals and wash them. Next, crush the petals and place them in a small cup. Pour warm water over them and let them sit until the water turns pink or red. Finally, strain the liquid to get your indicator.
In simple words: You can make indicators by soaking crushed flower petals in warm water. The colored water you get is the indicator.

Exam Tip: When describing experiments, write down clear, numbered steps (like collection, washing, crushing, heating, and filtering) to get full marks.

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: You can easily find the baking soda solution, but you cannot tell the vinegar apart from the sugar solution using only turmeric. This is because turmeric paper turns red only when it touches basic liquids. Vinegar is acidic and sugar water is neutral, so neither of them will change the yellow color of the turmeric paper. Only the basic baking soda will change the paper to red.
In simple words: Turmeric only changes to red with bases. It will identify the baking soda, but both vinegar and sugar will keep the paper yellow.

Exam Tip: Explain that turmeric is a selective indicator that only shows a color change in basic solutions, remaining unaffected by both acids and neutral substances.

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: Liquid X is a basic solution because red rose extract turns green only in bases. Amla juice is highly acidic. When you add a lot of amla juice to liquid X, it will neutralize the base. Once there is more acid than base, the entire solution becomes acidic, and the red rose indicator will turn red.
In simple words: Liquid X is a base. Adding lots of sour amla juice makes the whole mixture acidic, so the green color changes to red.

Exam Tip: Be sure to state the nature of Liquid X (basic) first, then explain the neutralization process and the final color change (red).

Question 12. Observe and analyse the information given in the following flowchart. Complete the missing information. (Flowchart shows poor plant health due to soil condition)
Answer: The missing flowchart information can be completed as follows:
- If the soil is acidic: It can be treated by adding bases such as quicklime or slaked lime to neutralize the acidity.
- If the soil is basic: It can be treated by adding organic matter like compost or manure, which releases natural acids.
- Testing soil nature: An acid-base indicator, such as litmus solution mixed with soil and water, or a soil testing kit, can be used.
In simple words: Acidic soil needs bases like lime, and basic soil needs organic compost. You can test soil using simple litmus paper.

Exam Tip: Remember the remedies: acidic soil needs quicklime/slaked lime, while basic soil needs organic matter (compost).

Class 7 Science Curiosity Chapter 2 Very Short Answer Type Questions

Question 1. What surprising thing happened with Ashwin and Keerthi at the science fair?
Answer: When a liquid was sprayed onto their blank white papers, hidden words saying "Welcome to the Wonderful World of Science" suddenly appeared. This amazing event got them very excited and curious.
In simple words: Hidden letters appeared on their blank papers when they sprayed them with a liquid.

Exam Tip: Mention the specific message that appeared on the paper to show you read the textbook story carefully.

Question 2. What is litmus obtained from?
Answer: Litmus is a natural dye that comes from plants called lichens. We can get it either as a liquid mixture or as small paper strips in red and blue colors.
In simple words: Litmus comes from lichens. It is used as a liquid or paper strips to test for acids and bases.

Exam Tip: The keyword "lichens" is essential for full marks. Always specify that it is available as blue and red paper strips.

Question 3. What does a substance turning blue litmus paper red indicate?
Answer: When blue litmus paper turns red, it shows that the liquid is acidic.
In simple words: Turning blue litmus paper red means the liquid is an acid.

Exam Tip: Remember the mnemonic: "Blue to Red = Acid" (BAR: Blue to Red is Acid).

Question 4. What does a basic substance feel like when touched?
Answer: A basic liquid usually feels slippery or soapy to the touch, just like wet soap or a mixture of baking soda.
In simple words: Bases feel slippery and soapy when you rub them between your fingers.

Exam Tip: "Slippery" or "soapy" are the key descriptive words examiners look for when defining the feel of bases.

Question 5. What is an olfactory indicator?
Answer: Olfactory indicators are materials that change how they smell when they are mixed with an acid or a base. Onion is a great example of this.
In simple words: These are indicators that change their scent instead of their color when put in acids or bases.

Exam Tip: Always give "onion" or "vanilla extract" as examples when describing olfactory indicators.

Class 7 Science Curiosity Chapter 2 Short Answer Type Questions

Question 1. What do we learn by grouping samples into Group A, B, and C in the litmus test?
Answer: Sorting samples into these groups helps us easily classify them. Group A contains liquids that turn blue litmus red (acids). Group B contains liquids that turn red litmus blue (bases). Group C has liquids that do not change either paper (neutral).
In simple words: Grouping helps us separate liquids into acids, bases, and neutral solutions based on how they change litmus paper.

