Exploring-substances-acidic-basic-and-neutral : Class 7 : Science : Question and Answers
SUMMARY
This chapter introduces us to three important types of substances in our daily life: acids, bases, and neutral substances.
Acids taste sour. Examples: lemon juice, vinegar, tamarind, curd. They turn blue litmus paper red.
Bases taste bitter and feel soapy to touch. Examples: baking soda, lime water, soap solution. They turn red litmus paper blue.
Neutral substances are neither acidic nor basic. Examples: water, common salt, sugar solution.
The chapter explains the use of indicators like litmus paper, turmeric, and china rose, which help us identify whether a substance is acidic or basic.
It also discusses how acids and bases can be harmful in concentrated form, but many are useful in medicines, food preservation, cleaning, and agriculture.
Finally, the chapter highlights the importance of maintaining a balance, since both excessive acidity and alkalinity can be harmful to living beings and the environment.
SHORT QUESTION ANSWERS
Q1. What is an acid?
Ans: A substance that tastes sour and turns blue litmus red.
Q2. Give two examples of natural acids.
Ans: Lemon juice, vinegar.
Q3. What is a base?
Ans: A substance that tastes bitter, feels soapy, and turns red litmus blue.
Q4. Name two examples of bases.
Ans: Baking soda, soap solution.
Q5. What are neutral substances?
Ans: Substances that are neither acidic nor basic, e.g., water, salt.
Q6. Which indicator is most commonly used?
Ans: Litmus paper.
Q7. What colour does turmeric turn in a base?
Ans: Red-brown.
Q8. What colour does litmus paper turn in acid?
Ans: Blue to red.
Q9. What colour does litmus paper turn in base?
Ans: Red to blue.
Q10. What is an indicator?
Ans: A substance that shows whether another substance is acidic or basic.
Q11. Give two natural indicators.
Ans: Turmeric, china rose.
Q12. Give two synthetic indicators.
Ans: Methyl orange, phenolphthalein.
Q13. What are mineral acids?
Ans: Acids prepared from minerals, e.g., hydrochloric acid, sulphuric acid.
Q14. What are organic acids?
Ans: Acids found in plants and animals, e.g., citric acid, lactic acid.
Q15. Which acid is found in ant sting?
Ans: Formic acid.
Q16. Which acid is present in curd?
Ans: Lactic acid.
Q17. Which base is used in making soap?
Ans: Sodium hydroxide.
Q18. Which base is used in antacids?
Ans: Magnesium hydroxide.
Q19. Which acid is used in car batteries?
Ans: Sulphuric acid.
Q20. Which acid is present in vinegar?
Ans: Acetic acid.
Q21. What is neutralisation?
Ans: The reaction between an acid and a base to form salt and water.
Q22. Write the products of acid + base reaction.
Ans: Salt + water.
Q23. Give an example of neutralisation in daily life.
Ans: Antacid tablets neutralise stomach acid.
Q24. Why is toothpaste basic?
Ans: To neutralise the acids in the mouth.
Q25. What is the pH scale?
Ans: A scale used to measure acidity or alkalinity of a substance.
Q26. What is the pH of pure water?
Ans: 7 (neutral).
Q27. What type of soil is treated with lime?
Ans: Acidic soil.
Q28. What type of soil is treated with organic manure?
Ans: Basic soil.
Q29. Which indicator is used in titration?
Ans: Phenolphthalein.
Q30. What colour does phenolphthalein show in base?
Ans: Pink.
Q31. What colour does methyl orange show in acid?
Ans: Red.
Q32. Name a strong acid.
Ans: Hydrochloric acid.
Q33. Name a weak acid.
Ans: Citric acid.
Q34. Name a strong base.
Ans: Sodium hydroxide.
Q35. Name a weak base.
Ans: Ammonium hydroxide.
Q36. Which acid is present in tomatoes?
Ans: Oxalic acid.
Q37. Which base is used in glass making?
Ans: Sodium carbonate.
Q38. Which base is used in baking?
Ans: Sodium bicarbonate (baking soda).
Q39. Which acid is present in oranges?
Ans: Citric acid.
Q40. Why should strong acids be handled carefully?
Ans: Because they are highly corrosive and can cause burns.
LONG QUESTION ANSWERS
Q1. Define acids. Explain their properties with examples.
Ans: Acids are substances that taste sour, turn blue litmus red, and react with bases to form salt and water. Examples: lemon juice (citric acid), vinegar (acetic acid).
Q2. Define bases. Explain their properties with examples.
Ans: Bases are substances that taste bitter, feel soapy, and turn red litmus blue. They react with acids to form salt and water. Examples: baking soda, sodium hydroxide.
Q3. Explain neutral substances with examples.
Ans: Neutral substances are neither acidic nor basic, e.g., pure water, salt, sugar solution. They do not change the colour of indicators.
Q4. What are indicators? Explain natural and synthetic indicators with examples.
Ans: Indicators detect acids or bases. Natural: turmeric, china rose. Synthetic: phenolphthalein, methyl orange.
Q5. Differentiate between acids and bases.
