Chapter.4. Acids, Bases and Salts, The Living World, Class – VII, DAV Board, The Best Solutions.

Keywords To Remember

1. Acid – A substance that tastes sour and releases hydrogen ions (H⁺) in water.
2. Base – A substance that tastes bitter, feels soapy, and releases hydroxide ions (OH⁻) in water.
3. Alkali – A base that is soluble in water, such as sodium hydroxide.
4. Mineral Acids – Acids prepared from minerals found in the earth (e.g., HCl, H₂SO₄).
5. Organic Acids – Naturally occurring acids found in plants or animals (e.g., citric acid, acetic acid).
6. Strong Acid – An acid that completely dissociates in water (e.g., hydrochloric acid).
7. Weak Acid – An acid that partially dissociates in water (e.g., acetic acid).
8. Strong Base – A base that completely dissociates in water (e.g., sodium hydroxide).
9. Weak Base – A base that partially dissociates in water (e.g., ammonium hydroxide).
10. Concentrated Acid – An acid with very little water content.
11. Dilute Acid – An acid with high water content, made by adding water to a concentrated acid.
12. Indicator – A substance that changes colour in the presence of an acid or base.
13. Litmus – A natural indicator that turns red in acids and blue in bases.
14. Turmeric – A natural indicator that remains yellow in acids and turns red in bases.
15. Phenolphthalein – A chemical indicator that is colourless in acids and turns pink in bases.
16. China Rose Indicator – Natural indicator that turns dark pink in acids and green in bases.
17. Neutralisation Reaction – A chemical reaction between an acid and a base, producing salt and water.
18. Salt – A compound formed during the neutralisation of an acid with a base.
19. Neutral Salt – A salt formed from a strong acid and a strong base; does not affect litmus.
20. Acidic Salt – A salt formed from a strong acid and a weak base; turns blue litmus red.
21. Basic Salt – A salt formed from a weak acid and a strong base; turns red litmus blue.
22. Sodium Chloride – A neutral salt commonly known as table salt (NaCl).
23. Sodium Acetate – A basic salt formed from sodium hydroxide and acetic acid (CH₃COONa).
24. Aluminium Chloride – An acidic salt formed from HCl and aluminium hydroxide (AlCl₃).
25. Electric Conductivity – The ability of a solution to conduct electricity; shown by salt solutions.
26. Lactic Acid – Organic acid found in sour milk and curd.
27. Citric Acid – Organic acid found in citrus fruits like lemon and orange.
28. Tartaric Acid – Organic acid found in grapes, tamarind, and gooseberries.
29. Acetic Acid – Organic acid found in vinegar (CH₃COOH).
30. Oxalic Acid – Organic acid found in guava and spinach.
31. Formic Acid – Acid found in ant stings and bee stings.
32. Hydrochloric Acid (HCl) – A strong mineral acid found in the stomach and used in industry.
33. Sulphuric Acid (H₂SO₄) – Known as the “King of Chemicals,” widely used industrial acid.
34. Nitric Acid (HNO₃) – A strong acid used in fertilizers and explosives.
35. Sulphurous Acid (H₂SO₃) – A weaker mineral acid used as a preservative.
36. Phosphoric Acid (H₃PO₄) – A strong acid used in soft drinks and fertilizers.
37. Carbonic Acid (H₂CO₃) – Weak acid formed when carbon dioxide dissolves in water.
38. Antacid – A mild base used to neutralise excess acid in the stomach.
39. Calcium Carbonate (CaCO₃) – Main component of marble; reacts with acid rain.
40. Acid Rain – Rainwater made acidic due to dissolved gases like CO₂, SO₂, and NO₂.
41. Sodium Hydroxide (NaOH) – A strong base; also an alkali.
42. Potassium Hydroxide (KOH) – A strong base used in soaps and detergents.
43. Ammonium Hydroxide (NH₄OH) – A weak base used in household cleaners.
44. Calcium Sulphate (CaSO₄) – A salt formed from calcium hydroxide and sulphuric acid.
45. Sodium Nitrate (NaNO₃) – A neutral salt used in fertilizers.
46. Magnesium Nitrate (Mg(NO₃)₂) – A salt formed from magnesium hydroxide and nitric acid.
47. Electrolyte – A solution that can conduct electricity due to the presence of ions.
48. LED Bulb Test – A method to check conductivity using a light bulb in a circuit.
49. Petri Dish – A shallow dish used to observe reactions in science experiments.
50. Graphite Rods – Used in conductivity experiments as electrodes.

