Extra Questions for Chapter 2 Exploring Substances: Acidic, Basic and Neutral Class 7 Science Curiosity
Class 7 Science The Ever-Evolving World of Science Extra Questions
Very Short Answer Questions
Question 1. Which acid is present in an ant sting?
Answer
Formic acid is present in an ant sting.
Question 2. While playing in a park, a child was stung by a wasp. Some elders suggested applying a paste of baking soda and others lemon juice as a remedy. Which remedy do you think is appropriate and why?
Answer
Wasp sting inject a liquid in the skin which is acidic in nature. Hence, baking soda is the appropriate remedy as it is basic in nature and neutralises the acid.
Question 3. Name two other natural indicators besides litmus and red rose extract.
Answer
Beetroot and purple cabbage are natural indicators. They change colour when mixed with acidic or basic substances.
Question 4. What is the nature of distilled water?
Answer
The Distilled water is neutral.
Question 5. In a reaction given below, how can we get solid sodium chloride from its solution?
Hydrochloric acid + Sodium hydroxide ➝ Sodium chloride + Water
Answer
Through the process of evaporation we can get solid sodium chloride from its solution. We must evaporate the water in a container over a burner until all the water from the mixture evaporates.
Question 6. Form a sentence using the following words: baking soda, ant bite, moist, effect, neutralised, rubbing.
Answer
The effect of an ant bite can be neutralised by rubbing moist baking soda.
Question 7. Why is litmus better natural indicator than other natural indicators?
Answer
Litmus give wide range of colour change from weak acid to strong acid and from weak base to strong base. Other natural indicator, on the other hand, do not show a wide colour change.
Question 8. What colour change does turmeric paper show when it comes in contact with a base?
Answer
Turmeric paper, which is yellow, turns red when it touches a basic substance. It does not change colour with acidic or neutral substances.
Question 9. Give examples of some acids and bases
Answer
Curd, lemon juice, vinegar, orange juice etc. are acids and baking soda, lime water etc. are bases.
Question 10. Red litmus paper is dipped in a solution; it remains red, what is the nature of the solution?
Answer
Red litmus paper when dipped in a solution, if it remains red then the nature of the solution is neutral.
Question 11. Rena is trying to wash turmeric stain on her cloth with soap, she noticed the stain colour changed to red, explain why?
Answer
Turmeric is a natural indicator which when reacts with bases turns it into red colour; here soap solution is basic so it turns red.
Short Answer Questions
Question 1. Tom rubs a solution between his fingers and feels soapy, what is the nature of that solution?
Answer
The solution that feels soapy when rubbed between fingers is basic. This characteristic indicates that it contains substances known as bases, which typically have a bitter taste and a slippery feel.
Question 2. Give some examples of acids and bases that we encounter in day-to-day life.
Answer
Acids - Curd, lemon juice, vinegar, orange juice etc.
Base - baking soda, lime water etc.
Question 3. Why factory waste is neutralised before disposing it of in the water bodies?
Answer
The wastes from many factories often contain acids. If these acids enter water bodies, they can harm aquatic life, including fish and other organisms. To prevent this, factory wastes are neutralised by adding basic substances, which helps to make the water safe.
Question 4. What is litmus and how is it used to identify acids and bases?
Answer
Litmus is a natural material made from lichens and comes as blue and red paper strips. It helps identify substances: acids turn blue litmus paper red, and bases turn red litmus paper blue.
Question 5. What is the effect of the China rose indicator on acidic and basic solutions?
Answer
The China rose indicator has distinct effects on solutions:
- In acidic solutions, it turns a dark pink (magenta) colour.
- In basic solutions, it changes to green.
Question 6. Why do curd, lemon juice, orange juice, and vinegar taste sour?
Answer
Curd, lemon juice, orange juice, and vinegar taste sour because they contain acids.
- These substances have a chemical nature that is acidic.
- The term acid comes from the Latin word acere, meaning sour.
