CBSE Class 10 Science Acids bases and salts Sure Shot Questions Set 02

Question. CuO + (X) → CuSO₄ + H₂O. Here (X) is -
(A) CuSO₄
(B) HCl
(C) H₂SO₄
(D) HNO₃
Answer : C

ASSIGNMENT QUESTIONS SET - 2
CHAPTER - 2
ACIDS, BASES AND SALTS

1. The colour of neutral litmus solution is [1 Mark]
(a) red
(b) blue
(c) purple
(d) yellow

Answer: (c) purple

1. Litmus solution is a natural indicator extracted from lichens.
2. In a neutral medium, its natural colour is purple.
3. Hence, the correct option is (c).

Teacher's Note:
a) Litmus turns red in acidic solutions and blue in basic solutions.
b) The neutral purple colour is observed before any acid or base is added.

2. Which of the following indicators is an olfactory indicator? [1 Mark]
(a) litmus
(b) vanilla
(c) turmeric
(d) phenolphthalein

Answer: (b) vanilla

1. Olfactory indicators are substances whose odour changes in acidic or basic media.
2. Vanilla extract retains its characteristic smell in acidic medium but loses its smell in basic medium.
3. Hence, the correct option is (b).

Teacher's Note:
a) Onion and clove oil are other common examples of olfactory indicators.
b) Litmus and turmeric are natural acid-base indicators, while phenolphthalein is a synthetic indicator.

3. Which one is suitable method to find the accurate pH value? [1 Mark]
(a) pH meter
(b) pH paper
(c) Universal indicator
(d) Litmus solution

Answer: (a) pH meter

1. A pH meter is an electronic device that measures the electrical potential difference between electrodes immersed in a solution.
2. It gives precise and accurate numerical pH values up to two decimal places.
3. Hence, the correct option is (a).

Teacher's Note:
a) pH paper and universal indicator give approximate colour-matched pH values.
b) Litmus solution only indicates whether a solution is acidic or basic, not the exact pH.

4. Which one of the following statements is correct about universal indicator? [1 Mark]
(a) It is a mixture of HCl and NaOH
(b) It is a mixture of many indicators
(c) It is a solution of phenolphthalein in alcohol
(d) It is a solution of phenolphthalein in water.

Answer: (b) It is a mixture of many indicators

1. A universal indicator consists of a blend of several different indicators.
2. It exhibits a wide range of distinct colour changes over a wide pH range from 0 to 14.
3. Hence, the correct option is (b).

Teacher's Note:
a) It shows different colours at different concentrations of hydrogen ions in solution.
b) Do not confuse it with single indicators like phenolphthalein or litmus.

5. Which of the following properties are shown by dilute HCl? [1 Mark]
(1) It turns blue litmus red
(2) It turns red litmus blue
(3) It reacts with zinc and a gas is evolved
(4) It reacts with solid sodium carbonate to give brisk effervescence
(a) 1 and 2
(b) 1 and 3
(c) 1, 3 and 4
(d) 2, 3 and 4

Answer: (c) 1, 3 and 4

1. Being an acid, dilute HCl turns blue litmus red, satisfying statement (1).
2. It reacts with reactive metals like zinc to evolve hydrogen gas, satisfying statement (3).
3. It reacts with metal carbonates like sodium carbonate to release carbon dioxide gas with brisk effervescence, satisfying statement (4).
4. Hence, the correct option is (c).

Teacher's Note:
a) Statement (2) is incorrect because bases turn red litmus blue, not acids.
b) Always test each property independently when evaluating multiple statements.

6. A teacher gave two test tubes - one containing water and the other containing sodium hydroxide solution to two students. Then he asked them to identify the test tube containing sodium hydroxide solution. Which one of the following can be used for correctly identifying the test tube containing the solution of sodium hydroxide? [1 Mark]
(a) Blue litmus
(b) Red litmus
(c) Sodium carbonate solution
(d) Dilute HCl

Answer: (b) Red litmus

1. Sodium hydroxide is a strong base, whereas water is neutral.
2. Bases turn red litmus paper blue, while neutral water has no effect on red litmus.
3. Hence, the correct option is (b).

Teacher's Note:
a) Blue litmus will remain blue in both water and sodium hydroxide solution.
b) Red litmus is the diagnostic indicator for identifying alkaline solutions.

7. Metallic oxides are ________ in nature, but non-metallic oxides are __________ in nature.
The information in which alternative completes the given statement? [1 Mark]
(a) neutral, acidic
(b) acidic, basic
(c) basic, neutral
(d) basic, acidic

Answer: (d) basic, acidic

1. Metallic oxides dissolve in water to form metal hydroxides, which release hydroxide ions and are basic in nature.
2. Non-metallic oxides dissolve in water to form acids, which release hydrogen ions and are acidic in nature.
3. Hence, the correct option is (d).

Teacher's Note:
a) Remember that metal oxides are basic and non-metal oxides are acidic.
b) Some metal oxides like aluminium oxide and zinc oxide are amphoteric.

8. When a drop of unknown solution X is placed on a strip of pH paper, a deep red colour is produced. This sample is which one of these? [1 Mark]
(a) NaOH
(b) HCl
(c) Water
(d) CH3COOH

Answer: (b) HCl

1. A deep red colour on pH paper indicates a strongly acidic solution with a pH close to 1 or 2.
2. Hydrochloric acid (HCl) is a strong acid that produces a deep red colour.
3. Acetic acid (CH3COOH) is a weak acid and produces a orange or light red colour.
4. Hence, the correct option is (b).

Teacher's Note:
a) Strong acids yield lower pH values and deeper red colours on the universal pH scale.
b) Weak acids like acetic acid show weaker acidic colours like orange-red.

9. A student tests a sample drinking water and reports its pH value as 6 at room temperature. Which one of the following might have been added in water? [1 Mark]
(a) Calcium chloride
(b) Sodium chloride
(c) Sodium bicarbonate
(d) Bleaching powder

Answer: (a) Calcium chloride

1. Pure water has a pH of 7 at room temperature.
2. A pH value of 6 indicates a slightly acidic solution, which can result from dissolved acidic salts or carbon dioxide.
3. Salts of strong acids and weak bases, such as calcium chloride or certain mineral salts, can impart a slightly acidic character to water.
4. Hence, the correct option is (a).

Teacher's Note:
a) Sodium chloride forms a neutral solution with a pH of 7.
b) Sodium bicarbonate forms a basic solution with a pH greater than 7.

10. Solid sodium bicarbonate was placed on a strip of pH paper. The color of the strip [1 Mark]
(a) turned red
(b) did not change
(c) turned green and slightly yellow
(d) turned pink

Answer: (b) did not change

1. Indicator colour changes on pH paper require moisture to allow hydrogen ions to dissociate and interact with the indicator dye.
2. Solid sodium bicarbonate is dry and lacks the aqueous medium required for ionisation.
3. Therefore, placing dry solid sodium bicarbonate on dry pH paper produces no colour change.
4. Hence, the correct option is (b).

Teacher's Note:
a) This is a classic conceptual question testing the necessity of water for acid-base reactions.
b) If a solution of sodium bicarbonate were used instead, it would turn the pH paper blue.

11. Four drops of red litmus solution were added to each of the following samples. Which one turns red litmus blue? [1 Mark]
(a) Alcohol
(b) Distilled water
(c) Sodium hydroxide sol
(d) HCl

Answer: (c) Sodium hydroxide sol

1. Red litmus turns blue in basic or alkaline mediums.
2. Sodium hydroxide solution is a strong base.
3. Hence, the correct option is (c).

Teacher's Note:
a) Alcohol and distilled water are neutral and will not change the colour of red litmus.
b) HCl is an acid and will keep red litmus red.

12. The pH of which of the following samples can not be found directly using pH paper? [1 Mark]
(a) Lemon juice
(b) Dilute HCl
(c) Solid sodium bicarbonate
(d) Solution of a detergent.

Answer: (c) Solid sodium bicarbonate

1. pH paper requires an aqueous medium (presence of hydrogen ions in solution) to show color change.
2. Solid sodium bicarbonate does not dissociate into ions in solid state, so its pH cannot be tested directly using pH paper without dissolving it in water.
3. Hence, the correct option is (c).

Teacher's Note:
a) pH paper works only when moisture or an aqueous solution is present.
b) Solid salts do not exhibit pH until dissolved in water.

13. Which of the following natural sources contains oxalic acid? [1 Mark]
(a) lemon
(b) orange
(c) tomato
(d) tamarind

Answer: (c) tomato

1. Lemon and orange contain citric acid.
2. Tamarind contains tartaric acid.
3. Tomato naturally contains oxalic acid.
4. Hence, the correct option is (c).

Teacher's Note:
a) Memorize common natural sources and their corresponding organic acids.
b) Tomatoes and spinach are well-known sources of oxalic acid.

