Acids and Bases Exercise Solutions | KalviGavani

Q: What is the pH scale?

The pH scale measures how acidic or basic a substance is, ranging from 0 to 14. pH 7 is neutral, below 7 is acidic, and above 7 is basic.

Exercise Solutions Available

Step-by-step answers for all textbook questions in this chapter

View Solutions →

I. Choose the Best Answer (MCQs)

Q1. Which of the following is an acid?

(a) NaOH (b) HCl (c) NaCl (d) CaO

Answer(b) HCl HCl (hydrochloric acid) is an acid — it releases H⁺ ions in water. NaOH is a base, NaCl is a salt, and CaO is a basic oxide.

Q2. Blue litmus turns red in

(a) Acidic solution (b) Basic solution (c) Neutral solution (d) Salt solution

Answer(a) Acidic solution Blue litmus paper turns red when dipped in an acidic solution. This is a standard test for acids.

Q3. The pH of a neutral solution is

(a) 0 (b) 7 (c) 14 (d) 1

Answer(b) 7 A neutral solution (like pure water) has a pH of 7. pH below 7 indicates acidity; pH above 7 indicates basicity.

Q4. Which of the following is a natural indicator?

(a) Phenolphthalein (b) Methyl orange (c) Turmeric (d) Litmus

Answer(c) Turmeric Turmeric is a natural indicator. It turns red in basic solutions and remains yellow in acidic/neutral solutions. Note: Litmus is also derived from lichens (natural), but among the given options, turmeric is the most commonly cited natural indicator. Actually, litmus is also natural — but in the Samacheer textbook, turmeric is specifically listed as a natural indicator.

Q5. When an acid reacts with a metal, it produces

(a) Salt and water (b) Salt and hydrogen gas (c) Salt and carbon dioxide (d) Salt and oxygen

Answer(b) Salt and hydrogen gas Acid + Metal → Salt + Hydrogen gas. Example: Zn + H₂SO₄ → ZnSO₄ + H₂↑

The hydrogen gas burns with a pop sound when a burning splinter is brought near it.

Q6. The reaction between an acid and a base is called

(a) Oxidation (b) Reduction (c) Neutralisation (d) Decomposition

Answer(c) Neutralisation Neutralisation is the reaction between an acid and a base that produces salt and water. Example: HCl + NaOH → NaCl + H₂O.

Q7. Phenolphthalein turns ______ in a basic solution.

(a) Colourless (b) Pink (c) Red (d) Yellow

Answer(b) Pink Phenolphthalein is colourless in acidic and neutral solutions but turns pink in basic solutions.

Q8. Which of the following is the strongest acid?

(a) pH 2 (b) pH 5 (c) pH 7 (d) pH 10

Answer(a) pH 2 Lower the pH, stronger the acid. pH 2 indicates a very acidic solution, making it the strongest acid among the options.

II. Fill in the Blanks

1. Acids taste sour. 2. Bases taste bitter and feel soapy/slippery to touch. 3. The pH scale ranges from 0 to 14. 4. A solution with pH less than 7 is acidic. 5. A solution with pH greater than 7 is basic (alkaline). 6. The neutralisation reaction produces salt and water. 7. Blue litmus turns red in acidic solutions. 8. Red litmus turns blue in basic solutions. 9. Phenolphthalein is colourless in acidic solution and pink in basic solution. 10. Lemon juice is an example of acidic substance (contains citric acid).

III. True or False

Q1. All acids are corrosive.

AnswerFalse — Not all acids are corrosive. Dilute acids like citric acid (in lemon juice) and acetic acid (in vinegar) are mild and safe. Only strong concentrated acids like sulphuric acid and nitric acid are corrosive.

Q2. Bases turn red litmus blue.

AnswerTrue — This is a fundamental property of bases. Red litmus paper turns blue when dipped in a basic solution.

Q3. Neutralisation produces salt and water only.

AnswerTrue — The general reaction is: Acid + Base → Salt + Water. This is the definition of neutralisation.

Q4. The pH of gastric juice is around 7.

AnswerFalse — Gastric juice contains hydrochloric acid (HCl) and has a pH of about 1.5, which is highly acidic. This acidic environment helps in digestion and kills bacteria.

Q5. Metal oxides are generally basic in nature.

AnswerTrue — Metal oxides (like Na₂O, CaO, MgO) dissolve in water to form bases. Example: CaO + H₂O → Ca(OH)₂ (calcium hydroxide).

Q6. Non-metal oxides are generally acidic in nature.

AnswerTrue — Non-metal oxides (like CO₂, SO₂, P₂O₅) dissolve in water to form acids. Example: CO₂ + H₂O → H₂CO₃ (carbonic acid).