Ans: Acids: sour, turn blue litmus red, contain H⁺ ions. Bases: bitter, soapy, turn red litmus blue, contain OH⁻ ions.
Q6. Explain mineral acids and organic acids with examples.
Ans: Mineral acids are made from minerals (HCl, H₂SO₄). Organic acids are found in plants/animals (citric, lactic acid).
Q7. Write five uses of acids.
Ans: In car batteries (H₂SO₄), vinegar in food preservation, citric acid in fruits, hydrochloric acid in cleaning, nitric acid in fertilizers.
Q8. Write five uses of bases.
Ans: Sodium hydroxide in soap, calcium hydroxide in whitewashing, magnesium hydroxide in antacids, ammonium hydroxide in cleaning, sodium carbonate in glass making.
Q9. Explain the neutralisation reaction with an example.
Ans: Acid + Base → Salt + Water. Example: HCl + NaOH → NaCl + H₂O.
Q10. Explain the importance of neutralisation in daily life.
Ans: Antacids neutralise stomach acid, lime neutralises acidic soil, baking soda neutralises bee stings, toothpaste neutralises mouth acids.
Q11. What is the role of pH in our body?
Ans: Human blood has a pH of about 7.4. A change in pH can cause diseases. Antacids maintain stomach pH balance.
Q12. What is the role of acids in plants and animals?
Ans: Citric acid in citrus fruits, oxalic acid in tomatoes, formic acid in ants, lactic acid in milk.
Q13. Explain the harmful effects of strong acids.
Ans: Corrosive, cause burns, damage materials, pollute environment, cause acid rain.
Q14. Explain the harmful effects of strong bases.
Ans: Corrosive, harmful to skin, damage fabrics, pollute water and soil.
Q15. Explain acid rain and its effects.
Ans: Acid rain is caused by sulphuric and nitric acids from industries. It damages crops, buildings, soil, and aquatic life.
Q16. Why is toothpaste basic?
Ans: Toothpaste neutralises acids in the mouth formed by bacteria, protecting teeth from decay.
Q17. How does turmeric act as an indicator?
Ans: Turmeric remains yellow in acids but turns red-brown in bases.
Q18. How does china rose act as an indicator?
Ans: In acids it turns pink/red, in bases it turns green.
Q19. Explain neutralisation in agriculture.
Ans: Lime is added to acidic soil, while organic manure is added to basic soil to balance pH.
Q20. Explain the role of antacids.
Ans: Antacids like milk of magnesia neutralise excess HCl in the stomach, curing acidity.
Q21. Explain the laboratory preparation of an acid.
Ans: HCl can be prepared by dissolving hydrogen chloride gas in water.
Q22. Explain the laboratory preparation of a base.
Ans: NaOH is prepared by passing electricity through sodium chloride solution (electrolysis).
Q23. Write differences between strong and weak acids.
Ans: Strong acids (HCl, H₂SO₄) completely ionise in water. Weak acids (citric, acetic) partially ionise.
Q24. Write differences between strong and weak bases.
Ans: Strong bases (NaOH, KOH) ionise completely. Weak bases (NH₄OH) ionise partially.
Q25. Explain the role of acids in digestion.
Ans: HCl in the stomach helps digestion and kills harmful microbes.
Q26. Explain why acids are stored in glass or plastic bottles.
Ans: Because acids react with metals and would corrode metal containers.
Q27. How does pH affect plants?
Ans: Some plants grow only in acidic soil (tea), others in neutral soil (wheat). pH affects crop yield.
Q28. How does pH affect aquatic life?
Ans: Fish can survive only in water with proper pH (6.5–8.5). Acid rain makes water acidic, harming aquatic life.
Q29. What are ant stings? How are they treated?
Ans: Ant sting contains formic acid. It is treated with a base like baking soda.
Q30. What are bee stings? How are they treated?
Ans: Bee stings are acidic. Treated with baking soda paste.
Q31. Write differences between acid and neutral substances.
Ans: Acids are sour, change litmus red, contain H⁺ ions. Neutral substances are neither sour nor bitter, don’t change litmus.
Q32. Write differences between base and neutral substances.
Ans: Bases are bitter, soapy, change litmus blue, contain OH⁻ ions. Neutrals don’t change indicators.
Q33. Explain the role of acids in food preservation.
Ans: Vinegar (acetic acid) prevents food spoilage by killing bacteria.
Q34. Explain why milk turns sour.
Ans: Bacteria produce lactic acid, making milk sour.
Q35. Explain importance of acids and bases in industries.
Ans: Acids used in fertilizers, dyes, batteries. Bases used in soaps, glass, cleaning agents.
Q36. What is the importance of pH in medicine?
Ans: Drug effectiveness depends on pH. Wrong pH may harm the body.
Q37. What is the importance of neutralisation in factories?
Ans: Factory waste acids are neutralised with bases before disposal.
Q38. Explain the role of acids and bases in cleaning.
Ans: Acids clean metals, bases like NH₄OH clean glass.
Q39. Why are acids and bases dangerous for children?
Ans: They can burn skin, eyes, and cause poisoning if swallowed.
Q40. Why is balance of acids and bases important in life?