14. The acid found in an ant sting is [1 Mark]
(a) acetic acid
(b) citric acid
(c) tartaric acid
(d) methanoic acid

Answer: (d) methanoic acid

1. Ant stings inject an acidic liquid containing methanoic acid (also known as formic acid) into the skin.
2. This causes irritation and pain.
3. Hence, the correct option is (d).

Teacher's Note:
a) Methanoic acid has the chemical formula HCOOH.
b) Bee stings also inject methanoic acid.

15. To relieve pain caused due to acidity, we can take [1 Mark]
(a) sour milk
(b) lemon juice
(c) orange juice
(d) milk of magnesia

Answer: (d) milk of magnesia

1. Acidity in the stomach is caused by excess hydrochloric acid.
2. Milk of magnesia contains magnesium hydroxide, which is a mild base.
3. It neutralizes the excess acid and provides relief.
4. Hence, the correct option is (d).

Teacher's Note:
a) Antacids are basic substances used to neutralize stomach acid.
b) Magnesium hydroxide is sparingly soluble and safe for consumption.

16. What are the products obtained when potassium sulphate reacts with barium iodide in an aqueous medium? [1 Mark]
(a) KI and BaSO₄
(b) KI, Ba and SO₂
(c) K, I₂ and BaSO₄
(d) K, Ba, I₂ and SO₂

Answer: (a) KI and BaSO₄

1. This is a double displacement (precipitation) reaction.
2. Equation: K₂SO₄ + BaI₂ → BaSO₄ (white precipitate) + 2KI.
3. Hence, the correct option is (a).

Teacher's Note:
a) Barium sulphate forms an insoluble white precipitate.
b) Potassium iodide remains dissolved in the aqueous medium.

17. Which of the following salts is basic in nature? [1 Mark]
(a) NH₄NO₃
(b) Na₂CO₃
(c) Na₂SO₄
(d) NaCl

Answer: (b) Na₂CO₃

1. Sodium carbonate (Na₂CO₃) is a salt of a strong base (NaOH) and a weak acid (H₂CO₃).
2. Therefore, its aqueous solution is basic in nature.
3. Hence, the correct option is (b).

Teacher's Note:
a) Salts of strong acid and weak base are acidic.
b) Salts of strong base and weak acid are basic.

18. Which of the following salts has the minimum pH value? [1 Mark]
(a) (NH₄)₂SO₄
(b) NaHCO₃
(c) K₂SO₄
(d) NaCl

Answer: (a) (NH₄)₂SO₄

1. Ammonium sulphate is a salt of a strong acid (H₂SO₄) and a weak base (NH₄OH).
2. Its solution is acidic, having a pH less than 7.
3. Other options are neutral or basic salts with pH values equal to or greater than 7.
4. Hence, the correct option is (a).

Teacher's Note:
a) Lower pH indicates higher acidity.
b) Ammonium sulphate yields an acidic solution due to cationic hydrolysis.

19. You are given four unknown solutions I, II, III, and IV. The pH values of these solutions are found to be 3, 7, 8, and 10 respectively. Among the given solutions, which solution has the highest hydrogen ion concentration? [1 Mark]
(a) I
(b) II
(c) III
(d) IV

Answer: (a) I

1. Hydrogen ion concentration is inversely proportional to the pH value.
2. Solution I has the lowest pH value of 3 among all given options.
3. Therefore, it has the highest hydrogen ion concentration.
4. Hence, the correct option is (a).

Teacher's Note:
a) pH is the negative logarithm of hydrogen ion concentration.
b) Lower pH means stronger acidity and more H⁺ ions.

20. Which one of the following is required to identify the gas evolved when dilute hydrochloric acid reacts with zinc metal? [1 Mark]
(a) blue litmus paper
(b) red litmus paper
(c) a burning slinter
(d) lime water

Answer: (c) a burning slinter

1. Reaction of zinc with dilute HCl produces hydrogen gas.
2. Hydrogen gas burns with a pop sound when a burning splinter is brought near it.
3. Hence, the correct option is (c).

Teacher's Note:
a) This is the standard confirmatory test for hydrogen gas.
b) Splinter is sometimes misspelled as slinter in question papers.

21. Zinc reacts with an acid as well as with a base to liberate hydrogen. On the basis of this what should be the nature of the zinc metal? [1 Mark]
(a) basic
(b) acidic
(c) amphoteric
(d) neutral

Answer: (c) amphoteric

1. Substances that react with both acids and bases to form salt and water are called amphoteric substances.
2. Since zinc reacts with both acids and bases to liberate hydrogen, it is amphoteric in nature.
3. Hence, the correct option is (c).

Teacher's Note:
a) Zinc reacts with NaOH to form sodium zincate and hydrogen gas.
b) Amphoteric oxides and metals show dual acidic and basic properties.

22. When you test the solutions of sodium bicarbonate, sodium hydroxide, hydrochloric acid and acetic acid with universal indicator, in which case would you get a red colour? [1 Mark]
(a) sodium bicarbonate
(b) hydrochloric acid
(c) sodium hydroxide
(d) acetic acid

Answer: (b) hydrochloric acid

1. Universal indicator gives a red color in strongly acidic solutions.
2. Hydrochloric acid is a strong acid, whereas acetic acid is a weak acid (giving orange or dark pink color).
3. Hence, the correct option is (b).

Teacher's Note:
a) Strong acids give red color with universal indicator.
b) NaOH gives blue or purple, while sodium bicarbonate gives light blue.

23. The pH of a sample of pure water is 7 at room temperature. What is its pH when a pinch of solid sodium bicarbonate is dissolved in it? [1 Mark]
(a) vary near to 7
(b) less than 7
(c) more than 7
(d) exactly 7

Answer: (c) more than 7

1. Sodium bicarbonate is a salt of a strong base (NaOH) and a weak acid (H₂CO₃).
2. Dissolving it in pure water forms a mildly alkaline solution.
3. The pH of an alkaline solution is greater than 7.
4. Hence, the correct option is (c).

Teacher's Note:
a) Hydrolysis of sodium bicarbonate produces bicarbonate ions which create a basic medium.
b) Note the typo "vary near" in option (a) of the question paper.

24. If an unknown solution turns blue litmus red, then the pH of the solution is more likely to be [1 Mark]
(a) 12
(b) 10
(c) 7
(d) 4

Answer: (d) 4

1. A solution that turns blue litmus red is acidic in nature.
2. Acidic solutions have a pH value less than 7.
3. Among the given options, only 4 is less than 7.
4. Hence, the correct option is (d).

Teacher's Note:
a) Acids turn blue litmus red; bases turn red litmus blue.
b) pH 7 is neutral, while 10 and 12 are basic.

25. What is the pH of a 0.00001 molar HCl solution? [1 Mark]
(a) 1
(b) 9
(c) 5
(d) 4

Answer: (c) 5

1. Concentration of HCl = \(0.00001\) M = \(10^{-5}\) M.
2. Since HCl is a strong acid, [H⁺] = \(10^{-5}\) mol/L.
3. pH = \(-\log[H^+]\) = \(-\log(10^{-5})\) = 5.
4. Hence, the correct option is (c).

Teacher's Note:
a) Count the number of decimal places to find the negative power of 10.
b) pH calculation for strong acids is direct from concentration.

26. There are alternate acid base theories that define an acid as any species that can
{hint: According to Bronsted-Lowry theory, an acid is any species that can donate a proton to another species.} [1 Mark]
(a) donate a proton
(2) donate an electron
(c) accept a proton
(d) accept an electron

Answer: (a) donate a proton

1. According to the Bronsted-Lowry theory, an acid is defined as a proton (H⁺) donor.
2. A base is defined as a proton acceptor.
3. Hence, the correct option is (a).

Teacher's Note:
a) Note the typographical numbering error in option (2) instead of (b) in the question paper.
b) Bronsted-Lowry theory is broader than the Arrhenius concept of acids.

27. What happens when a solution of an acid is mixed with a solution of a base in a test tube? [1 Mark]
(i) The temperature of the solution increases
(ii) The temperature of the solution decreases
(iii) The temperature of the solution remains the same
(iv) Salt formation takes place
(a) (i) only
(b) (i) and (iii)
(c) (ii) and (iii)
(d) (i) and (iv)

Answer: (d) (i) and (iv)

1. Neutralization reaction occurs between an acid and a base.
2. This reaction is exothermic, which increases the temperature of the solution.
3. The products of this reaction are salt and water.
4. Hence, the correct option is (d).

Teacher's Note:
a) Neutralization is always an exothermic process releasing heat energy.
b) Salt and water are the universal products of an acid-base reaction.

28. An aqueous solution turns red litmus solution blue. Excess addition of which of the following solution would reverse the change? [1 Mark]
(a) Baking powder
(b) Lime
(c) Ammonium hydroxide solution
(d) Hydrochloric acid

Answer: (d) Hydrochloric acid

1. A solution that turns red litmus blue is basic in nature.
2. To reverse this change, an acidic solution must be added in excess.
3. Hydrochloric acid is a strong acid that turns blue litmus back to red.
4. Hence, the correct option is (d).

