Solutions - Acids, Bases and Salts

On Page 18

Question 1: You have been provided with 3 test tubes, one of them contains distilled water, and the other two contain an acidic solution and a basic solution, respectively. If you are given only red litmus paper, how will you identify the contents of each test tube?

Answer: First, we dip the red litmus paper in each of the 3 test tubes. The tube in which it turns blue indicates that the solution is basic. Next, we take the blue litmus paper formed above and dip it in the remaining two tubes. The tube in which it changes back to red indicates that the solution is acidic. The last tube, in which neither the red litmus turns blue nor the blue litmus turns red, contains distilled water.

On Page 22

Question 1: Why should curd and sour substances not be kept in brass and copper vessels?

Answer: Curd and sour substances are acidic in nature. The acid present in curd and sour substances reacts with the metal surface of brass and copper vessels, producing toxic compounds that are not fit for consumption. Therefore, curd and sour substances should not be kept in brass and copper vessels.

Question 2: Which gas is usually liberated when an acid reacts with a metal? Illustrate with an example. How will you test for the presence of this gas?

Answer: When an acid reacts with a metal, hydrogen gas (H2) is usually liberated. For example, when magnesium metal reacts with sulphuric acid, it forms magnesium sulphate and releases hydrogen gas: Mg(s) + H2SO4(aq) → MgSO4(aq) + H2(g) To test for the presence of this gas, we can pass it through soap solution and then bring a burning candle near a gas-filled bubble. The soap bubble bursts, and the hydrogen gas burns with a “pop” sound.

Question 3: Metal compound A reacts with dil. HCl to produce effervescence. The gas evolved extinguishes a burning candle. Write a balanced chemical equation for the reaction if one of the compounds formed is CaCl2.

Answer: According to the question, the reaction can be represented as: A + HCl(aq) → Effervescence (gas) → Extinguishes a burning candle Since the gas extinguishes a burning candle, it may be carbon dioxide (CO2). When metal carbonates and bicarbonates react with dilute acids, carbon dioxide is produced with brisk effervescence. Therefore, the balanced chemical equation can be written as: A + 2HCl(aq) → CaCl2(aq) + CO2(g) + H2O(l) Hence, compound A is calcium carbonate (CaCO3).

On Page 25

Question 1: Why do HCl, HNO3, etc., show acidic characters in aqueous solutions while solutions of compounds like alcohols and glucose do not show acidic character?

Answer: Acids like HCl and HNO3 completely ionize in water (aqueous solutions) to form hydronium ions (H3O+). This is why they show acidic behavior in aqueous solutions. On the other hand, compounds like alcohols and glucose do not ionize in aqueous solutions to give hydronium ions. Therefore, they do not show acidic character.

Question 2: Why does an aqueous solution of an acid conduct electricity?

Answer: An aqueous solution refers to a solution in water. When an acid is added to water, it completely ionizes to form positively charged cations and negatively charged anions. These ions can move towards opposite electrodes, allowing the solution to conduct electricity.

Question 3: Why does dry HCl gas not change the color of dry litmus paper?

Answer: Dry HCl gas cannot ionize. In order for a reaction to take place, dry HCl gas must first form ions. HCl ionizes in an aqueous medium to form H+ ions. Therefore, dry HCl gas does not change the color of dry litmus paper. To show its acidic behavior, it needs wet litmus paper.

Question 4: While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?

Answer: Diluting a concentrated acid is a highly exothermic reaction, generating a lot of heat. Care must be taken when mixing concentrated acid with water. The acid should always be added slowly to water with constant stirring. If water is added to the concentrated acid, the heat generated may cause the mixture to splash out and cause burns. The glass container may also break due to excessive local heating.

Question 5: How is the concentration of H3O+ ions affected when a solution of an acid is diluted?

Answer: When an acid is dissolved in water, it dissociates into hydronium ions (H3O+) and anions. When a solution is diluted, the volume of the solution increases, but the number of ions remains the same. As a result, the concentration of hydronium ions per unit volume decreases.

Question 6: How is the concentration of hydroxide ions (OH-) affected when excess base is dissolved in a solution of NaOH?

Answer: The concentration of hydroxide ions (OH-) increases when excess base is dissolved in a solution of NaOH.

On Page 28

Question 1: You have two solutions, A and B. The pH of solution A is 6, and the pH of solution B is 8. Which solution has a higher concentration of H+ ions? Which one is acidic and which one is basic?

Answer: Solution A is acidic in nature because its pH is less than 7, and solution B is basic in nature because its pH is greater than 7. The higher the pH of a solution, the lower the concentration of H+ ions. Therefore, solution A has a higher concentration of H+ ions.

Question 2: What effect does the concentration of H+ ions have on the nature of the solution?

Answer: The higher the concentration of H+ ions in a solution, the more acidic the solution is. The concentration of hydrogen ions is directly related to the acidity of the solution.

Question 3: Do basic solutions also have H+ ions? If yes, then why are they basic?

Answer: Yes, basic solutions also have H+ ions in addition to OH- ions, but the concentration of OH- ions will be much higher than that of H+ ions since it is a basic solution. The presence of a higher concentration of OH- ions gives the solution its basic nature.

Question 4: Under what soil condition do you think a farmer would treat the soil of his field with quicklime (CaO), slaked lime (Ca(OH)2), or chalk (CaCO3)?

Answer: Slaked lime, quicklime, and chalk are all basic in nature. Therefore, if the soil is acidic, the farmer will add these basic substances to make the soil pH neutral (pH = 7). Maintaining a pH value of 7 is crucial for the healthy growth of plants.