Something To Know

A. Fill in the blanks.

1. Acids which are present in plants and animals are called __________.

Answer –  organic acids

2. Bases taste __________and have a __________ feel.

Answer –  bitter, soapy

3. Acids turns the colour of blue litmus paper to __________ .

Answer –  red

4. The products of neutralisation reaction are __________ and __________.

Answer –  salt and water

5. Salts of nitric acid (HNO₃) are named as __________.

Answer –  nitrates

6. Sodium acetate (CH₃COONa) is a basic salt formed by the reaction of __________ and __________ .

Answer –  acetic acid and sodium hydroxide.

B. Match the following:

1. Lemon juice (a) Oxalic acid
2. Tamarind (b) Lactic acid
3. Vinegar (c) Citric acid
4. Red ants (d) Acetic acid
5. Sour milk (e) Tartaric acid
6. Guava (f) Formic: acid

Answer –

C. Tick (✓) the correct option.

1. Bases have a—

bitter taste and a rough feel

bitter taste and a soapy feel

sour taste and a rough feel

sour taste and a soapy feel

Answer – bitter taste and a soapy feel

2. An example of a natural indicator is—

methyl orange

phenolphthalein

ink

litmus

Answer – litmus

3. An acid, that contributes to the sour taste of some fruits, is

hydrochloric acid

sulphuric acid

citric acid

nitric acid

Answer – citric acid

4. Which of the following is a strong acid?

acetic acid

nitric acid

citric acid

carbonic acid

Answer – nitric acid

5. Substances, produced through a chemical reaction between acids and bases, are known as—

salts

antacids

indicators

alkalies

Answer – salts

6. An indicator, that turns red in a basic medium, is—

turmeric

blue litmus

phenolphthalein

hibiscus

Answer – phenolphthalein

7. The general taste, of acids and bases, is respectively—

sweet and salty

sour and sweet

sour and salty

sour and bitter

Answer – sour and bitter

D. Answer the following questions in brief.

1. What are mineral acids?

Answer – Mineral acids are acids that are derived from inorganic minerals or compounds, e.g., hydrochloric acid, nitric acid.

2. Give two examples each of mineral acids and organic acids.

Answer – Mineral acids: HCl (hydrochloric acid), H₂SO₄ (sulphuric acid)

Organic acids: Citric acid, Acetic acid

3. Name any two substances that can be used as indicators.

Answer – Litmus, turmeric

4. Write the meaning of the term ‘neutralisation reaction.’

Answer – A neutralisation reaction is a chemical reaction in which an acid and a base react to form salt and water.

5. Give any two properties of salts.

Answer – Salts are usually crystalline in nature.

They are generally soluble in water and can conduct electricity in aqueous solution.

6. Classify the following salts as neutral, acidic or basic. Also, write their names.

(a) Na}PO4

Answer – Basic salt; Sodium phosphate

(b) K,CO,

Answer – Basic salt; Potassium carbonate

(c) NH4NO,

Answer – Acidic salt; Ammonium nitrate

E. Answer the following questions

1. All alkalies are bases but all bases are not alkalies’. Justify this statement.

Answer – Alkalies are bases that dissolve in water to form OH⁻ ions. Not all bases are water-soluble, so while all alkalies are bases, not all bases qualify as alkalies.