Teacher's Note:
a) Bases turn red litmus blue, while acids turn blue litmus red.
b) Baking powder, lime, and ammonium hydroxide are all basic.

29. During the preparation of hydrogen chloride gas on a humid day, the gas is usually passed through the guard tube containing calcium chloride. The role of calcium chloride taken in the guard tube is to [1 Mark]
(a) absorb the evolved gas
(b) moisten the gas
(c) absorb moisture from the gas
(d) absorb Cl^- ions from the evolved gas

Answer: (c) absorb moisture from the gas

1. Calcium chloride is a well known drying agent.
2. On a humid day, air contains moisture that must be removed from the hydrogen chloride gas.
3. The guard tube dries the gas before its collection.
4. Hence, the correct option is (c).

Teacher's Note:
a) Anhydrous calcium chloride has high affinity for water molecules.
b) It does not react chemically with hydrogen chloride gas.

30. Compounds such as alcohols and glucose contain hydrogen but are not categorized as acids. Describe an activity to prove it. [3 Marks]

Answer:
1. Fix two iron nails on a rubber cork and place the cork in a 100 mL beaker.
2. Connect the nails to the two terminals of a 6 volt battery through a bulb and a switch.
3. Pour some dilute glucose or alcohol solution in the beaker and switch on the current; observe that the bulb does not glow.
4. This proves that alcohols and glucose do not dissociate into hydrogen ions in aqueous solution, hence they are not acids.

Teacher's Note:
a) Acids conduct electricity due to the presence of free mobile ions in solution.
b) Glucose and alcohol dissolve as molecules, not as ions.
c) Always use distilled water for preparing solutions in this activity.

31. Why does distilled water not conduct electricity, whereas rain water does? [2 Marks]

Answer:
1. Distilled water is pure and free from ionic impurities, so it lacks free ions to conduct electricity.
2. Rain water dissolves acidic gases like carbon dioxide and sulfur dioxide from the atmosphere, forming carbonic acid and other ions.
3. These free ions enable rain water to conduct electricity.

Teacher's Note:
a) Electrical conductivity in liquids requires charged particles.
b) Pure water is a poor conductor of electricity.

32. Why do acids not show acidic behavior in the absence of water? [2 Marks]

Answer:
1. Acids show acidic behavior due to the presence of hydrogen ions.
2. Hydrogen ions are produced only in the presence of water, where the acid molecules dissociate.
3. In the absence of water, separation of hydrogen ions from acid molecules does not occur.

Teacher's Note:
a) Water helps polar covalent bonds of acids to break into ions.
b) Dry HCl gas does not change the color of dry litmus paper.

33. Five solutions A, B, C, D and E when tested with universal indicator showed pH as 4, 1, 11, 7 and 9, respectively, which solution is: a) neutral b) strongly alkaline? c) strongly acidic d) weakly acidic e) weakly alkaline
Arrange the pH in increasing order of hydrogen ion concentration. [3 Marks]

Answer:
1. Solution D (pH 7) is neutral, and Solution C (pH 11) is strongly alkaline.
2. Solution B (pH 1) is strongly acidic, Solution A (pH 4) is weakly acidic, and Solution E (pH 9) is weakly alkaline.
3. pH values in increasing order of hydrogen ion concentration: 11, 9, 7, 4, 1.

Teacher's Note:
a) Lower pH values correspond to higher hydrogen ion concentrations.
b) pH 7 is neutral; values below 7 are acidic and above 7 are basic.
c) Strong acids have very low pH while strong bases have very high pH.

34. What is a neutralization reaction? Give two examples. [2 Marks]

Answer:
1. The reaction between an acid and a base to give salt and water is called a neutralization reaction.
2. Example 1: \(\text{NaOH} + \text{HCl} \rightarrow \text{NaCl} + \text{H}_2\text{O}\)
3. Example 2: \(\text{KOH} + \text{HNO}_3 \rightarrow \text{KNO}_3 + \text{H}_2\text{O}\)

Teacher's Note:
a) In neutralization, hydrogen ions combine with hydroxide ions to form water.
b) These reactions are generally exothermic.

35. What happens when an acid or base is mixed with water? [2 Marks]

Answer:
1. Mixing an acid or base with water results in dilution, decreasing the concentration of ions per unit volume.
2. This process is highly exothermic, releasing heat energy continuously.

Teacher's Note:
a) Acid must always be added slowly to water with constant stirring.
b) Adding water to concentrated acid can cause the mixture to splash and cause burns.

36. Equal lengths of magnesium ribbons are taken in test tubes A and B. Hydrochloric acid is added to test tube A, while acetic acid is added to test tube B. The concentrations taken for both the acids are same in which test tube the reaction occur more vigorously and why? [3 Marks]

Answer:
1. The reaction will occur more vigorously in test tube A.
2. Hydrochloric acid is a strong acid that ionizes completely, producing a higher concentration of hydrogen ions.
3. Acetic acid is a weak acid that ionizes partially, resulting in a lower concentration of hydrogen ions.

Teacher's Note:
a) Rate of a chemical reaction is directly proportional to the concentration of hydrogen ions.
b) Strong acids react faster than weak acids at the same molar concentration.
c) Effervescence of hydrogen gas will be visibly faster in test tube A.

37. Fresh milk has a pH of 6. How does the pH change as it turns to curd? Explain your answer. [2 Marks]

Answer:
1. The pH will decrease below 6 as milk turns to curd.
2. During curd formation, lactose sugar in milk converts into lactic acid, increasing the hydrogen ion concentration.

Teacher's Note:
a) Lactic acid is produced by the action of bacteria on milk.
b) An increase in acid concentration lowers the pH value.

38. A milkman adds a very small amount of baking soda to fresh milk.
a) Why does he shift the pH of the fresh milk from 6 to slightly alkaline?
b) Why does this milk take a long time to set as curd? [2 Marks]

Answer:
a) He makes the milk slightly alkaline so that it does not easily turn sour due to lactic acid formation.
b) The added base must first be neutralized by the lactic acid produced before the milk can become acidic enough to set as curd.

Teacher's Note:
a) Baking soda is basic and neutralizes acids as they form.
b) This is a common practice by milkmen to increase shelf life in warm weather.

39. Why does tooth decay start when the pH of mouth is lower than 5.5? [2 Marks]

Answer:
1. Tooth enamel is made of calcium phosphate, which is the hardest substance in the body and does not dissolve in water.
2. However, it gets corroded when the pH inside the mouth falls below 5.5 due to acid production by bacteria acting on food particles.

Teacher's Note:
a) Bacteria degrade leftover sugar in the mouth to produce acids.
b) Using toothpaste, which is generally basic, helps neutralize excess acid and prevent decay.

40. How does the flow of acid rain water into a river make the survival of aquatic life in a river difficult? [2 Marks]

Answer:
1. Acid rain lowers the pH of river water, making it acidic.
2. Lower pH levels and acidic conditions make the survival of aquatic animals and plants extremely difficult.

Teacher's Note:
a) Normal rainwater has a pH of about 5.6 due to dissolved carbon dioxide, but acid rain has a much lower pH.
b) Aquatic organisms are sensitive to narrow pH ranges and perish when water becomes too acidic.

41. Dry hydrogen chloride gas does not turn blue litmus whereas hydrochloric acid does. Why? [2 Marks]

Answer:
1. Dry HCl gas does not contain hydrogen ions (H⁺) in free form as it does not dissociate without water.
2. Hydrochloric acid in aqueous solution produces hydrogen ions (H⁺) which are responsible for acidic behaviour and the color change of litmus.

Teacher's Note:
a) Acids show acidic properties only in the presence of water.
b) Always test for dry gases using moist litmus paper.

42. What is meant by “water of crystallization” of a substance? Describe an activity to demonstrate water of crystallization. [3 Marks]

Answer:
1. Water of crystallization is the fixed number of water molecules present in one formula unit of a salt.
2. Activity: Take a few crystals of copper sulphate in a dry boiling tube and heat it; the blue crystals turn white and water droplets appear on the walls of the tube.
3. Add 2 to 3 drops of water to the white anhydrous copper sulphate obtained, and the blue colour is restored.

Teacher's Note:
a) Copper sulphate pentahydrate contains five molecules of water of crystallization.
b) Heating removes this chemically bound water, changing the crystal structure and color.
c) This reaction is reversible, which helps demonstrate hydration and dehydration.

43. Plaster of paris should be stored in a moisture - proof container. Explain why? [2 Marks]

Answer:
1. Plaster of Paris absorbs moisture from the air to form a hard solid mass called gypsum.
2. This sets the material and renders it useless for its intended applications.

Teacher's Note:
a) The chemical reaction is \(\text{CaSO}_4\cdot\frac{1}{2}\text{H}_2\text{O} + 1\frac{1}{2}\text{H}_2\text{O} \rightarrow \text{CaSO}_4\cdot2\text{H}_2\text{O}\).
b) Proper airtight packaging prevents premature setting.