Exam Tip: Clearly define the characteristic reaction of each group (Group A = acidic, Group B = basic, Group C = neutral) to get full points.

Question 2. How does red rose extract work as an indicator?
Answer: Red rose extract changes colors depending on what it touches. It turns red in acidic solutions, shifts to green in basic solutions, and stays its original color in neutral liquids. This makes it a great natural indicator.
In simple words: Red rose juice turns red with acids, turns green with bases, and does not change with neutral liquids.

Exam Tip: Remember to state that the color remains unchanged in neutral solutions to show a complete understanding.

Question 3. How is turmeric used as an indicator?
Answer: To use turmeric, we mix it with water to make a paste and spread it on paper strips. The yellow turmeric paper turns bright red when dipped in basic liquids. However, it stays yellow in both acidic and neutral liquids.
In simple words: Yellow turmeric paper turns red only when it touches basic liquids like soap water.

Exam Tip: Highlight that turmeric stays yellow with acids and neutral solutions; it only changes color in the presence of a base.

Question 4. What happens in a neutralisation reaction?
Answer: During neutralization, an acid and a base react together and cancel each other out. This reaction produces salt and water, and it also releases heat, making the mixture warm.
In simple words: An acid and a base mix to make salt and water. This reaction also releases heat.

Exam Tip: Do not forget to mention "release of heat" and the formation of both "salt and water" in your definition.

Question 5. Why did moist baking soda help Ashwin after an ant bite?
Answer: When an ant bites, it injects a chemical called formic acid into the skin. Applying moist baking soda, which is basic, neutralizes the acid and stops the sharp, stinging pain.
In simple words: The ant bite has acid. Wet baking soda is a base that cancels the acid and stops the sting.

Exam Tip: Name the specific acid injected by the ant ("formic acid") to secure full marks on this common exam question.

Class 7 Science Curiosity Chapter 2 Descriptive Answer Type Questions

Question 1. Explain how soil treatment helps plants grow better.
Answer: Plants need soil that is neither too acidic nor too basic to grow well. If the soil is too acidic, farmers add quicklime, which is basic, to neutralize the acid. If the soil is too basic, they mix in organic matter like manure or compost, which releases natural acids. Keeping the soil balanced allows plants to absorb nutrients easily and grow healthy.
In simple words: Adding lime fixes acidic soil, and adding compost fixes basic soil. This balance helps plants grow well.

Exam Tip: Be clear on which treatment goes with which soil condition: use lime for acidic soil and compost for basic soil.

Question 2. How does neutralisation help in industrial waste treatment?
Answer: Waste from factories is often highly acidic and can kill fish and other life if dumped directly into lakes and rivers. To prevent this, factories treat the acidic waste with basic materials to neutralize it. This makes the waste safe before it enters water bodies, keeping aquatic life safe.
In simple words: Factories mix bases into their acidic waste to neutralize it so it does not harm fish in rivers.

Exam Tip: Emphasize that neutralizing the acidic waste with basic materials protects aquatic ecosystems and prevents water pollution.

Question 3. Why can’t turmeric be used to test acidic substances?
Answer: Turmeric only changes its color from yellow to red when it reacts with bases. Since it does not react or change color with acids or neutral liquids, we cannot use it to identify an acid. It can only tell us if something is basic.
In simple words: Turmeric stays yellow in both acids and water. Since it does not change, it cannot help us identify acids.

Exam Tip: Clearly state that turmeric remains yellow in both acidic and neutral conditions, meaning it is only effective for identifying bases.

Question 4. What does the hydrangea flower’s colour tell about soil?
Answer: Hydrangea flowers act like a natural indicator for soil. If the soil is acidic, these flowers bloom with a blue color. If the soil is basic, they bloom pink or red. Gardeners can change the soil's acid levels to make the flowers grow in the color they want.
In simple words: Hydrangeas are blue in acidic soil and pink or red in basic soil. Their color shows how acidic the soil is.

Exam Tip: Memorize the specific colors for hydrangeas: blue for acidic soil and pink/red for basic soil. This is a common objective question.

Question 5. Why should we not taste unknown substances during experiments?
Answer: Many chemicals in the laboratory are highly toxic, corrosive, or poisonous. Tasting unknown substances can burn your mouth or make you very sick. To remain safe, never taste anything in the science lab unless your teacher tells you to.
In simple words: Lab chemicals can be dangerous or toxic. Never taste them because they can hurt you.

Exam Tip: Safety is paramount in science labs. Always write about the risks of toxicity and chemical burns in safety-related answers.