2. Suggest an activity that can help one to decide whether a given solution is acidic or basic in nature.

Answer – Dip a strip of blue and red litmus paper in the solution. If blue turns red, the solution is acidic. If red turns blue, it’s basic.

3. Write chemical equations for the following reactions:

(a) Calcium hydroxide reacts with nitric acid.

Answer – Ca(OH)₂ + 2HNO₃ → Ca(NO₃)₂ + 2H₂O

(b) Acetic acid reacts with calcium hydroxide.

Answer – 2CH₃COOH + Ca(OH)₂ → (CH₃COO)₂Ca + 2H₂O

(c) Hydrochloric acid reacts with sodium hydroxide.

Answer – HCl + NaOH → NaCl + H₂O

(d) Ammonium hydroxide reacts with sulphuric acid.

Answer – 2NH₄OH + H₂SO₄ → (NH₄)₂SO₄ + 2H₂O

4. State the difference between neutral, acidic and basic salts. Give one example of each.

Answer –

5. Describe an activity to show that solutions of salts, in water, can conduct electricity.

Answer – Take a beaker with salt solution.

Insert two graphite electrodes connected to a bulb.

The bulb glows, showing salt solutions conduct electricity.

Value Based Question

The Physical Training (PT) teacher noticed that two of her students were having very heated arguments with each other. She immediately went to them, calmed them down and made them sit together. She listened patiently to both of them and then explained to them the merits, of the ‘other point of view’ and the importance of ‘team work*. The students agreed to follow her advice and instructions.

The chemistry teacher, who was watching all this, remarked that she sees a lot of similarity between the role of the PT teacher and the ‘neutralisation reactions’ between acids and bases.

1. State the values displayed by the PT teacher.

Answer – Patience, understanding, conflict resolution, empathy, teamwork.

2. Do you agree with the remarks of the chemistry teacher? Give reason.

Answer – Yes, because just like neutralisation reduces the effect of both acids and bases, the PT teacher reduced the conflict between the students and restored harmony.

3. Write the chemical equations for two neutralisation reactions, giving names of all the compounds relevant to the reaction.

Answer – (i) HCl + NaOH → NaCl + H₂O (Hydrochloric acid + Sodium hydroxide → Sodium chloride + Water)
(ii) H₂SO₄ + 2KOH → K₂SO₄ + 2H₂O (Sulphuric acid + Potassium hydroxide → Potassium sulphate + Water)

Something To Do

1. Common names of some substances are given below. Find out the chemical formulae, chemical names and the uses of these substances, and write them in the table given below:

Common Name             Formula                         Chemical Name                      Use

Caustic soda  

Caustic potash  

Baking soda  

Washing soda  

Limestone  

Quicklime  

Lime water  

Plaster of paris  

Blue vitriol (neela thotha)  

Answer –

2. Solve the crossword puzzle with the help of the clues given below:

Across -»

1. A basic salt present in washing powders.
2. A weak acid present in vinegar.

Down i

2. An acid whose salts are called sulphites.
3. A substance used to reduce acidity in our stomach.
4. An acid present in oranges and lemons.

Answer – Across:

1. Washing soda
2. Acetic acid

Down:
2. Sulphurous acid
3. Antacid
5. Citric acid

3. Some acids, bases and salts are commonly used in our daily life. Find out the names of such substances, specify whether they are acids, bases or salts and write their uses.

