Acids Bases and Salts — Class 7 Science NCERT Solutions (Free)
Free step-by-step NCERT solutions for Class 7 Science chapter "Acids Bases and Salts" — 8 important questions with detailed answers for CBSE board exam preparation.
TL;DR: Free step-by-step NCERT solutions for Class 7 Science chapter "Acids Bases and Salts" — 8 important questions with detailed answers for CBSE board exa…
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NCERT Solutions for Class 7 Science — Acids Bases and Salts. Step-by-step answers to all 8 textbook questions from this chapter, written for CBSE board preparation and free to use.
Acids Bases and Salts — All 8 Questions Solved
Q1. Define an acid.
An acid is a substance that tastes sour and turns blue litmus paper red.
Examples: Hydrochloric acid (HCl), Sulphuric acid (H₂SO₄), Acetic acid (CH₃COOH).
Acids release H⁺ ions in solution.
Q2. Define a base.
A base is a substance that tastes bitter and slippery, and turns red litmus paper blue.
Examples: Sodium hydroxide (NaOH), Ammonia (NH₃), Calcium hydroxide (Ca(OH)₂).
Bases release OH⁻ ions in solution.
Q3. What is the pH scale? What is the pH of a neutral solution?
pH scale measures the acidity or basicity of a substance.
It ranges from 0 to 14.
PH < 7: Acidic; pH = 7: Neutral; pH > 7: Basic
Water is neutral with pH = 7.
Q4. What is neutralisation?
Neutralisation is the reaction between an acid and a base to form salt and water.
Acid + Base → Salt + Water
Example: HCl + NaOH → NaCl + H₂O
The pH becomes 7 (neutral) after neutralisation.
Q5. Define a salt. Give three examples.
A salt is an ionic compound formed by the reaction between an acid and a base.
Examples:
- Sodium chloride (NaCl) - table salt
- Copper sulphate (CuSO₄) - blue in colour
- Ammonium chloride (NH₄Cl)
Q6. Name two natural indicators used to test acids and bases.
Natural indicators:
- Litmus: Red litmus paper turns blue in base; blue litmus turns red in acid
- Turmeric: Yellow turmeric paper turns red in base
- Methyl orange: Changes colour from red (acid) to yellow (base)
Q7. What happens when hydrochloric acid reacts with sodium hydroxide?
HCl + NaOH → NaCl + H₂O
This is a neutralisation reaction.
Acid donates H⁺ ions and base donates OH⁻ ions.
They combine to form water, and the salt (NaCl) remains in solution.
The temperature of the solution increases (exothermic reaction).
Q8. How can we use neutralisation to treat indigestion?
Indigestion is caused by excess acid in the stomach.
Antacids (mild bases like calcium carbonate or magnesium hydroxide) are used to neutralise the excess acid.
This brings relief and the pH inside the stomach returns to normal.
Acids Bases and Salts — Practice MCQs with Answers
Attempt these 20 multiple-choice questions after working through the solutions above. Each carries the correct option and the reasoning behind it, so a wrong answer tells you which idea to revisit.
MCQ 1. Which of the following substances typically has a sour taste?
- A. Sugar
- B. Salt
- C. Vinegar
- D. Baking soda
Answer: C. Vinegar — Acids are known for their sour taste. Vinegar contains acetic acid, which gives it a characteristic sour taste.
MCQ 2. Which of the following substances feels soapy to touch?
- A. Lemon juice
- B. Curd
- C. Soap solution
- D. Salt water
Answer: C. Soap solution — Bases often feel soapy or slippery to touch. Soap solutions are basic in nature due to the presence of bases.
MCQ 3. What color does blue litmus paper turn when dipped into a solution of lemon juice?
- A. Blue
- B. Red
- C. Green
- D. Yellow
Answer: B. Red — Lemon juice is acidic. Acids are known to turn blue litmus paper red.
MCQ 4. What color does red litmus paper turn when dipped into a solution of baking soda?
- A. Red
- B. Blue
- C. Green
- D. No change
Answer: B. Blue — Baking soda solution is basic. Bases turn red litmus paper blue.
MCQ 5. The acid found in tamarind is:
- A. Lactic acid
- B. Citric acid
- C. Tartaric acid
- D. Acetic acid
Answer: C. Tartaric acid — Tamarind naturally contains tartaric acid, which is responsible for its sour taste.
MCQ 6. Milk of magnesia, commonly used as an antacid, primarily contains which base?
- A. Sodium hydroxide
- B. Potassium hydroxide
- C. Magnesium hydroxide
- D. Calcium hydroxide
Answer: C. Magnesium hydroxide — Milk of magnesia is a suspension of magnesium hydroxide, a mild base used to neutralize excess stomach acid.