44. What is baking powder? How does it make the cake soft and spongy? [3 Marks]

Answer:
1. Baking powder is a mixture of baking soda (sodium hydrogen carbonate) and a mild edible acid like tartaric acid.
2. When mixed with water or heated during baking, carbon dioxide gas is produced.
3. The trapped carbon dioxide bubbles cause bread or cake to rise, making them soft and spongy.

Teacher's Note:
a) The acid neutralizes the bitter sodium carbonate produced during heating.
b) Equation: \(\text{NaHCO}_3 + \text{H}^+ \rightarrow \text{CO}_2 + \text{H}_2\text{O} + \text{Sodium salt of acid}\).
c) Baking powder differs from baking soda, which lacks the added edible acid.

45. Give two important uses of washing soda and baking soda. [2 Marks]

Answer:
1. Washing soda is used in glass, soap, and paper industries, and for softening hard water.
2. Baking soda is used to make baking powder and as an ingredient in antacids to neutralize stomach acidity.

Teacher's Note:
a) Washing soda formula is \(\text{Na}_2\text{CO}_3\cdot10\text{H}_2\text{O}\).
b) Baking soda formula is \(\text{NaHCO}_3\).

46. WHO AM I? [5 Marks]
> I can roughly measure pH value from 0-14.
> I am called antichlor and am used to remove excess chlorine from clothes when treated with bleaching powder.
> I am a product of gypsum and am used to making chalks and fire proof materials.
> I am a compound of calcium and can be used for disinfecting drinking water as well as for decolourisation.
> I give different smell in acid and base solution.
> I am an oxide capable of showing properties for both acids and bases.
> I am a covalent compound and conducts electricity in aqueous medium.
> I am a salt of potassium hydroxide and nitric acid.
> I am the term used when a solid becomes liquid when exposed to moist air.
> I am derived from tomato and turn blue litmus into red.

Answer:
1. Universal indicator
2. Sulphur dioxide (or Sodium thiosulphate)
3. Plaster of Paris
4. Bleaching powder (Calcium oxychloride)
5. Olfactory indicator
6. Amphoteric oxide
7. Hydrochloric acid (HCl)
8. Potassium nitrate
9. Deliquescence
10. Oxalic acid

Teacher's Note:
a) This question tests multiple core concepts from the entire chapter in a riddle format.
b) Each clue points directly to a specific chemical substance, property, or term.
c) Amphoteric oxides include aluminum oxide and zinc oxide.
d) Deliquescent substances absorb moisture from air and dissolve in it.

47. The colour of methyl orange indicator in acidic medium is : [1 Mark]
(A) Yellow
(B) green
(C) orange
(D) red

Answer: (D) red

1. Methyl orange turns red in acidic medium.
2. Its neutral colour is orange and basic colour is yellow.
3. Hence, the correct option is (D).

Teacher's Note:
a) Indicators show colour changes depending on the hydrogen ion concentration.
b) Remember the sequence: red in acid, yellow in base.

48. The colour of phenolphthalein indicator in basic solution is : [1 Mark]
(A) Yellow
(B) green
(C) pink
(D) orange

Answer: (C) pink

1. Phenolphthalein is colourless in acidic and neutral solutions.
2. It turns pink in basic solutions.
3. Hence, the correct option is (C).

Teacher's Note:
a) Phenolphthalein is a synthetic indicator.
b) It is commonly used in acid-base titrations.

49. What is the colour methyl orange in alkaline medium? [1 Mark]
(A) orange
(B) yellow
(C) red
(D) blue

Answer: (B) yellow

1. Methyl orange exhibits a yellow colour in basic or alkaline mediums.
2. In acidic medium, it turns red.
3. Hence, the correct option is (B).

Teacher's Note:
a) Alkalies are water-soluble bases.
b) Keep track of indicator colour changes to avoid confusion between acid and base observations.

50. A solution turns red litmus blue, its pH will be : [1 Mark]
(A) 1
(B) 4
(C) 5
(D) 10

Answer: (D) 10

1. A solution that turns red litmus blue is basic in nature.
2. Basic solutions have a pH greater than 7.
3. Among the choices, 10 is the only value greater than 7.
4. Hence, the correct option is (D).

Teacher's Note:
a) pH scale ranges from 0 to 14.
b) Acids have pH less than 7, neutral substances have pH equal to 7.

51. A solution reacts with crushed egg-shells to give a gas that turns lime-water Milky, the solution contains : [1 Mark]
(A) NaCl
(B) HCl
(C) LiCl
(D) KCl

Answer: (B) HCl

1. Egg-shells contain calcium carbonate, which reacts with acids to release carbon dioxide gas.
2. Carbon dioxide turns lime water milky due to the formation of insoluble calcium carbonate.
3. Among the given options, HCl is an acid.
4. Hence, the correct option is (B).

Teacher's Note:
a) Metal carbonates react with acids to form salt, water, and carbon dioxide.
b) Lime water test confirms the evolution of carbon dioxide gas.

52. Why is universal indicator a better one than litmus paper? [1 Mark]
(A) Litmus paper can only be used for acids.
(B) Litmus paper can only be used for alkalis.
(C) Universal indicator goes green if something is neutral.
(D) Universal indicator is useful for all ranges of pH of the solution.

Answer: (D) Universal indicator is useful for all ranges of pH of the solution.

1. Litmus paper only indicates whether a solution is acidic or basic without giving its strength.
2. Universal indicator consists of a mixture of several indicators that show different colours at different pH values across the entire 0-14 range.
3. Hence, the correct option is (D).

Teacher's Note:
a) Universal indicator helps determine the exact strength of an acid or base.
b) It provides a complete colour spectrum corresponding to pH values.

53. Water soluble bases are known as? [1 Mark]
(A) neutral
(B) base
(C) acid
(D) alkali

Answer: (D) alkali

1. Bases that dissolve in water are specially termed alkalis.
2. All alkalis are bases, but all bases are not alkalis.
3. Hence, the correct option is (D).

Teacher's Note:
a) Examples of alkalis include sodium hydroxide and potassium hydroxide.
b) They release hydroxide ions (OH^-) in aqueous solution.

54. Which one of the following pairs of substances when mixed together produces table salt? [1 Mark]
(a) Sodium thiosulphate and sulphur dioxide
(b) Hydrochloric acid and sodium hydroxide
(c) Chlorine and oxygen
(d) Nitric acid and sodium hydrogen carbonate

Answer: (b) Hydrochloric acid and sodium hydroxide

1. HCl + NaOH → NaCl + H₂O.
2. This is a neutralisation reaction that gives common salt.

Teacher's Note:
a) A strong acid and a strong base always give a neutral salt.

55. What colour would hydrochloric acid (pH = 1) turn universal indicator? [1 Mark]
(a) Orange
(b) Purple
(c) Yellow
(d) Red

Answer: (d) Red

1. A pH of 1 is strongly acidic.
2. Universal indicator shows red for very low pH values.

Teacher's Note:
a) Red is the colour for pH 1 on the universal indicator colour chart.

56. Which one of the following medicines is used for treating indigestion? [1 Mark]
(a) Antibiotic
(b) Analgesic
(c) Antacid
(d) Antiseptic

Answer: (c) Antacid

Indigestion is caused by excess acid in the stomach. An antacid is a base that neutralises the excess acid.

Teacher's Note:
a) Antibiotics treat infections and analgesics relieve pain; neither neutralises acid.

57. If magnesium reacts with hydrochloric acid, what gas is produced? [1 Mark]
(a) Hydrogen
(b) Oxygen
(c) Carbon dioxide
(d) Chlorine

Answer: (a) Hydrogen

Mg + 2HCl → MgCl₂ + H₂. Acids react with metals to give hydrogen gas.

Teacher's Note:
a) The gas burns with a pop sound, which is the test for hydrogen.

58. Which of the following is the most accurate way of representing neutralisation? [1 Mark]
(a) Acid + base → neutral solution
(b) Acid + base → salt + water
(c) Acid + base → sodium chloride + hydrogen
(d) Acid + base → acidic solution

Answer: (b) Acid + base → salt + water

Neutralisation always gives a salt and water. The salt is not always sodium chloride.

Teacher's Note:
a) Option (a) describes the result, not the reaction.

59. Classify the following examples as acid, base or salt: Mg(OH)₂, KCl, HCl, H₃PO₄, HBr, Al(OH)₃, KNO₂, NaCl, Ba(OH)₂, HFO₄. [1 Mark]

Answer:
Mg(OH)₂: base
KCl: salt
HCl: acid
H₃PO₄: acid
HBr: acid
Al(OH)₃: base
KNO₂: salt
NaCl: salt
Ba(OH)₂: base
HFO₄: acid

Teacher's Note:
Acids give H⁺ ions, bases give OH^- ions, and salts are formed by neutralisation.