Answer –

Additional Questions And Answers – 01

Fill in the blanks (with answers)

1. The acid present in curd is __________.
   Answer: lactic acid
2. __________ is used to test whether a solution is acidic or basic.
   Answer: Indicator
3. An acid that is commonly used in car batteries is __________.
   Answer: sulphuric acid
4. A base that is used in antacids is __________.
   Answer: magnesium hydroxide
5. A solution of baking soda is __________ in nature.
   Answer: basic
6. The chemical name of table salt is __________.
   Answer: sodium chloride
7. Acids produce __________ ions in aqueous solution.
   Answer: hydrogen (H⁺)
8. The base present in soap is __________.
   Answer: sodium hydroxide
9. A neutralisation reaction produces __________ and __________.
   Answer: salt, water
10. A red litmus paper remains red in __________ solution.
    Answer: acidic

Multiple Choice Questions (with answers)

1. Which acid is used in the pickling of metals?
   a) Acetic acid
   b) Hydrochloric acid
   c) Nitric acid
   d) Sulphuric acid
   Answer: b) Hydrochloric acid
2. Which base is used in making soaps?
   a) Calcium hydroxide
   b) Magnesium hydroxide
   c) Sodium hydroxide
   d) Ammonium hydroxide
   Answer: c) Sodium hydroxide
3. A substance which changes its colour in an acidic or basic medium is called—
   a) Solute
   b) Neutraliser
   c) Indicator
   d) Solvent
   Answer: c) Indicator
4. Which of the following is a strong base?
   a) Ammonium hydroxide
   b) Magnesium hydroxide
   c) Sodium hydroxide
   d) Calcium hydroxide
   Answer: c) Sodium hydroxide
5. The acid found in tomato is—
   a) Tartaric acid
   b) Citric acid
   c) Oxalic acid
   d) Acetic acid
   Answer: c) Oxalic acid

True/False (with answers)

1. Blue litmus turns red in bases.
   Answer: False
2. Acids are sour in taste.
   Answer: True
3. All salts are neutral in nature.
   Answer: False
4. Phenolphthalein turns pink in acidic solution.
   Answer: False
5. Turmeric turns red in acidic medium.
   Answer: False
6. Bases produce hydroxide ions in aqueous solution.
   Answer: True
7. Sodium chloride is formed by neutralisation reaction.
   Answer: True

Short Answer Questions (with answers)

1. What is an indicator?
   Answer: An indicator is a substance that changes its colour in acidic or basic solutions and helps identify their nature.
2. Why should acids be handled with care?
   Answer: Acids are corrosive and can cause burns or damage materials, hence they should be handled with care.
3. Name a natural and a synthetic indicator.
   Answer: Natural indicator – Litmus, Synthetic indicator – Phenolphthalein
4. What is the chemical formula of baking soda?
   Answer: NaHCO₃ (Sodium hydrogen carbonate)
5. Name two bases used in daily life.
   Answer: Sodium hydroxide (soap making), Magnesium hydroxide (antacid)

Competency-Based Questions (Reasoning & Assertion)

1. Assertion (A): Blue litmus paper turns red in acids.
   Reason (R): Acids release hydroxide ions in solution.
   a) Both A and R are true and R is the correct explanation.
   b) Both A and R are true but R is not the correct explanation.
   c) A is true but R is false.
   d) A is false but R is true.
   Answer: c) A is true but R is false.
2. Assertion (A): Vinegar is used as a food preservative.
   Reason (R): It contains acetic acid, which is weak but inhibits microbial growth.
   Answer: a) Both A and R are true and R is the correct explanation.

Higher Order Thinking Skill (HOTS) Questions

1. Why do acidic foods stored in metal containers sometimes become poisonous?
   Answer: Acids react with metals to form metal salts, some of which may be toxic, making the food poisonous.
2. If a person accidentally swallows an acid, why is a basic substance given immediately?
   Answer: A basic substance neutralises the acid, reducing its harmful effects on the body.
3. Why do factories use acid-resistant containers for storing acids?
   Answer: To prevent corrosion and leakage caused by the acids reacting with the container materials.

