Unit Test (Solution): Exploring Substances: Acidic, Basic, and Neutral | Science (Curiosity) Class 7 - New NCERT PDF Download

Maximum Marks: 30
Attempt all questions

• Question numbers 1 to 7 carry 1 mark each.
• Question numbers 8 to 12 carry 2 marks each.
• Question numbers 13 to 15 carry 3 marks each.
• Question numbers 16 carry  4 marks each.

Q1. Which of the following substances turns blue litmus paper to red?  (1 Mark)
(i) Soap solution
(ii) Lemon juice
(iii) Sugar solution
(iv) Salt solution 
Ans: (ii) Lemon juice

Substances that are acidic in nature, such as lemon juice, turn blue litmus paper to red.

Q2. The yellow colour of turmeric paper changes to ______ in a basic solution. (1 Mark)
Ans: red

Turmeric paper turns red when exposed to basic substances, indicating their basic nature.

Q3. Which of the following is a natural acid-base indicator? (1 Mark)
(i) Vinegar
(ii) Red rose extract
(iii) Baking soda
(iv) Tap water
Ans: (ii) Red rose extract

Red rose extract changes colour in acidic and basic solutions, making it a natural acid-base indicator.

Q4. What is the result of a neutralisation reaction? (1 Mark)
Ans: Salt and water

A neutralisation reaction between an acid and a base produces salt, water, and heat.

Q5. Which indicator can be used to identify basic substances but not acidic ones? (1 Mark)
(i) Blue litmus
(ii) Red litmus
(iii) Turmeric paper
(iv) Red rose extract
Ans: (iii) Turmeric paper

Turmeric paper changes colour only in basic solutions, making it suitable for identifying bases.

Q6. Substances that do not change the colour of litmus paper are called ______. (1 Mark)
Ans: neutral

Neutral substances, like sugar solution, do not affect the colour of red or blue litmus paper.

Q7. Why is moist baking soda used to relieve an ant bite? (1 Mark)
(i) It neutralises the acidic liquid
(ii) It cools the skin
(iii) It prevents infection
(iv) It reduces swelling
Ans: (i) It neutralises the acidic liquid

Moist baking soda, a base, neutralises the acid named formic acid injected by an ant bite, relieving pain.

Q8. Explain how red rose extract acts as an acid-base indicator. (2 Mark)
Ans: 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.

Q9. Why is litmus considered an acid-base indicator? (2 Mark)
Ans: 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.

Q10. How does neutralisation help in treating acidic soil? (2 Mark)
Ans: 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.

Q11. Describe the feel and taste of basic substances. (2 Mark)
Ans: 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.

Q12. What are olfactory indicators, and give one example? (2 Mark)
Ans: 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.

Q13. Explain the process of preparing turmeric paper and its use as an indicator. (3 Mark)
Ans: 

• 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.

Q14. What is Neutralisation Reaction? Describe how neutralisation reactions are applied in daily life with two examples. (3 Mark)
Ans: When the solution of an acid is mixed with the solution of a base in suffi cient 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.

Q15. How can you identify whether a substance is acidic, basic, or neutral using indicators? (3 Mark)
Ans: 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.

Q16. Read the passage below and answer the questions that follow. (4 Mark)
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.
Ans: 

• (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.