- The acids present in these foods are natural acids.
Question 7. Look at the given reaction.
Hydrochloric acid + Sodium hydroxide (base) → Sodium chloride (salt) + Water
Sodium chloride formed in this reaction remains in solution form. Can we get solid sodium chloride from this solution? Suggest a method (if any).
Answer
We can get solid sodium chloride by evaporation method. Evaporation is the process by which water changes from a liquid to a gas or vapour. Rate of evaporation increases with temperature.
Question 8. What do you mean by neutral solution? Give examples.
Answer
Neutral solutions do not change the colour of either red or blue litmus paper. These solutions are neither acidic nor basic.
- Sugar solution
- Distilled water
These examples illustrate the characteristics of neutral solutions.
Question 9. Why a turmeric stain on my white shirt is turned to red when it is washed with soap.
Answer
A turmeric stain on a white shirt turns red when washed with soap due to the basic nature of the soap solution.
Here’s a brief explanation:
- Turmeric acts as a natural indicator.
- In a basic solution, turmeric changes from yellow to red.
- Soap solutions are typically basic, causing this colour change.
Question 10. Is the distilled water acidic/basic/neutral? How would you verify it?
Answer
Distilled water is considered neutral in nature. You can verify this by using litmus paper:
- Use red litmus paper and blue litmus paper.
- Place distilled water on both papers.
- No colour change will occur on either paper.
This indicates that distilled water is neutral.
Question 11. How is red rose extract prepared and how does it act as an indicator?
Answer
Red rose extract is made by crushing fresh rose petals, soaking them in hot water, and filtering the liquid. It changes colour to red in acidic substances and green in basic substances, helping to identify them.
Question 12. What are olfactory indicators and give an example?
Answer
Olfactory indicators are substances whose smell changes when mixed with acids or bases. For example, a cloth soaked in onion juice loses its smell when put in acidic tamarind water or basic baking soda solution.
Question 13. What are salts? Give examples.
Answer
Salts are the ionic compounds generally formed by neutralisation of an acid with base. They can be acidic, basic as well as neutral. Example acidic salts: sodium bicarbonate, basic salts: magnesium chloride, neutral salt: sodium chloride.
Question 14. Define indicators along with examples.
Answer
Indicators are special type of substance that are used to taste whether a substance is acidic or basic in nature. It change the colour of acidic or basis substances when added into it. Turmeric, litmus, etc. are some natural indicators.
Question 15. What do you mean by neutral substance, explain with examples?
Answer
The substances which are neither acidic, nor basic are called neutral substance. These substances neither turn blue litmus red nor red litmus blue, for example distilled water, sugar solution etc.
Question 16. Explain why factory waste should be neutralised before disposing it into the water bodies.
Answer
The wastes of many factories contain acids. If they are allowed to flow into the water bodies, the acids will kill fish and other organisms. The factory wastes are, therefore, neutralised by adding basic substances into it.
Question 17. Describe the process of neutralisation with the help of an example.
Answer
Neutralisation is a process in which an acid solution when mixed with base solution, react with each other to produce a salt and water along with generation of heat. Salt so produced, may be acidic, basic or neutral in nature. In this process the acidic nature of the acid and the basic nature of the base are destroyed.
Acid + base → salt + water (heat is evolved)
For example: HCl + NaOH→ NaCl + H2O.
Hydrochloric acid + Sodium hydroxide → Sodium chloride + Water
Question 18. Explain the process and treatment of an ant bite.
Answer
When an ant bites, it injects an acidic liquid called formic acid into the skin, causing burning pain, redness, and irritation. The effect of the acid can be neutralized by applying a mild base to the affected area.
Two common treatments are:
- Baking soda solution: Baking soda (sodium hydrogen carbonate) neutralizes the acid and reduces the pain and irritation.
- Calamine solution: Calamine contains zinc carbonate, which also neutralizes the acidic effect and provides relief.