Long Answer Questions (50–60 words each)

1. Explain how neutralisation reaction is helpful in our daily life.
   Answer: Neutralisation reactions are useful in many ways, such as treating acidity in the stomach using antacids, treating stings from bees or wasps using baking soda or vinegar, and improving soil quality by neutralising acidic or basic nature with lime or gypsum. These reactions help maintain balance and reduce harmful effects.
2. Discuss the role of acids and bases in agriculture and industries.
   Answer: In agriculture, acids and bases help adjust soil pH for better crop growth. For example, lime (a base) is used to neutralise acidic soils. In industries, acids like sulphuric acid are used in fertilizer production and bases like sodium hydroxide are used in soap and paper manufacturing, showing their wide utility.
3. Write a short note on salts and their classification with examples.
   Answer: Salts are formed by the reaction of an acid and a base. They are classified as neutral (NaCl), acidic (NH₄Cl), or basic (CH₃COONa) depending on the strength of the acid and base that react. Salts are widely used in food preservation, manufacturing, and even in medical treatments.

Additional Questions And Answers – 02

Fill in the Blanks

1. Materials that taste sour due to the presence of chemicals called acids include lemon, vinegar, and ________.

Answer: Tamarind

2. Acids prepared from minerals present in the earth are called ________ acids.

Answer: Mineral

3. Citric acid is found in fruits like orange, lemon, and ________.

Answer: Amla

4. Bases that are soluble in water are called ________.

Answer: Alkalies

5. An indicator that turns red in an acidic medium and blue in a basic medium is ________.

Answer: Litmus

6. A reaction between an acid and a base that forms salt and water is called a ________ reaction.

Answer: Neutralisation

7. Salts formed by the reaction of a strong acid and a strong base are called ________ salts.

Answer: Neutral

8. The chemical formula of acetic acid, found in vinegar, is ________.

Answer: CH3COOH

9. Sulphuric acid is often referred to as the ________ of chemicals.

Answer: King

10. The indicator phenolphthalein is colourless in an acidic medium and turns ________ in a basic medium.

Answer: Pink

Multiple Choice Questions (MCQs)

1. Which of the following is a mineral acid?
2. a) Citric acid
3. b) Acetic acid
4. c) Hydrochloric acid
5. d) Formic acid

Answer: c) Hydrochloric acid

2. What type of acid is found in the sting of bees and red ants?
3. a) Tartaric acid
4. b) Formic acid
5. c) Lactic acid
6. d) Oxalic acid

Answer: b) Formic acid

3. Which indicator turns dark pink in an acidic medium and green in a basic medium?
4. a) Litmus
5. b) Turmeric
6. c) China rose
7. d) Phenolphthalein

Answer: c) China rose

4. What is the product of a neutralisation reaction between sodium hydroxide and hydrochloric acid?
5. a) Sodium sulphate and water
6. b) Sodium chloride and water
7. c) Sodium nitrate and water
8. d) Sodium carbonate and water

Answer: b) Sodium chloride and water

5. Which of the following is a strong base?
6. a) Ammonium hydroxide
7. b) Aluminium hydroxide
8. c) Sodium hydroxide
9. d) Copper hydroxide

Answer: c) Sodium hydroxide

6. What happens to the color of turmeric when it is added to a basic medium?
7. a) Turns yellow
8. b) Turns red
9. c) Turns green
10. d) Remains yellow

Answer: b) Turns red

7. Which salt is formed from the reaction of calcium hydroxide and sulphuric acid?
8. a) Calcium chloride
9. b) Calcium nitrate
10. c) Calcium sulphate
11. d) Calcium carbonate

Answer: c) Calcium sulphate

8. What type of salt is aluminium chloride (AlCl3)?
9. a) Neutral salt
10. b) Acidic salt
11. c) Basic salt
12. d) Organic salt

Answer: b) Acidic salt

9. Why does acid rain affect marble buildings like the Taj Mahal?
10. a) It makes the marble shiny
11. b) It reacts with calcium carbonate in marble
12. c) It cleans the marble surface
13. d) It strengthens the marble

Answer: b) It reacts with calcium carbonate in marble

10. Which of the following is a property of salts in water?
11. a) They do not conduct electricity
12. b) They conduct electricity
13. c) They are insoluble
14. d) They turn acidic