60. Fill in the following blanks: (a) A ______ taste is a characteristic property of all acids in aqueous solution. (b) Acids react with some metals to produce ______ gas. (c) Aqueous acid solutions conduct electricity because they have ______. [1 Mark]

Answer:
(a) sour
(b) hydrogen
(c) ions (H⁺ and anions)
(d) The remaining blanks of the question are: Acid reacts with base to produce a ________ and water. Answer: salt
(e) Acid turns methyl orange to ________ colour. Answer: red
(f) Bases tend to taste ________ and feel ________. Answer: bitter; soapy
(g) Aqueous basic solutions conduct electricity because they have ________. Answer: ions (OH^- and cations)
(h) Bases react with ________ to produce a salt and ________. Answer: acids; water
(i) Bases turn phenolphthalein to ________ colour. Answer: pink

These are the basic properties of acids in water.

Teacher's Note:
a) Students often write "sour" for every acid, but the sour taste is typical of food acids and not a safe test in the laboratory.

61. Match the following: (a) Plaster of Paris, (b) Gypsum, (c) Bleaching powder, (d) Baking soda, (e) Washing soda, with (1) CaOCl₂, (2) NaHCO₃, (3) Na₂CO₃, (4) CaSO₄.½H₂O, (5) CaSO₄.2H₂O. [1 Mark]
(a) a-4, b-5, c-1, d-2, e-3
(b) a-5, b-4, c-1, d-3, e-2
(c) a-4, b-5, c-2, d-1, e-3
(d) a-1, b-2, c-3, d-4, e-5

Answer: (a) a-4, b-5, c-1, d-2, e-3

Plaster of Paris is CaSO₄.½H₂O, gypsum is CaSO₄.2H₂O, bleaching powder is CaOCl₂, baking soda is NaHCO₃ and washing soda is Na₂CO₃.

Teacher's Note:
a) Each formula should be memorised with its common name.

62. Which of the following salts does not contain water of crystallisation? [1 Mark]
(a) Blue vitriol
(b) Baking soda
(c) Washing soda
(d) Gypsum

Answer: (b) Baking soda

Blue vitriol (CuSO₄.5H₂O), washing soda (Na₂CO₃.10H₂O) and gypsum (CaSO₄.2H₂O) are hydrated salts. Baking soda (NaHCO₃) is anhydrous.

Teacher's Note:
a) Blue vitriol is another name for copper sulphate pentahydrate.

63. Sodium carbonate is a basic salt because it is a salt of [1 Mark]
(A) strong acid and strong base
(B) weak acid and weak base
(C) strong acid and weak base
(D) weak acid and strong base

Answer: (D) weak acid and strong base

1. Sodium carbonate (Na₂CO₃) is formed by the neutralization of a weak acid (carbonic acid, H₂CO₃) and a strong base (sodium hydroxide, NaOH).
2. Salts of weak acids and strong bases yield a basic solution with pH greater than 7.
3. Hence, the correct option is (D).

Teacher's Note:
a) The strength of parent acid and base determines salt hydrolysis.
b) Do not confuse sodium carbonate with sodium chloride, which is neutral.

64. Calcium phosphate is present in tooth enamel. Its nature is [1 Mark]
(A) basic
(B) acidic
(C) neutral
(D) amphoteric

Answer: (A) basic

1. Tooth enamel is made of calcium phosphate, which is a calcium salt of phosphoric acid.
2. Being a salt derived from a relatively weak phosphoric acid and a strong base (calcium hydroxide), calcium phosphate is basic in nature.
3. Hence, the correct option is (A).

Teacher's Note:
a) Tooth decay starts when mouth pH falls below 5.5 due to acid production by bacteria.
b) Basic nature protects teeth from moderate neutral conditions.

65. A sample of soil is mixed with water and allowed to settle. The clear supernatant solution turns the pH paper yellowish-orange. Which of the following would change the colour of this pH paper to greenish-blue? [1 Mark]
(A) Lemon juice
(B) Vinegar
(C) Common salt
(D) An antacid

Answer: (D) An antacid

1. A yellowish-orange colour on pH paper indicates an acidic medium (pH around 4 to 5).
2. A greenish-blue colour on pH paper indicates a basic medium (pH around 9 to 10).
3. An antacid is basic in nature and will increase the pH, turning the paper greenish-blue.
4. Hence, the correct option is (D).

Teacher's Note:
a) pH paper shows distinct colours across the pH scale from 0 to 14.
b) Lemon juice and vinegar are acidic, while common salt is neutral.

66. Which of the following gives the correct increasing order of acidic strength? [1 Mark]
(A) Water < Acetic acid < Hydrochloric acid
(B) Water < Hydrochloric acid < Acetic acid
(C) Acetic acid < Water < Hydrochloric acid
(D) Hydrochloric acid < Water < Acetic acid

Answer: (A) Water < Acetic acid < Hydrochloric acid

1. Water is nearly neutral with the lowest concentration of hydrogen ions among the three.
2. Acetic acid is a weak organic acid that dissociates partially in aqueous solution.
3. Hydrochloric acid is a strong mineral acid that dissociates completely in water, giving the highest hydrogen ion concentration.
4. Hence, the correct option is (A).

Teacher's Note:
a) Acidic strength depends on the degree of dissociation and hydrogen ion concentration.
b) Mineral acids are generally stronger than organic carboxylic acids.

67. If a few drops of a concentrated acid accidentally spills over the hand of a student, what should be done? [1 Mark]
(A) Wash the hand with saline solution
(B) Wash the hand immediately with plenty of water and apply a paste of sodium hydrogencarbonate
(C) After washing with plenty of water apply solution of sodium hydroxide on the hand
(D) Neutralise the acid with a strong alkali

Answer: (B) Wash the hand immediately with plenty of water and apply a paste of sodium hydrogencarbonate

1. Water washes away and dilutes the concentrated acid immediately to reduce its corrosive action.
2. Applying a weak base like sodium hydrogencarbonate neutralises any residual acid safely.
3. Strong alkalis like sodium hydroxide must never be used as they cause severe chemical burns themselves.
4. Hence, the correct option is (B).

Teacher's Note:
a) Safety protocol requires immediate washing with copious amounts of running water.
b) Neutralisation using weak bases prevents exothermic injury caused by strong alkalis.

68. Sodium hydrogencarbonate when added to acetic acid evolves a gas. Which of the following statements are true about the gas evolved? [1 Mark]
(i) It turns lime water milky
(ii) It extinguishes a burning splinter
(iii) It dissolves in a solution of sodium hydroxide
(iv) It has a pungent odour
(A) (i) and (ii)
(B) (i), (ii) and (iii)
(C) (ii), (iii) and (iv)
(D) (i) and (iv)

Answer: (B) (i), (ii) and (iii)

1. The reaction between sodium hydrogencarbonate and acetic acid produces carbon dioxide gas (CO₂).
2. Carbon dioxide turns lime water milky due to the formation of insoluble calcium carbonate, and it extinguishes a burning splinter.
3. Carbon dioxide is an acidic oxide, so it dissolves in sodium hydroxide solution.
4. CO₂ is odourless, so statement (iv) is incorrect.
5. Hence, the correct option is (B).

Teacher's Note:
a) Metal hydrogencarbonates react with acids to liberate carbon dioxide gas.
b) Carbon dioxide does not support combustion and turns lime water milky.

69. Common salt besides being used in kitchen can also be used as the raw material for making [1 Mark]
(i) washing soda
(ii) bleaching powder
(iii) baking soda
(iv) slaked lime
(A) (i) and (ii)
(B) (i), (ii) and (iv)
(C) (i) and (iii)
(D) (i), (iii) and (iv)

Answer: (C) (i) and (iii)

1. Common salt (NaCl) is the primary raw material for manufacturing washing soda (sodium carbonate decahydrate) and baking soda (sodium hydrogencarbonate) via the Solvay process.
2. Bleaching powder is prepared using chlorine gas, which is a byproduct of the chlor-alkali process involving sodium chloride.
3. Slaked lime is prepared by adding water to quicklime, which does not use common salt.
4. Hence, the correct option is (C) as statements (i) and (iii) directly represent products derived from common salt.

Teacher's Note:
a) Common salt is a vital chemical feedstock in industrial chemistry.
b) Note that chlorine from brine electrolysis is used for bleaching powder, but washing soda and baking soda are direct sodium compounds derived from salt.

70. One of the constituents of baking powder is sodium hydrogen carbonate, the other constituent is [1 Mark]
(A) hydrochloric acid
(B) tartaric acid
(C) acetic acid
(D) sulphuric acid

Answer: (B) tartaric acid

1. Baking powder is a mixture of sodium hydrogencarbonate and a mild edible solid acid such as tartaric acid.
2. When baking powder mixes with water in dough, tartaric acid reacts with sodium hydrogencarbonate to release carbon dioxide gas.
3. The released carbon dioxide makes cakes and bread rise, making them soft and spongy.
4. Hence, the correct option is (B).