Applying either of these substances helps neutralize the acid and reduce the discomfort caused by the ant bite.
Question 19. Explain how red rose extract acts as an acid-base indicator.
Answer
Red rose extract changes colour depending on the nature of the substance it is exposed to. In acidic solutions, it turns a shade of red, while in basic solutions, it turns green. This colour change makes it an effective natural acid-base indicator, as it helps identify whether a substance is acidic, basic, or neutral by comparing the resulting colour with known standards.
Question 20. Why is litmus considered an acid-base indicator?
Answer
Litmus is considered an acid-base indicator because it changes colour in acidic and basic solutions. Blue litmus turns red in acidic solutions, and red litmus turns blue in basic solutions. This distinct colour change allows litmus to reliably indicate the acidic or basic nature of a substance.
Question 21. How does neutralisation help in treating acidic soil?
Answer
Neutralisation helps treat acidic soil by adding a basic substance, such as lime (calcium hydroxide), to the soil. The base reacts with the acidic components, forming neutral products like water and salts. This balances the soil's pH, making it more suitable for plant growth and improving crop health.
Question 22. Describe the feel and taste of basic substances.
Answer
Basic substances generally feel soapy or slippery when rubbed between fingers, as seen with baking soda solution. They typically taste bitter, although not all bitter substances are bases (e.g., bitter gourd is not basic). These properties help identify basic substances, but tasting should only be done with known, safe substances.
Question 23. What are olfactory indicators, and give one example?
Answer
Olfactory indicators are substances whose odour changes in acidic or basic environments, helping identify the nature of a solution. An example is onion, which, when exposed to acidic substances like tamarind water, may retain or alter its odour differently compared to basic substances like baking soda solution.
Question 24. Which acid is injected into our body by an ant sting that causes pain?
Answer
- The acid injected into our body by an ant sting is formic acid.
- When an ant bites or stings, it releases formic acid into the skin, which causes a burning sensation and pain.
- Formic acid is responsible for the irritation and discomfort caused by ant stings.
Question 25. Gurbir observed that most of the fish in the pond of her village were gradually dying. He also observed that the wastes of a factory in their village are flowing into the pond which probably caused the fish to die.
(a) Explain why the fish were dying?
(b) If the factory waste is acidic in nature, how can it be neutralised?
Answer
(a) If the wastes of a factory flow into waterbodies, it can cause a threat to the lives of sea creatures and to anybody who drink the water. Since factory wastes may contain acids or bases and it can kill the fish.
(b) If the factory waste is acidic in nature, it can be neutralised by adding basic substances.
Long Answer Questions
Question 1. State a few properties of acids.
Answer
Properties of acids:
- Acids are sour in taste
- Acid turns blue litmus red.
- China rose indicator turns acidic solutions to dark pink (magenta).
- Turmeric indicator does not change its colour with acid
- Gives burning sensation.
- Acids are usually sticky.
- When the solution is acidic, phenolphthalein remains colourless.
Question 2. A farmer was unhappy because of his low crop yield. He discussed the problem with an agricultural scientist and realised that the soil of his field was either too acidic or too basic. What remedy would you suggest the farmer to neutralise the soil?
Answer
The reason for low crop yield in farmer’s fields was that the soil at a place is either too acidic or too basic.- The soil may be acidic or basic naturally.
- The excessive use of fertilisers in the fields also makes the soil too acidic.
- When the soil is too acidic, it is treated with a base such as quicklime (calcium oxide) or slaked lime (calcium hydroxide) which neutralises the excess acid present in the soil and reduces its acidic nature.
- If the soil is too basic, then decaying organic matter (called manure or compost) is added to it.
- The decaying organic matter releases acid which neutralises the excess bases present in the soil and reduces its basic nature.
Question 3. Why does a turmeric stain on a white shirt turn to red when it is washed with soap?
Answer
Turmeric contains a natural yellow pigment called curcumin that changes colour depending on the pH of the solution.- When turmeric stains a shirt, it remains yellow in neutral or acidic conditions.