Answer: b) They conduct electricity

True/False Questions

1. All acids are sour in taste.

Answer: True

2. Bases are sweet in taste and have a soapy feel.

Answer: False (Bases are bitter in taste)

3. Sodium hydroxide is an alkali because it is insoluble in water.

Answer: False (Sodium hydroxide is an alkali because it is soluble in water)

4. Litmus turns blue in an acidic medium.

Answer: False (Litmus turns red in an acidic medium)

5. Concentrated acids should be diluted by adding water directly to the acid.

Answer: False (Acid should be added to water slowly to dilute it)

6. A neutralisation reaction produces only salt as a product.

Answer: False (It produces salt and water)

7. Salts in their solid state can conduct electricity.

Answer: False (Salts conduct electricity in molten state or in solution)

8. Sodium acetate is a basic salt.

Answer: True

9. Mineral acids are naturally occurring acids found in plants and animals.

Answer: False (Mineral acids are prepared from minerals in the earth)

10. Acid rain is caused by gases like carbon dioxide and sulphur dioxide mixing with rainwater.

Answer: True

Short Questions and Answers

1. What are organic acids?

Answer: Organic acids are naturally occurring acids found in plants and animals, such as citric acid in lemons.

2. Name two mineral acids.

Answer: Hydrochloric acid (HCl) and sulphuric acid (H2SO4).

3. What is an alkali?

Answer: An alkali is a base that is soluble in water, such as sodium hydroxide (NaOH).

4. How does litmus paper help identify acids and bases?

Answer: Litmus paper turns red in an acidic medium and blue in a basic medium, helping identify acids and bases.

5. What is a neutralisation reaction?

Answer: A neutralisation reaction is a chemical reaction between an acid and a base that forms salt and water.

6. Why is sulphuric acid called the ‘King of Chemicals’?

Answer: Sulphuric acid is called the ‘King of Chemicals’ because it is used in the manufacture of a wide variety of materials.

7. What is the effect of acid rain on marble buildings?

Answer: Acid rain reacts with calcium carbonate in marble, causing damage and yellowing, as seen in the Taj Mahal.

8. Name one example of a basic salt.

Answer: Sodium acetate (CH3COONa).

9. What happens to phenolphthalein in a basic medium?

Answer: Phenolphthalein turns pink in a basic medium.

10. Why do solutions of salts in water conduct electricity?

Answer: Solutions of salts in water conduct electricity because the salt dissociates into ions, which can carry electric current.

Competency-Based Questions (Reasoning and Assertion)

1. Assertion (A): Lemon juice turns blue litmus paper red. Reason (R): Lemon juice contains citric acid, which is an acid.
2. a) Both A and R are true, and R is the correct explanation of A.
3. b) Both A and R are true, but R is not the correct explanation of A.
4. c) A is true, but R is false.
5. d) A is false, but R is true.

Answer: a) Both A and R are true, and R is the correct explanation of A.

2. Assertion (A): Sodium chloride is a neutral salt. Reason (R): Sodium chloride is formed by the reaction of a strong acid and a strong base.
3. a) Both A and R are true, and R is the correct explanation of A.
4. b) Both A and R are true, but R is not the correct explanation of A.
5. c) A is true, but R is false.
6. d) A is false, but R is true.

Answer: a) Both A and R are true, and R is the correct explanation of A.

3. Assertion (A): Aluminium chloride solution turns blue litmus paper red. Reason (R): Aluminium chloride is a basic salt.
4. a) Both A and R are true, and R is the correct explanation of A.
5. b) Both A and R are true, but R is not the correct explanation of A.
6. c) A is true, but R is false.
7. d) A is false, but R is true.

Answer: c) A is true, but R is false. (Aluminium chloride is an acidic salt.)