MCQ 7. A farmer observes that the soil in his field is too acidic. Which of the following substances would he most likely add to neutralize the soil?
- A. Vinegar
- B. Common salt
- C. Slaked lime
- D. Lemon juice
Answer: C. Slaked lime — Slaked lime (calcium hydroxide) is a base. Adding a base helps to neutralize the acidity of the soil, making it suitable for crops.
MCQ 8. What color does turmeric solution turn when added to a solution of soap?
- A. Yellow
- B. Reddish-brown
- C. Green
- D. Blue
Answer: B. Reddish-brown — Turmeric is a natural indicator that turns reddish-brown in basic solutions like soap solution, and remains yellow in acidic or neutral solutions.
MCQ 9. When China rose indicator is added to a highly acidic solution, what color would it show?
- A. Green
- B. Red
- C. Dark Pink/Magenta
- D. Yellow
Answer: C. Dark Pink/Magenta — China rose indicator turns dark pink or magenta in acidic solutions and green in basic solutions.
MCQ 10. Which of these is a neutral substance?
- A. Orange juice
- B. Pure water
- C. Detergent solution
- D. Curd
Answer: B. Pure water — Pure water is neither acidic nor basic; it is a neutral substance.
MCQ 11. The reaction between an acid and a base is called:
- A. Oxidation
- B. Reduction
- C. Neutralization
- D. Precipitation
Answer: C. Neutralization — The reaction between an acid and a base, which produces salt and water, is known as a neutralization reaction.
MCQ 12. What are the main products formed when an acid reacts with a base?
- A. Acid and Base
- B. Salt and Water
- C. Acid and Salt
- D. Base and Water
Answer: B. Salt and Water — In a neutralization reaction, an acid reacts with a base to form salt and water as the primary products.
MCQ 13. An ant sting injects formic acid into the skin. Which common household substance can be used to neutralize its effect?
- A. Vinegar
- B. Lemon juice
- C. Baking soda
- D. Common salt
Answer: C. Baking soda — Baking soda (sodium hydrogen carbonate) is a mild base. It neutralizes the formic acid injected by an ant sting, reducing irritation.
MCQ 14. A wasp sting is alkaline in nature. Which of the following would be suitable to apply to the affected area?
- A. Calamine solution
- B. Baking soda solution
- C. Vinegar
- D. Soap solution
Answer: C. Vinegar — Since a wasp sting is alkaline, an acidic substance like vinegar (acetic acid) can be applied to neutralize it.
MCQ 15. Which of the following is a strong mineral acid?
- A. Citric acid
- B. Lactic acid
- C. Hydrochloric acid
- D. Acetic acid
Answer: C. Hydrochloric acid — Hydrochloric acid (HCl) is a strong mineral acid, unlike the others which are weak organic acids.
MCQ 16. A solution turns phenolphthalein indicator pink. What is the nature of the solution?
- A. Acidic
- B. Basic
- C. Neutral
- D. Slightly acidic
Answer: B. Basic — Phenolphthalein indicator turns pink in basic solutions and remains colorless in acidic or neutral solutions.
MCQ 17. A solution shows no change with blue litmus paper, but turns turmeric paper reddish-brown. What is the nature of the solution?
- A. Acidic
- B. Basic
- C. Neutral
- D. Cannot be determined
Answer: B. Basic — If blue litmus paper shows no change, the solution is either basic or neutral. However, turmeric paper turning reddish-brown confirms that the solution is basic.
MCQ 18. Which acid is present in curd?
- A. Citric acid
- B. Lactic acid
- C. Tartaric acid
- D. Oxalic acid
Answer: B. Lactic acid — Curd contains lactic acid, which is produced when bacteria ferment the lactose sugar in milk.
MCQ 19. Which of these bases is also known as 'caustic soda'?
- A. Calcium hydroxide
- B. Magnesium hydroxide
- C. Sodium hydroxide
- D. Ammonium hydroxide
Answer: C. Sodium hydroxide — Sodium hydroxide (NaOH) is a strong base commonly known as caustic soda.
MCQ 20. Factory wastes are often highly acidic. Before discharging them into water bodies, they are usually treated with:
- A. More acid
- B. Bleaching powder
- C. Basic substances
- D. Alcohol
Answer: C. Basic substances — Factory wastes often contain harmful acids. They are neutralized by adding basic substances before discharge to prevent damage to aquatic life and the environment.
How to Use These Solutions
Attempt each question yourself first and only then compare with the worked answer. Marks in the CBSE board exam are awarded for the METHOD as much as the final result, so reproduce the steps rather than memorising the last line. Where a solution states a law, a formula or a definition, learn that wording — examiners look for it.
Related practice for this chapter: chapter MCQs, previous-year questions, revision notes and sample papers.