Teacher's Note:
a) Baking soda alone produces sodium carbonate on heating, which tastes bitter; tartaric acid neutralises it.
b) Edible organic acids are essential components of culinary baking powder formulations.

71. To protect tooth decay we are advised to brush our teeth regularly. The nature of the tooth paste commonly used is [1 Mark]
(a) acidic
(b) neutral
(c) basic
(d) corrosive

Answer: (c) basic

Toothpastes are basic, so they neutralise the acid that causes tooth decay.

Teacher's Note:
a) Corrosive describes a property, not an acid-base nature.

72. Which of the following statements is correct about an aqueous solution of an acid and of a base? (i) Higher the pH, stronger the acid (ii) Higher the pH, weaker the acid (iii) Lower the pH, stronger the base (iv) Lower the pH, weaker the base [1 Mark]
(a) (i) and (iii)
(b) (ii) and (iii)
(c) (i) and (iv)
(d) (ii) and (iv)

Answer: (d) (ii) and (iv)

A higher pH means a weaker acid, so (ii) is correct. A lower pH means a weaker base, so (iv) is correct.

Teacher's Note:
a) Strong acids have low pH values, and strong bases have high pH values.

73. The pH of the gastric juices released during digestion is [1 Mark]
(a) less than 7
(b) more than 7
(c) equal to 7
(d) equal to 0

Answer: (a) less than 7

Gastric juice contains hydrochloric acid, so its pH is below 7 (about 2).

Teacher's Note:
a) A pH of exactly 0 is not the typical value for gastric juice.

74. Which of the following phenomena occur when a small amount of acid is added to water? (i) Ionisation (ii) Neutralisation (iii) Dilution (iv) Salt formation [1 Mark]
(a) (i) and (ii)
(b) (i) and (iii)
(c) (ii) and (iii)
(d) (ii) and (iv)

Answer: (b) (i) and (iii)

Adding acid to water ionises the acid and dilutes it. No base is present, so there is no neutralisation or salt formation.

Teacher's Note:
a) Dilution must be done by adding acid to water, never water to acid.

75. Which one of the following can be used as an acid-base indicator by a visually impaired student? [1 Mark]
(a) Litmus
(b) Turmeric
(c) Vanilla essence
(d) Petunia leaves

Answer: (c) Vanilla essence

Vanilla is an olfactory indicator. Its smell changes with a base, so it can be used without seeing a colour change.

Teacher's Note:
a) Litmus, turmeric and petunia leaves all depend on colour change, which is not usable without sight.

76. Which of the following substances will not give carbon dioxide on treatment with dilute acid? [1 Mark]
(a) Marble
(b) Limestone
(c) Baking soda
(d) Lime

Answer: (d) Lime

Marble and limestone are CaCO₃, and baking soda is NaHCO₃. Each gives CO₂ with acid. Lime (CaO or Ca(OH)₂) does not contain carbonate.

Teacher's Note:
a) Lime is a base, so it reacts with acid to give salt and water only.

77. Which of the following is acidic in nature? [1 Mark]
(a) Lime juice
(b) Human blood
(c) Lime water
(d) Antacid

Answer: (a) Lime juice

Lime juice contains citric acid and has a pH below 7. Human blood is slightly basic, lime water is basic and antacids are basic.

Teacher's Note:
a) Human blood has a pH of 7.35 to 7.45, which is slightly basic.

78. In an attempt to demonstrate electrical conductivity through an electrolyte, the apparatus shown in the figure was set up. Which of the following statement(s) is(are) correct? (i) Bulb will not glow because the electrolyte is not acidic (ii) Bulb will glow because NaOH is a strong base and furnishes ions for conduction (iii) Bulb will not glow because circuit is incomplete (iv) Bulb will not glow because it depends upon the type of electrolytic solution [1 Mark]
(a) (i) and (iii)
(b) (ii) and (iv)
(c) (ii) only
(d) (iv) only

Answer: (c) (ii) only

NaOH is a strong base that dissociates into Na⁺ and OH^- ions, so it conducts electricity and the bulb glows. A conducting electrolyte need not be an acid, and the circuit is complete when the bulb glows.

Teacher's Note:
a) The figure shows a conductivity circuit with NaOH solution as the electrolyte.

79. Which of the following is used for the dissolution of gold? [1 Mark]
(a) Hydrochloric acid
(b) Sulphuric acid
(c) Nitric acid
(d) Aqua regia

Answer: (d) Aqua regia

Aqua regia, a mixture of concentrated hydrochloric acid and nitric acid in a 3:1 ratio, dissolves gold.

Teacher's Note:
a) Neither hydrochloric acid nor nitric acid alone dissolves gold.

80. Which of the following is not a mineral acid? [1 Mark]
(a) Hydrochloric acid
(b) Citric acid
(c) Sulphuric acid
(d) Nitric acid

Answer: (b) Citric acid

Mineral acids are obtained from rocks and minerals: HCl, H₂SO₄ and HNO₃. Citric acid is an organic acid found in fruits.

Teacher's Note:
a) Organic acids are weaker than mineral acids.

81. Which among the following is not a base? [1 Mark]
(a) NaOH
(b) KOH
(c) NH₄OH
(d) C₂H₅OH

Answer: (d) C₂H₅OH

C₂H₅OH is ethanol, an alcohol. It does not release hydroxide ions in water.

Teacher's Note:
a) Ethanol contains an -OH group, but it is not ionic and does not act as a base.

82. Which of the following statements is not correct? [1 Mark]
(a) All metal carbonates react with acid to give a salt, water and carbon dioxide
(b) All metal oxides react with water to give salt and acid
(c) Some metals react with acids to give salt and hydrogen
(d) Some non-metal oxides react with water to form an acid

Answer: (b) All metal oxides react with water to give salt and acid

Most metal oxides are basic and react with water to form bases, not acids. Non-metal oxides such as CO₂ and SO₂ form acids.

Teacher's Note:
a) Option (b) is incorrect because metal oxides give bases with water.

83. Which of the following is(are) true when HCl (g) is passed through water? (i) It does not ionise in the solution as it is a covalent compound (ii) It ionises in the solution (iii) It gives both hydrogen and hydroxyl ions in the solution (iv) It forms hydronium ions in the solution due to the combination of hydrogen ions with water molecules [1 Mark]
(a) (i) only
(b) (iii) only
(c) (ii) and (iv)
(d) (iii) and (iv)

Answer: (c) (ii) and (iv)

HCl ionises in water to give H⁺ and Cl^-, and H⁺ combines with water to form H₃O⁺. HCl does not give hydroxyl ions.

Teacher's Note:
a) HCl is covalent as a gas but ionises in water, which is why it is a strong acid.

84. Which of the following statements is true for acids? [1 Mark]
(a) Bitter and change red litmus to blue
(b) Sour and change red litmus to blue
(c) Sour and change blue litmus to red
(d) Bitter and change blue litmus to red

Answer: (c) Sour and change blue litmus to red

1. Acids are sour in taste.
2. They turn blue litmus paper red.
3. Hence, the correct option is (c).

Teacher's Note:
a) Acids release hydrogen ions in aqueous solution.
b) Remember the basic indicator colour change property.

85. Which of the following are present in a dilute aqueous solution of hydrochloric acid? [1 Mark]
(a) H₃O⁺ + Cl^-
(b) H₃O⁺ + OH^-
(c) Cl^- + OH^-
(d) unionised HCl

Answer: (a) H₃O⁺ + Cl^-

1. Hydrochloric acid is a strong acid that completely dissociates in water.
2. Hydrogen ions combine with water molecules to form hydronium ions (H₃O⁺) along with chloride ions (Cl^-).
3. Hence, the correct option is (a).

Teacher's Note:
a) Strong acids dissociate completely in aqueous solutions.
b) Free ions are responsible for electrical conductivity.

86. Identify the correct representation of reaction occurring during chloralkali process [1 Mark]
(a) 2NaCl(l) + 2H₂O(l) → 2NaOH(l) + Cl₂(g) + H₂(g)
(b) 2NaCl(aq) + 2H₂O(aq) → 2NaOH(aq) + Cl₂(g) + H₂(g)
(c) 2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(aq) + H₂(aq)
(d) 2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g)

Answer: (d) 2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g)

1. Brine is an aqueous solution of sodium chloride.
2. Electricity is passed through aqueous sodium chloride, yielding dissolved sodium hydroxide, chlorine gas at the anode, and hydrogen gas at the cathode.
3. Hence, the correct option is (d).

Teacher's Note:
a) Physical states like aqueous solution and liquid water must be represented accurately.
b) Chlorine and hydrogen are evolved as gases.