IV. Match the Following

Column I	Column II	Answer
Hydrochloric acid	HCl	Hydrochloric acid — HCl
Sulphuric acid	H₂SO₄	Sulphuric acid — H₂SO₄
Nitric acid	HNO₃	Nitric acid — HNO₃
Sodium hydroxide	NaOH	Sodium hydroxide — NaOH
Calcium hydroxide	Ca(OH)₂	Calcium hydroxide — Ca(OH)₂
Acetic acid	CH₃COOH	Acetic acid — CH₃COOH

V. Short Answer Questions

Q1. What are acids? Give four examples. Answer: Acids are substances that release hydrogen ions (H⁺) when dissolved in water. They taste sour, turn blue litmus red, and have a pH less than 7.

Examples: 1. Hydrochloric acid (HCl) — found in gastric juice 2. Sulphuric acid (H₂SO₄) — used in batteries 3. Acetic acid (CH₃COOH) — found in vinegar 4. Citric acid — found in citrus fruits (lemon, orange)

Q2. What are bases? Give four examples. Answer: Bases are substances that release hydroxide ions (OH⁻) when dissolved in water. They taste bitter, feel soapy to touch, turn red litmus blue, and have a pH greater than 7.

Examples: 1. Sodium hydroxide (NaOH) — used in soap making 2. Potassium hydroxide (KOH) — used in alkaline batteries 3. Calcium hydroxide (Ca(OH)₂) — used in whitewashing 4. Ammonium hydroxide (NH₄OH) — used in cleaning

Q3. What are indicators? Name two natural indicators. Answer: Indicators are substances that change colour in acidic or basic solutions, helping us identify whether a substance is an acid or a base.

Two natural indicators: 1. Litmus: Extracted from lichens. Turns red in acid and blue in base. 2. Turmeric: Remains yellow in acidic/neutral solutions but turns reddish-brown in basic solutions.

Q4. What is neutralisation? Give one example. Answer: Neutralisation is the reaction between an acid and a base to form salt and water.

Example: HCl + NaOH → NaCl + H₂O

In this reaction, the acidic properties of HCl and the basic properties of NaOH are neutralised, producing sodium chloride (common salt) and water.

Q5. What is the pH scale? What does a pH of 7 indicate? Answer: The pH scale is a measure of the hydrogen ion concentration in a solution. It ranges from 0 to 14.

pH less than 7 → Acidic solution (lower pH = stronger acid)
pH = 7 → Neutral solution (pure water)
pH greater than 7 → Basic solution (higher pH = stronger base)

A pH of 7 indicates a neutral solution — neither acidic nor basic. Pure water has a pH of 7.

Q6. What happens when an acid reacts with a metal? Give an example. Answer: When an acid reacts with a metal, it produces salt and hydrogen gas. The hydrogen gas burns with a pop sound when a burning splinter is brought near it.

Example: Zn + H₂SO₄ → ZnSO₄ + H₂↑

Zinc reacts with dilute sulphuric acid to form zinc sulphate (salt) and hydrogen gas.

Q7. What happens when a base reacts with a metal? Give an example. Answer: When a base reacts with certain metals, it produces salt and hydrogen gas. However, only some metals (like aluminium and zinc) react with strong bases.

Example: 2Al + 2NaOH + 2H₂O → 2NaAlO₂ + 3H₂↑

Aluminium reacts with sodium hydroxide and water to form sodium aluminate and hydrogen gas.

VI. Detailed Answer Questions

Q1. Explain the different types of indicators used to identify acids and bases. Answer: Indicators are substances that change colour in acidic or basic solutions. They are of two types:

Natural Indicators: 1. Litmus: Extracted from lichens. Blue litmus turns red in acid; red litmus turns blue in base. It is purple in neutral solutions. 2. Turmeric: Remains yellow in acid and neutral solutions; turns reddish-brown in basic solutions. 3. Red cabbage juice: Turns red in acid, green in base, and purple in neutral solutions. 4. China rose petals: Turn dark pink/magenta in acid and green in base.

Synthetic Indicators: 1. Phenolphthalein: Colourless in acid and neutral solutions; turns pink in basic solutions. 2. Methyl orange: Turns red in acidic solutions and yellow in basic solutions. 3. Bromothymol blue: Turns yellow in acid and blue in base; green in neutral.

Indicator	Acid	Base
Litmus (blue)	Red	Blue
Litmus (red)	Red	Blue
Phenolphthalein	Colourless	Pink
Methyl orange	Red	Yellow
Turmeric	Yellow	Reddish-brown

Q2. Describe the reactions of acids with different substances with equations. Answer:

(a) Acid + Metal → Salt + Hydrogen gas Zn + H₂SO₄ → ZnSO₄ + H₂↑ The hydrogen gas burns with a pop sound.

(b) Acid + Metal carbonate → Salt + Water + Carbon dioxide CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑ The carbon dioxide gas turns limewater milky.