- However, soap solutions are basic (alkaline) in nature.
- When the turmeric stain comes in contact with the basic soap solution, the curcumin reacts and changes its colour to red.
Question 4. Read the passage below and answer the questions that follow.
Aarav and Kaira attended a science fair on National Science Day. At the entrance, they were given a white paper, which remained blank until a volunteer sprayed a liquid on it, revealing the message: "Welcome to the Wonderful World of Science." This sparked their curiosity. Later at the ‘Colourful World of Substances’ stall, they discovered natural indicators like turmeric, red rose extract, and litmus paper being used to test substances such as lemon juice, baking soda, and soap solution.
They decided to recreate the magic message at home. Aarav coated a card with turmeric paste and let it dry. Kaira used a cotton bud dipped in a solution to write on it. When they sprayed another solution over the card, the message turned dark red!
Answer the following:
(a) Name the two solutions likely used for writing the message and for spraying the card.
(b) Why did the message turn red only after spraying?
(c) What does this activity demonstrate about turmeric?
(d) Is turmeric a suitable indicator for all substances? Explain briefly.
Answer
(a) The writing was likely done using soap solution (a basic substance), and the spray was made of turmeric solution.
(b) The message turned red because turmeric changes colour in the presence of a base. When turmeric (yellow) comes into contact with a base like soap, it turns reddish-brown.
(c) This activity demonstrates that turmeric acts as a natural indicator for basic substances.
(d) No, turmeric cannot detect acidic or neutral substances. It only changes colour in the presence of a base and thus is not a universal indicator.
Question 5. Ashwin, Keerthi and their friend Gurdip were provided with a test, each containing Red rose solution which was pink in colour. Ashwin added 2 drops of solution ‘A’ in his test tube and got dark pink colour. Keerthi added 2 drops of solution ‘S’to her test tube and got green colour. Gurdip added 2 drops of solution ‘C but could not get any change in colour. Suggest the possible cause for the variation in their results.
Answer
Red rose (gudhal) is an acid-base indicator. Red rose solution in different mediums shows the following changes:- Red rose → acid → Gives dark pink colour, hence A is an acidic solution.
- Red rose → base → Gives a green colour, hence 6 is a basic solution.
- China rose neutral → No change in colour, hence C is a neutral solution.
Question 6. State few properties of acids.
Answer
Acids are substances known for their sour taste (though tasting them is not safe). The word "acid" is derived from the Latin word 'ACERE', meaning sour.
Properties of Acids:
- Sour in taste.
- Turn blue litmus paper red.
- Turn China rose solution dark pink or magenta.
Examples of acidic substances include curd, vinegar, lemon, and orange juice, which all contain natural acids.
Question 7. State few properties of bases.
Answer
Bases are substances with a bitter taste and a slippery feel.
Properties of Bases:
- Bitter in taste.
- Turn red litmus paper blue.
- Turn turmeric paper reddish-brown.
- Turn China rose solution green.
Some common examples of bases include baking soda, milk of magnesia, and soap. The chemical nature of these substances is referred to as basic.
Question 8. A shopkeeper has a few bottles of soft drink in his shop. But, unfortunately, these are not labelled. He has to serve the drinks on the demand of customers. One customer wants acidic drink; another wants basic and third one wants neutral drink. How will he decide which drink is to be served to whom?
Answer
On the basis of the property of acids and bases, he can differentiate all the drinks.- An acidic drink will have a sour taste and will turn blue litmus red.
- A basic drink will turn red litmus red.
- A neutral drink will not change the colour of red or blue litmus.
- Apart from this, he can also use turmeric solution for checking the drinks.
Question 9. What are the differences between acids and bases?