4. Assertion (A): Bases have a bitter taste and a soapy feel. Reason (R): Bases react with acids to form salt and water.
5. a) Both A and R are true, and R is the correct explanation of A.
6. b) Both A and R are true, but R is not the correct explanation of A.
7. c) A is true, but R is false.
8. d) A is false, but R is true.

Answer: b) Both A and R are true, but R is not the correct explanation of A.

5. Assertion (A): Turmeric turns red in an acidic medium. Reason (R): Turmeric is a natural indicator that changes color based on the medium.
6. a) Both A and R are true, and R is the correct explanation of A.
7. b) Both A and R are true, but R is not the correct explanation of A.
8. c) A is true, but R is false.
9. d) A is false, but R is true.

Answer: d) A is false, but R is true. (Turmeric remains yellow in an acidic medium and turns red in a basic medium.)

High Order Thinking Skill (HOTS) Questions and Answers

1. Why should concentrated acid be diluted by adding acid to water and not the other way around?

Answer: Adding acid to water slowly prevents a sudden exothermic reaction, which could cause splashing or boiling. Adding water to acid can lead to rapid heat generation, increasing the risk of accidents.

2. How does a neutralisation reaction help in treating excess stomach acid?

Answer: Excess stomach acid (hydrochloric acid) causes discomfort. Antacids, containing a mild base, neutralise the excess acid, forming salt and water, thus relieving uneasiness, nausea, and pain in the stomach.

3. Why does a solution of sodium acetate change the color of red litmus to blue?

Answer: Sodium acetate is a basic salt formed from a weak acid (acetic acid) and a strong base (sodium hydroxide). Its solution in water is basic, so it changes red litmus to blue, indicating a basic medium.

4. How does acid rain cause the yellowing of the Taj Mahal?

Answer: Acid rain, formed by gases like sulphur dioxide mixing with rainwater, reacts with the calcium carbonate in the Taj Mahal’s marble. This reaction forms soluble salts that wash away, causing yellowing and damage to the marble.

5. Why do solutions of salts conduct electricity while solid salts do not?

Answer: In solution, salts dissociate into ions that can move and carry electric current. In the solid state, ions in salts are fixed in a lattice and cannot move, so solid salts do not conduct electricity.

Long Questions and Answers (50-60 words)

1. Explain the difference between mineral acids and organic acids with examples.

Answer: Mineral acids are prepared from minerals in the earth, like hydrochloric acid (HCl) and sulphuric acid (H2SO4). Organic acids occur naturally in plants and animals, such as citric acid in lemons and acetic acid in vinegar. Mineral acids are typically stronger, while organic acids are weaker and found in food sources.

2. Describe how litmus can be used to identify acids and bases with examples.

Answer: Litmus is an indicator that turns red in an acidic medium and blue in a basic medium. For example, dipping blue litmus in lemon juice (acidic) turns it red, while dipping red litmus in soap solution (basic) turns it blue, helping identify the nature of the substance.

3. What is a neutralisation reaction? Give an example with a chemical equation.

Answer: A neutralisation reaction occurs when an acid reacts with a base to form salt and water. For example, sodium hydroxide reacts with hydrochloric acid to form sodium chloride and water: NaOH + HCl → NaCl + H2O. This reaction cancels the acidic and basic properties of the reactants.

4. How are salts classified based on their reaction with litmus? Explain with examples.

Answer: Salts are classified as neutral, acidic, or basic. Neutral salts, like sodium chloride, do not change litmus color. Acidic salts, like aluminium chloride, turn blue litmus red. Basic salts, like sodium acetate, turn red litmus blue. This classification depends on whether the salt is formed from strong or weak acids and bases.

5. What are the effects of acid rain on marble buildings, and how is it formed?

Answer: Acid rain forms when gases like sulphur dioxide from factories mix with rainwater, creating acids. It reacts with calcium carbonate in marble buildings like the Taj Mahal, causing yellowing and damage. This chemical reaction dissolves the marble, leading to erosion and discoloration over time.