(c) Acid + Metal bicarbonate → Salt + Water + Carbon dioxide NaHCO₃ + HCl → NaCl + H₂O + CO₂↑

(d) Acid + Base → Salt + Water (Neutralisation) HCl + NaOH → NaCl + H₂O

(e) Acid + Metal oxide → Salt + Water CuO + H₂SO₄ → CuSO₄ + H₂O

Metal oxides are basic in nature, so they react with acids to form salt and water.

Q3. Describe the reactions of bases with different substances with equations. Answer:

(a) Base + Metal → Salt + Hydrogen gas (only some metals like Al and Zn) 2Al + 2NaOH + 2H₂O → 2NaAlO₂ + 3H₂↑

(b) Base + Non-metal oxide → Salt + Water Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

Non-metal oxides are acidic in nature, so they react with bases to form salt and water.

(c) Base + Acid → Salt + Water (Neutralisation) NaOH + HCl → NaCl + H₂O

(d) Base + Ammonium salt → Salt + Water + Ammonia gas NaOH + NH₄Cl → NaCl + H₂O + NH₃↑ Ammonia gas has a pungent smell and turns moist red litmus blue.

Q4. Explain the importance of neutralisation in daily life with examples. Answer: Neutralisation has several important applications in daily life:

1. Treating ant bites: Ant stings contain formic acid (methanoic acid). Rubbing baking soda (sodium bicarbonate, a mild base) on the sting neutralises the acid and reduces pain.

2. Treating acidity: Excess hydrochloric acid in the stomach causes acidity. Antacids (like milk of magnesia, Mg(OH)₂) are basic substances that neutralise the excess acid and provide relief.

3. Soil treatment: If soil is too acidic (due to excessive use of fertilisers), farmers add quicklime (CaO) or slaked lime (Ca(OH)₂) to neutralise it and improve crop growth.

4. Factory waste treatment: Factory waste often contains acids. Before releasing it into water bodies, it is treated with bases like calcium hydroxide to neutralise the acids and prevent water pollution.

5. Tooth decay: Bacteria in the mouth produce acids that cause tooth decay. Toothpastes are slightly basic and help neutralise these acids, protecting tooth enamel.

Q5. What is the importance of pH in everyday life? Answer:

1. Digestive system: The stomach contains hydrochloric acid (pH ~1.5) that helps in digestion and kills bacteria. When stomach acid increases (acidity), antacids are used to neutralise it.

2. Tooth decay: Mouth bacteria produce acids that lower the pH below 5.5, causing tooth decay. Brushing with basic toothpaste neutralises the acid and protects teeth.

3. Soil pH: Plants grow best at specific pH ranges. If soil is too acidic, farmers add lime (CaO) to raise pH. If too basic, they add organic matter.

4. Rainwater: Normal rainwater has a pH of about 5.6 (slightly acidic due to dissolved CO₂). Acid rain (pH below 5.6) damages buildings, crops, and aquatic life.

5. Blood pH: Human blood has a pH of about 7.35-7.45. Any significant change can be life-threatening.

6. Self-defence: Bee stings contain methanoic acid (formic acid). Baking soda solution neutralises it. Wasp stings are alkaline and are treated with vinegar (acetic acid).

Common Mistakes

Confusing acid and base litmus reactions: Acids turn blue litmus red; bases turn red litmus blue. Remember: "Blue to Red = Acid (BRA)."
Saying all bases dissolve in water: Not all bases are soluble in water. Bases that dissolve in water are called alkalis. For example, NaOH is an alkali, but Ca(OH)₂ is only slightly soluble.
Confusing metal and non-metal oxides: Metal oxides are basic (e.g., Na₂O, CaO). Non-metal oxides are acidic (e.g., CO₂, SO₂).
Forgetting to balance equations: Always check that atoms and charges are balanced in chemical equations.
Thinking pH 0 means no acidity: pH 0 is the most acidic (highest H⁺ concentration), not neutral. pH 7 is neutral.
Confusing indicators: Phenolphthalein is colourless in acid and pink in base. Methyl orange is red in acid and yellow in base. These are opposites and easy to mix up.

Exam Checklist

[ ] Can you define acids and bases with examples?
[ ] Can you list natural and synthetic indicators and their colour changes?
[ ] Can you write balanced equations for acid-metal, acid-carbonate, and neutralisation reactions?
[ ] Can you explain the pH scale and interpret pH values?
[ ] Can you describe at least five daily-life applications of neutralisation?
[ ] Can you distinguish between metal oxides (basic) and non-metal oxides (acidic)?
[ ] Can you identify acidic and basic solutions using different indicators?
[ ] Can you explain why antacids, soil treatment, and acid rain are related to neutralisation?
[ ] Can you write the products when acids react with metals, carbonates, and metal oxides?
[ ] Can you explain the importance of pH in digestion, dental health, and agriculture?

Frequently Asked Questions

Acids taste sour, turn blue litmus red, release H⁺ ions in water, and have pH less than 7. Bases taste bitter, turn red litmus blue, release OH⁻ ions in water, and have pH greater than 7.