87. What will be the action of the following substances on litmus paper? Dry HCl gas, Moistened NH₃ gas, Lemon juice, Carbonated soft drink, Curd, Soap solution. [3 Marks]

Answer:
1. Dry HCl gas: No action on dry litmus paper because it does not dissociate into ions without water.
2. Moistened NH₃ gas: Turns red litmus blue as it is basic in nature.
3. Lemon juice, Carbonated soft drink, and Curd: Turn blue litmus red as they are acidic in nature.
4. Soap solution: Turns red litmus blue as it is basic in nature.

Teacher's Note:
a) Acids and bases show their properties only in the presence of water or aqueous medium.
b) A common student trap is assuming dry HCl gas is acidic to dry litmus.
c) Ammonia is a gas that dissolves in moisture to form hydroxide ions.

88. Name the acid present in ant sting and give its chemical formula. Also give the common method to get relief from the discomfort caused by the ant sting. [2 Marks]

Answer:
1. The acid present in ant sting is methanoic acid (formic acid) and its chemical formula is HCOOH.
2. Relief can be obtained by applying a mild base such as baking soda to neutralise the acid.

Teacher's Note:
a) Natural stings inject organic acids into the skin.
b) Neutralisation reactions counteract the painful effects effectively.

89. A student prepared solutions of (i) an acid and (ii) a base in two separate beakers. She forgot to label the solutions and litmus paper is not available in the laboratory. Since both the solutions are colourless, how will she distinguish between the two? [2 Marks]

Answer:
1. She can use an olfactory indicator like vanilla extract, onion juice, or clove oil, or a synthetic indicator like phenolphthalein or methyl orange.
2. When phenolphthalein is added, the basic solution turns pink while the acidic solution remains colourless.

Teacher's Note:
a) Alternative indicators help identify acidic and basic media without litmus.
b) Phenolphthalein is colourless in acidic medium and pink in basic medium.

90. How would you distinguish between baking powder and washing soda by heating? [2 Marks]

Answer:
1. On heating baking powder, carbon dioxide gas is evolved which turns lime water milky due to the decomposition of sodium hydrogen carbonate.
2. On heating washing soda (sodium carbonate decahydrate), water molecules of crystallization are lost first to form a white powder without evolving carbon dioxide gas that turns lime water milky.

Teacher's Note:
a) Baking powder contains sodium hydrogen carbonate which releases carbon dioxide on heating.
b) Washing soda loses its water of crystallization upon strong heating.

91. Salt - A commonly used in bakery products on heating gets converted into another salt B which itself is used for removal of hardness of water and a gas C is evolved. The gas C when passed through lime water, turns it milky. Identify A, B and C. [3 Marks]

Answer:
1. Salt A is sodium hydrogen carbonate (NaHCO₃), commonly known as baking soda.
2. Salt B is sodium carbonate (Na₂CO₃), which is used for removing permanent hardness of water.
3. Gas C is carbon dioxide (CO₂), which turns lime water milky due to the formation of insoluble calcium carbonate.

Teacher's Note:
a) Thermal decomposition of sodium hydrogen carbonate yields sodium carbonate, water, and carbon dioxide.
b) Carbon dioxide reacts with calcium hydroxide to form a white precipitate.
c) Recognizing chemical tests for gases is essential for identification questions.

92. In one of the industrial processes used for manufacture of sodium hydroxide, a gas X is formed as by product. The gas X reacts with lime water to give a compound Y which is used as a bleaching agent in chemical industry. Identify X and Y giving the chemical equation of the reactions involved. [3 Marks]

Answer:
1. Gas X is chlorine gas (Cl₂) and compound Y is bleaching powder, calcium oxychloride (CaOCl₂).
2. Reaction of chlorine with dry slaked lime: Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O
3. Chloralkali process equation: 2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g)

Teacher's Note:
a) The chloralkali process produces sodium hydroxide, chlorine, and hydrogen.
b) Bleaching powder is manufactured by the action of chlorine on dry slaked lime.
c) Chemical equations must always be balanced properly with state symbols.

93. What are strong and weak acids? In the following list of acids, separate strong acids from weak acids. Hydrochloric acid, citric acid, acetic acid, nitric acid, formic acid, sulphuric acid. [3 Marks]

Answer:
1. Strong acids are those acids that dissociate almost completely in aqueous solution to produce a high concentration of hydronium ions. Weak acids dissociate only partially.
2. Strong acids from the list: Hydrochloric acid, nitric acid, sulphuric acid.
3. Weak acids from the list: Citric acid, acetic acid, formic acid.

Teacher's Note:
a) Degree of ionization determines whether an acid is strong or weak.
b) Mineral acids are generally strong, while organic acids are generally weak.
c) Clear classification helps in understanding chemical reactivity and pH values.

94. When zinc metal is treated with a dilute solution of a strong acid, a gas is evolved, which is utilised in the hydrogenation of oil. Name the gas evolved. Write the chemical equation of the reaction involved and also write a test to detect the gas formed. [3 Marks]

Answer:
1. The gas evolved is hydrogen gas (H₂).
2. Chemical equation: Zn + 2HCl → ZnCl₂ + H₂
3. Test to detect: Bring a burning candle near the mouth of the gas container; the gas burns with a pop sound.

Teacher's Note:
a) Reaction of active metals with acids produces hydrogen gas and a salt.
b) The characteristic pop sound is the standard confirmatory test for hydrogen gas.
c) Hydrogen finds major industrial use in the hydrogenation of vegetable oils.

95. In the following schematic diagram for the preparation of hydrogen gas as shown in below Figure, what would happen if following changes are made? [2 Marks]

Answer:
1. If zinc granules are replaced by zinc dust, the rate of evolution of hydrogen gas will increase due to a larger surface area.
2. If dilute hydrochloric acid is used in place of dilute sulphuric acid, hydrogen gas will still be evolved at a similar rate.

Teacher's Note:
a) Surface area directly affects the rate of chemical reactions involving solids.
b) Different strong mineral acids react similarly with active metals to produce hydrogen gas.

(a) In place of zinc granules, same amount of zinc dust is taken in the test tube [1 Mark]

Answer:
1. The rate of evolution of hydrogen gas will be faster.
2. Zinc dust has a larger surface area compared to zinc granules, which increases the rate of the chemical reaction.

Teacher's Note:
a) Surface area is directly proportional to the rate of a heterogeneous reaction.
b) Smaller particle size exposes more reactant atoms for effective collisions.
c) Mentioning the increased surface area is essential for full marks.

(b) Instead of dilute sulphuric acid, dilute hydrochloric acid is taken [1 Mark]

Answer:
1. There will be no change in the observation.
2. Hydrogen gas will continue to evolve with the formation of zinc chloride instead of zinc sulphate.

Teacher's Note:
a) Both dilute acids provide hydrogen ions to react with reactive metals.
b) The gaseous product remains the same regardless of the acid anion used.

(c) In place of zinc, copper turnings are taken [1 Mark]

Answer:
1. No reaction will take place and no hydrogen gas will evolve.
2. Copper is less reactive than hydrogen and cannot displace hydrogen from dilute acids.

Teacher's Note:
a) Refer to the reactivity series of metals to determine displacement feasibility.
b) Metals placed below hydrogen in the series do not evolve hydrogen gas with dilute acids.

(d) Sodium hydroxide is taken in place of dilute sulphuric acid and the tube is heated [1 Mark]

Answer:
1. Hydrogen gas will be evolved along with the formation of sodium zincate.
2. Chemical equation: Zn + 2NaOH → Na₂ZnO₂ + H₂ (g).

Teacher's Note:
a) Active metals like zinc can react with strong bases upon heating to liberate hydrogen gas.
b) This is a specific property of amphoteric metals reacting with alkalis.

96. For making cake, baking powder is taken. If at home your mother uses baking soda instead of baking powder in cake, [3 Marks]
(a) how will it affect the taste of the cake and why?
(b) how can baking soda be converted into baking powder?
(c) what is the role of tartaric acid added to baking soda?

Answer:
1. (a) The cake will taste bitter because baking soda is basic in nature and produces sodium carbonate on heating, which is bitter.
2. (b) Baking soda (sodium hydrogen carbonate) can be converted into baking powder by mixing it with a mild edible acid such as tartaric acid.
3. (c) Tartaric acid neutralizes the sodium carbonate formed during heating, preventing the bitter taste and releasing carbon dioxide gas which makes the cake soft and spongy.

Teacher's Note:
a) Baking soda is chemically sodium hydrogen carbonate (NaHCO₃).
b) Baking powder is a mixture of baking soda and a mild edible acid.
c) Always mention both the neutralization of base and the release of carbon dioxide.

97. A metal carbonate X on reacting with an acid gives a gas which when passed through a solution Y gives the carbonate back. On the other hand, a gas G that is obtained at anode during electrolysis of brine is passed on dry Y, it gives a compound Z, used for disinfecting drinking water. Identify X, Y, G and Z. [3 Marks]

Answer:
1. X is Calcium carbonate (CaCO₃) or any metal carbonate.
2. Y is Calcium hydroxide solution or lime water (Ca(OH)₂).
3. G is Chlorine gas (Cl₂).
4. Z is Bleaching powder (CaOCl₂).