Ans: To maintain health, protect environment, and support agriculture and industry.
MCQS WITH ANSWERS
1. Acids taste:
a) Bitter
b) Sweet
c) Sour
d) Salty
Ans: c) Sour
2. Bases taste:
a) Bitter
b) Sweet
c) Sour
d) Salty
Ans: a) Bitter
3. Litmus turns red in:
a) Base
b) Neutral
c) Acid
d) None
Ans: c) Acid
4. Litmus turns blue in:
a) Acid
b) Base
c) Neutral
d) None
Ans: b) Base
5. Vinegar contains:
a) Lactic acid
b) Acetic acid
c) Citric acid
d) Oxalic acid
Ans: b) Acetic acid
6. Lemon contains:
a) Oxalic acid
b) Acetic acid
c) Citric acid
d) Lactic acid
Ans: c) Citric acid
7. Tomato contains:
a) Citric acid
b) Oxalic acid
c) Formic acid
d) Acetic acid
Ans: b) Oxalic acid
8. Curd contains:
a) Lactic acid
b) Acetic acid
c) Citric acid
d) Formic acid
Ans: a) Lactic acid
9. Ant sting contains:
a) Acetic acid
b) Citric acid
c) Formic acid
d) Lactic acid
Ans: c) Formic acid
10. Soap solution is:
a) Acidic
b) Basic
c) Neutral
d) None
Ans: b) Basic
11. Water is:
a) Acidic
b) Basic
c) Neutral
d) Salty
Ans: c) Neutral
12. Which acid is used in batteries?
a) Nitric acid
b) Hydrochloric acid
c) Sulphuric acid
d) Acetic acid
Ans: c) Sulphuric acid
13. Turmeric turns red in:
a) Acid
b) Base
c) Neutral
d) Water
Ans: b) Base
14. China rose in acid turns:
a) Green
b) Pink/red
c) Blue
d) Colourless
Ans: b) Pink/red
15. China rose in base turns:
a) Red
b) Green
c) Pink
d) Yellow
Ans: b) Green
16. Toothpaste is:
a) Acidic
b) Basic
c) Neutral
d) Sour
Ans: b) Basic
17. Reaction of acid + base gives:
a) Salt + water
b) Only salt
c) Only water
d) Acid + base
Ans: a) Salt + water
18. Neutralisation is an example of:
a) Physical change
b) Chemical change
c) Neutral change
d) Natural change
Ans: b) Chemical change
19. Acid rain contains:
a) H₂SO₄ and HNO₃
b) HCl only
c) Citric acid
d) Acetic acid
Ans: a) H₂SO₄ and HNO₃
20. Which base is used in whitewashing?
a) NaOH
b) Ca(OH)₂
c) NH₄OH
d) Mg(OH)₂
Ans: b) Ca(OH)₂
21. Antacids contain:
a) Mg(OH)₂
b) HCl
c) NaOH
d) Ca(OH)₂
Ans: a) Mg(OH)₂
22. Milk turning sour is due to:
a) Citric acid
b) Lactic acid
c) Acetic acid
d) Oxalic acid
Ans: b) Lactic acid
23. Acidic soil is treated with:
a) Lime
b) Vinegar
c) Ammonia
d) Baking powder
Ans: a) Lime
24. Basic soil is treated with:
a) Lime
b) Organic manure
c) NaOH
d) HCl
Ans: b) Organic manure
25. Which indicator is used in titration?
a) Turmeric
b) Phenolphthalein
c) Litmus
d) China rose
Ans: b) Phenolphthalein
26. Phenolphthalein in base is:
a) Pink
b) Red
c) Yellow
d) Green
Ans: a) Pink
27. Methyl orange in acid is:
a) Yellow
b) Red
c) Pink
d) Blue
Ans: b) Red
28. pH of water is:
a) 6
b) 7
c) 8
d) 5
Ans: b) 7
29. Strong acid example:
a) Citric acid
b) HCl
c) Vinegar
d) Acetic acid
Ans: b) HCl
30. Weak acid example:
a) HCl
b) H₂SO₄
c) Citric acid
d) Nitric acid
Ans: c) Citric acid
31. Strong base example:
a) NaOH
b) NH₄OH
c) Ca(OH)₂
d) Mg(OH)₂
Ans: a) NaOH
32. Weak base example:
a) NaOH
b) KOH
c) NH₄OH
d) Ca(OH)₂
Ans: c) NH₄OH
33. Which gas is released when acid reacts with metal?
a) O₂
b) CO₂
c) H₂
d) N₂
Ans: c) H₂
34. Acid rain affects:
a) Soil
b) Buildings
c) Aquatic life
d) All of these
Ans: d) All of these
35. Which acid is used in cleaning rust?
a) Hydrochloric acid
b) Nitric acid
c) Acetic acid
d) Citric acid
Ans: a) Hydrochloric acid
36. Baking soda is:
a) Acid
b) Base
c) Neutral
d) Salt
Ans: b) Base
37. Which acid helps in digestion?
a) HCl
b) H₂SO₄
c) HNO₃
d) CH₃COOH
Ans: a) HCl