Class 7 Science Curiosity Chapter 2 Exploring Type Questions

Question 1. How could Ashwin’s “secret greeting card” trick work?
Answer: Ashwin's trick used turmeric paste as a base layer on the paper. He wrote his message using a basic soap solution, which turned the turmeric paper red. When sprayed with an acidic liquid or water, the contrast highlighted the red letters, revealing the message.
In simple words: Yellow turmeric paper turns red when written on with soap. This creates a hidden message that appears in red.

Exam Tip: Identify the roles: turmeric acts as the indicator and soap solution is the basic writing substance.

Question 2. How can a gardener change the colour of hydrangea flowers?
Answer: A gardener can alter the flower colors by modifying the soil's pH. To get blue flowers, the gardener can make the soil more acidic by mixing in organic matter. To get pink or red flowers, they can make the soil basic by adding quicklime.
In simple words: Gardeners add compost to make the soil acidic for blue flowers, or lime to make it basic for pink flowers.

Exam Tip: Remember: Organic matter increases acidity (blue flowers), and lime increases basicity (pink/red flowers).

Question 3. How can you identify vinegar, baking soda solution, and sugar solution using turmeric paper?
Answer: First, dip the turmeric paper into all three solutions. The basic baking soda solution will turn the yellow paper red. The other two will keep the turmeric yellow. Next, to tell the remaining two apart, we can taste them safely since they are common kitchen items: vinegar will taste sour because it is acidic, and the sugar solution will taste sweet because it is neutral.
In simple words: Baking soda turns turmeric red. Since both vinegar and sugar keep it yellow, taste them to find the sour vinegar and sweet sugar.

Exam Tip: When safe kitchen ingredients are used, mild tasting is an acceptable way to differentiate between acidic (sour) and neutral (sweet) solutions if turmeric cannot.

Question 4. Why did bubbles form when Aman spilled vinegar on eggshells?
Answer: Vinegar contains acid that reacts with the calcium carbonate found in eggshells. This chemical reaction produces carbon dioxide gas, which rises up as bubbles. A basic soap solution does not react with eggshells, so it produces no bubbles.
In simple words: Acidic vinegar reacts with eggshells to make carbon dioxide gas. This gas creates the fizzing bubbles.

Exam Tip: The gas released in this reaction is carbon dioxide (\( CO_2 \)). This is a key chemistry fact to write down in exams.

Question 5. Why is neutralisation useful in everyday life?
Answer: Neutralization helps us solve many daily problems. It relieves painful ant bites, balances farm soil to grow better crops, treats factory waste to save fish, and cures stomach acidity using antacids. It plays a big role in protecting both people and nature.
In simple words: Neutralization cures stomach aches, treats farm soil, heals ant bites, and cleans up dirty factory waste before it hurts fish.

Exam Tip: Be prepared to list at least three real-life examples of neutralization (stomach acid, ant stings, soil treatment, or waste treatment) for long-answer questions.

Question. What is the main focus of Class 7 Science Curiosity Chapter 2 Exploring Substances: Acidic, Basic and Neutral?
Answer: This chapter teaches us how to identify and understand different substances in our daily environment. It explains how liquids can be acidic, basic, or neutral. We learn to test them using indicators like litmus paper, red rose extract, turmeric, and onions, while exploring neutralization reactions in nature and industry.
In simple words: This chapter helps us learn how to identify acids, bases, and neutral liquids using simple tests.

Exam Tip: Focus on understanding how different indicators change color in acidic, basic, and neutral solutions, as this is the core theme of the chapter.

Question. How do the activities in Class 7 Science Curiosity Chapter 2 help students understand acids, bases and neutrals better?
Answer: The hands-on activities let students test real everyday items like lemon juice, soap, and baking soda. By watching indicators change color or smell right in front of them, students learn by doing. This makes the chemical concepts easier to remember and understand compared to just reading about them.
In simple words: Doing real tests with soap and juices helps us see how indicators work, making science fun and easy to learn.

Exam Tip: In activity-based questions, focus on describing the observed color changes and what they prove about the substance's nature.

Question. Why is understanding neutralisation reactions important according to Class 7 Science Curiosity Chapter 2?
Answer: Understanding neutralization helps us manage chemical balances in our bodies, farms, and factories. For example, it explains how we can soothe an acidic ant sting with a basic paste, or clean up factory waste to keep rivers clean. Knowing how acids and bases cancel each other out allows us to solve everyday issues safely.
In simple words: Neutralization explains how to balance acids and bases to cure stings, treat farm soil, and protect fish from waste.

Exam Tip: Remember that neutralization always produces a salt, water, and heat energy. Make sure to include these key products in any discussion of neutralization reactions.