Answer
|
Acids |
Bases |
|
1. Acids are sour in taste. |
1. Bases are bitter in taste and soapy to touch |
|
2. The chemical nature of such substances is acidic. |
2. The nature of such substances is said to be basic. |
|
3. Acids turn blue litmus red. |
3. Bases turn red litmus blue. |
|
4. Acids are generally found in vinegar, curd, spinach, amla, citrus fruits, tamarind, grapes, unripe mangoes and lemons. |
4. Bases are generally found in lime water, soap, window cleaner and Milk of Magnesia. |
Question 10. Arnav is provided with three kinds of liquid of them one is sodium hydroxide; another is hydrochloric acid and third is a sugar solution. How will he identify them when he have only turmeric indicator.
Answer
- Turmeric is yellow in colour, when it is exposed to neutral (Sugar Solution) or acidic substances (Hydrochloric Acid) it will retain its yellow colouration.
- However, if turmeric is exposed to more alkaline substances (sodium hydroxide) it becomes a dark pink/red.
- So first we detect sodium hydroxide - a basic substance by a colour change from yellow to dark or red.
- Then will test for an acid or neutral substance with indication of no colour change.
- Now out of these two, we will mix one with already tested solution for basic substance - sodium hydroxide with dark or red colour, if on mixing the colour reverses back to yellow, the liquid is an acid and the remaining third liquid is neutral.
Question 11. What is Neutralisation Reaction? Describe how neutralisation reactions are applied in daily life with two examples.
Answer
When the solution of an acid is mixed with the solution of a base in sufficient quantity, we find that the resulting solution is neither acidic nor basic. Such reactions are called neutralisation reactions.
Acid + Base → Salt + Water + Heat
Daily life Applications of Neutralisation Reaction:
- When an ant bites, it injects formic acid, causing pain. Applying moist baking soda, a base, neutralises the acid, relieving the sting.
- In agriculture, acidic soil from excessive chemical fertilisers can hinder plant growth. Adding lime, a base, neutralises the soil’s acidity, restoring a balanced pH for healthy plant growth. These examples show how neutralisation addresses common issues effectively.
Question 12. How can you identify whether a substance is acidic, basic, or neutral using indicators?
Answer
To identify a substance’s nature, use acid-base indicators like litmus, turmeric, or red rose extract.
- With blue litmus paper, if it turns red, the substance is acidic; if it stays blue, test with red litmus.
- If red litmus turns blue, the substance is basic. If neither litmus changes, the substance is neutral.
- Alternatively, turmeric paper turns red in basic solutions but remains yellow in acidic or neutral ones.
- Red rose extract turns red in acidic solutions, green in basic ones, and stays unchanged in neutral ones.
- By observing these colour changes, the substance’s nature can be determined accurately.
Question 13. Which is the most commonly used natural indicator?
Answer
The most commonly used natural indicator is litmus. Key features of litmus include:
- Extracted from lichens.
- Appears mauve (purple) in distilled water.
- Turns red in acidic solutions.
- Turns blue in basic solutions.
- Available as a solution or as strips of paper, known as litmus paper.
- Typically found in red and blue varieties.
Question 14. Explain the process of preparing turmeric paper and its use as an indicator.
Answer
- To prepare turmeric paper, a spoonful of turmeric powder is mixed with water to form a paste, or fresh turmeric is ground.
- A piece of filter paper is dipped into this paste until it turns yellow, then removed and dried.
- The dried paper is cut into thin strips, creating turmeric paper.
- As an indicator, turmeric paper is used by placing a drop of a test substance on it.
- If the substance is basic, the yellow paper turns red, indicating a basic nature.
- It does not change colour in acidic or neutral substances, making it specific for detecting bases.
Question 15. Blue litmus paper is dipped in a solution. It remains blue. What is the nature of the solution? Explain.
Answer
The solution is either a base or neutral because blue litmus paper remains unchanged in both types of solutions. Here are the key points:
- If the litmus paper stays blue, it indicates that the solution does not contain acidic properties.
- Blue litmus paper will not change colour in a neutral solution.
Thus, the solution is confirmed to be either basic or neutral.