Teacher's Note:
a) Trace the chloralkali process where chlorine is released at the anode.
b) Calcium carbonate reacts with acid to give carbon dioxide, which turns lime water milky and reforms carbonate on prolonged passage.
c) Clearly state all four chemical identities to secure full marks.

98. A dry pellet of a common base B, when kept in open absorbs moisture and turns sticky. The compound is also a by-product of chloralkali process. Identify B. What type of reaction occurs when B is treated with an acidic oxide? Write a balanced chemical equation for one such solution. [3 Marks]

Answer:
1. Base B is Sodium hydroxide (NaOH).
2. The reaction that occurs when B is treated with an acidic oxide is a neutralization reaction.
3. Balanced chemical equation: 2NaOH + CO₂ → Na₂CO₃ + H₂O.

Teacher's Note:
a) Sodium hydroxide is deliquescent, hence it absorbs moisture from the air.
b) Acidic oxides react with bases to form salt and water, similar to acid-base neutralization.
c) Check that the chemical equation is fully balanced with proper state symbols or correct formulae.

99. A sulphate salt of Group 2 element of the Periodic Table is a white, soft substance, which can be moulded into different shapes by making its dough. When this compound is left in open for some time, it becomes a solid mass and cannot be used for moulding purposes. Identify the sulphate salt and why does it show such a behaviour? Give the reaction involved. [3 Marks]

Answer:
1. The sulphate salt is Plaster of Paris, chemically calcium sulphate hemihydrate (CaSO₄ · \(\frac{1}{2}\) H₂O).
2. It shows this behavior because on mixing with water, it sets into a hard solid mass due to hydration, forming gypsum.
3. Chemical reaction: CaSO₄ · \(\frac{1}{2}\) H₂O + 1 \(\frac{1}{2}\) H₂O → CaSO₄ · 2H₂O.

Teacher's Note:
a) Calcium belongs to Group 2 of the Periodic Table.
b) Plaster of Paris absorbs moisture from the air or water to recrystallize into gypsum.
c) Writing the correct chemical formula with water of crystallization is mandatory.

100. Identify the compound X on the basis of the reactions given below. Also, write the name and chemical formulae of A, B and C. [3 Marks]

[Figure: Box labeled Compound X connected with three arrows pointing right to circles A, B, and C with + Zn, + HCl, and + CH₃COOH respectively, all resulting in + H₂(g), + H₂O, and + H₂O]

Answer:
1. Compound X is Sodium hydroxide (NaOH).
2. Compound A is Sodium zincate, chemical formula Na₂ZnO₂.
3. Compound B is Sodium chloride, chemical formula NaCl.
4. Compound C is Sodium acetate, chemical formula CH₃COONa.

Teacher's Note:
a) Bases react with active metals, mineral acids, and organic acids to form respective salts.
b) Sodium hydroxide reacts with zinc to give sodium zincate and hydrogen.
c) Reaction with hydrochloric acid gives sodium chloride and water, and with acetic acid gives sodium acetate and water.

Question. Calcium hydroxide (slaked lime) is used in -
(A) Plastics and dyes
(B) Fertilizers
(C) Antacids
(D) White washing
Answer : D

Question. Reaction of an acid with a base is known as -
(A) decomposition
(B) combination
(C) redox reaction
(D) neutralization
Answer : D

Question. Acids gives -
(A) H+ in water
(B) OH– in water
(C) Both (A) & (B)
(D) None of these
Answer : A

Question. Caustic soda is the common name for -
(A) Mg(OH)₂
(B) KOH
(C) Ca(OH)₂
(D) NaOH
Answer : D

Question. Antacids contain -
(A) Weak base
(B) Weak acid
(C) Strong base
(D) Strong acid
Answer : A

Question. pH of Blood is -
(A) 6.4
(B) 7.4
(C) 4.7
(D) 5.2
Answer : B

Question. Which is a base and not an alkali ?
(A) NaOH
(B) KOH
(C) Fe(OH)₃
(D) none is true
Answer : C

Question. Common name of H₂SO₄ is -
(A) Oil of vitriol
(B) Muriatic acid
(C) Blue vitriol
(D) Green vitriol
Answer : A

Question. Materials used in the manufacture of bleaching powder are -
(A) lime stone and chlorine
(B) quick lime and chlorine
(C) slaked lime and HCl
(D) slaked lime and chlorine
Answer : D

Question. A solution has pH 9. On dilution the pH value
(A) decreases
(B) increases
(C) remain same
(D) none of these
Answer : A

Question. Which of the following is an example of basic salt solution ?
(A) CH₃COONa (aq)
(B) NH4Cl (aq)
(C) AlCl₃ (aq)
(D) (NH₄)₂SO₄ (aq)
Answer : A

Question. Nature of the aqueous solution of NaCl towards litmus is -
(A) acidic
(B) basic
(C) neutral
(D) none
Answer : C

Question. Nature of methyl orange is -
(A) acidic
(B) basic
(C) neutral
(D) none of these
Answer : B

Question. The amount of energy released in neutralisation reaction between strong acid and strong base is -
(A) 57.8 kJ/mol
(B) 57.1 kJ/mol
(C) 57.9 kJ/mol
(D) 56.1 kJ/mol
Answer : B

Question. Bleaching powder gives smell of chlorine because it -
(A) is unstable
(B) gives chlorine on exposure to atmosphere
(C) is a mixture of chlorine and slaked lime
(D) contains excess of chlorine
Answer : B

Question. Chemical formula of baking soda is -
(A) MgSO₄
(B) Na2CO₃
(C) NaHCO₃
(D) MgCO₃
Answer : C

Question. The chemical name of marble is -
(A) calcium carbonate
(B) magnesium carbonate
(C) calcium chloride
(D) calcium sulphate
Answer : A

Question. Materials used in the manufacture of bleaching powder are -
(A) lime stone and chlorine
(B) quick lime and chlorine
(C) slaked lime and HCl
(D) slaked lime and chlorine
Answer : D
 

Short Answer Questions

Question. What is a neutralization reaction? Give an examples.
Answer : A reaction in which an acid and base react with each other to give a salt and water is termed as neutralization reaction. In this reaction, energy is evolved in the form of heat.
(i) NaOH + HCl → NaCl + H₂O
(Base) (Acid) (Salt) (Water)
(ii) During indigestion (caused due to the production of excess of hydrochloric acid in the stomach),
we administer an antacid (generally milk of magnesia, which is basic in nature). The antacid neutralizes the excess of acids and thus gives relief from indigestion.
Example- 
Mg (OH)₂ + 2HCl→ MgCl₂ + 2H₂O

Question. What happens when nitric acid is added to egg shell? Write suitable chemical reaction.
Answer : Egg shells contain calcium carbonate. When nitric acid is added to it, carbon dioxide gas is evolved. The reaction can be given as
CaCO₃ + 2HNO₃ → Ca (NO₃)₂ + H₂O + CO₂

Question. Write the formula and give one use of each of the following compounda.
Plaster of Paris
b. Bleaching powder
c. Baking soda
Answer : a. CaSO₄ 1/2 H₂O, used to join fractured bones
b. CaOCl₂ , used to disinfect water
c. NaHCO₃ , making food items crispy and fluffy

Long Short Answer Questions

Question. Match the following pH values 1, 7, 10, 13 to the solutions given below:
• Milk of magnesia
• Gastric juices
• Brine
• Aqueous Sodium hydroxide.
Amit and Rita decided to bake a cake and added baking soda to the cake batter. Explain with a balanced reaction, the role of the baking soda. Mention any other use of baking soda.
Answer : • Milk of magnesia 10
• Gastric juices 1
• Brine 7
• Aqueous Sodium hydroxide 13
Baking soda undergoes thermal decomposition to form Na₂CO₃ ,CO₂ and H₂O ; CO₂ makes the cake fluffy & soft

Uses: Used in fire extinguishers/ antacid to neutralize excess acid in stomach / to neutralize the effect of acid in insect sting.

Question. (i) Four samples A, B, C and D change the colour of pH paper or solution to Green, Reddishpink, Blue and Orange. Their pH was recorded as 7, 2, 10.5 & 6 respectively. Which of the samples has the highest amount of Hydrogen ion concentration? Arrange the four samples in the decreasing order of their pH.
(ii) Rahul found that the Plaster of Paris, which he stored in a container, has become very hard and lost its binding nature. What is the reason for this? Also, write a chemical equation to represent the reaction taking place.
(iii) Give any one use of Plaster of Paris other than for plastering or smoothening of walls.
Answer : (i) a) B
b) C,A,D,B
(ii) Due to moisture in the atmosphere it converted into Gypsum
CaSO₄ + 1½ H₂O → CaSO₄.2H₂O
(iii) Making toys/dolls or statues /fixing broken limbs/making decorative materials.