On Page 33

Question 1: What is the common name of the compound CaOCl2?

Answer: The common name of the compound CaOCl2 is bleaching powder.

Question 2: Name the substance which, on treatment with chlorine, yields bleaching powder.

Answer: Dry slaked lime [Ca(OH)2] yields bleaching powder when treated with chlorine.

Question 3: Name the sodium compound used for softening hard water.

Answer: The sodium compound used for softening hard water is washing soda (sodium carbonate).

Question 4: What will happen if a solution of NaHCO3 is heated? Give the equation of the reaction involved.

Answer: When a solution of NaHCO3 is heated, it undergoes decomposition to produce carbon dioxide, water, and sodium carbonate. The reaction can be represented as: 2NaHCO3 → Na2CO3 + CO2 + H2O

Question 5: Write an equation to show the reaction between plaster of Paris and water.

Answer: The reaction between plaster of Paris (CaSO4·1/2H2O) and water can be represented as: CaSO4·1/2H2O + 1½H2O → 2CaSO4·2H2O (Gypsum)

Exercises

Question 1: When a solution turns red litmus paper blue, what is its pH likely to be? Answer: If the solution turns red litmus paper blue, it indicates that the solution is basic. The pH of a basic solution is greater than 7.

Question 2: A solution reacts with crushed eggshells to produce a gas that turns lime water milky. What does the solution contain? Answer: The solution contains hydrochloric acid (HCl). When HCl reacts with the calcium carbonate in crushed eggshells, it produces carbon dioxide (CO2) gas, which turns lime water milky.

Question 3: If 10 mL of a NaOH solution is completely neutralized by 8 mL of HCl solution, how much HCl solution would be required to neutralize 20 mL of the same NaOH solution? Answer: Since both solutions have the same strength, if we double the amount of the NaOH solution to 20 mL, we would need twice the amount of HCl solution. Therefore, 16 mL of the HCl solution would be required to neutralize it.

Question 4: Which type of medicine is used for treating indigestion? Answer: Antacids are used for treating indigestion.

Question 5: Write the word equation and balanced equation for the following reactions: (a) Dilute sulphuric acid reacts with zinc granules. Word equation: Zinc + dilute sulphuric acid → zinc sulphate + hydrogen gas Balanced equation: Zn + H2SO4 → ZnSO4 + H2

(b) Dilute hydrochloric acid reacts with magnesium ribbon. Word equation: Magnesium + dilute hydrochloric acid → magnesium chloride + hydrogen gas Balanced equation: Mg + 2HCl → MgCl2 + H2

(c) Dilute sulphuric acid reacts with aluminium powder. Word equation: Aluminium + dilute sulphuric acid → aluminium sulphate + hydrogen gas Balanced equation: 2Al + 3H2SO4 → Al2(SO4)3 + 3H2

(d) Dilute hydrochloric acid reacts with iron filings. Word equation: Iron + dilute hydrochloric acid → iron(II) chloride + hydrogen gas Balanced equation: Fe + 2HCl → FeCl2 + H2

Question 6: Compounds such as alcohols and glucose contain hydrogen but are not categorized as acids. Describe an activity to prove this. Answer: Alcohols and glucose do not ionize in a solution to produce H+ ions and, therefore, cannot conduct electricity. To demonstrate this, set up the following apparatus: a bulb, a 6V battery, a switch, a beaker, a nail, a solution of dilute hydrochloric acid (HCl), and rubber cork. Observe that the bulb glows when dilute HCl is added to the beaker but does not glow when alcohol or glucose solutions are added. This activity shows that alcohol and glucose cannot form ions in a solution and cannot conduct electricity, while dilute HCl solution forms H+ ions and can conduct electricity.

Question 7: Why does distilled water not conduct electricity, whereas rainwater does? Answer: Distilled water does not conduct electricity because it is free from dissolved ions. On the other hand, rainwater dissolves atmospheric gases such as CO2, SO2, and NO2, forming acids like H2CO3, H2SO4, and HNO3. Although the amount of these acids in rainwater is low, they can easily dissociate into ions, enabling rainwater to conduct electricity.

Question 8: Why do acids not exhibit acidic behavior in the absence of water? Answer: Acids require water to ionize and exhibit acidic behavior. Only in the presence of water can acids dissociate and form ions, which is essential for them to show acidic properties. For example, hydrochloric acid (HCl) ionizes in water to form H+ ions and chloride ions (Cl-).

Question 9: Five solutions, A, B, C, D, and E, showed pH values of 4, 1, 11, 7, and 9 respectively when tested with universal indicator. Determine which solution is (a) neutral, (b) strongly alkaline, (c) strongly acidic, (d) weakly acidic, and (e) weakly alkaline. Arrange the pH values in increasing order of H+ ion concentration. Answer: (a) Solution D is neutral because its pH is 7. (b) Solution C is strongly alkaline because its pH is 11. (c) Solution B is strongly acidic because its pH is 1. (d) Solution A is weakly acidic because its pH is 4. (e) Solution E is weakly alkaline because its pH is 9. The order of H+ ion concentration in increasing order is: C < E < D < A < B.

Question 10: Equal lengths of magnesium (Mg) ribbons are taken in test tubes A and B. HCl is added to test tube A, while acetic acid (CH3COOH) is added to test tube B. In which test tube will fizzing occur more vigorously and why? Answer: Fizzing will occur more vigorously in test tube A with magnesium ribbon and HCl. This is because HCl is a stronger acid than acetic acid. HCl produces a larger number of H+ ions, leading to a faster and more rapid release of hydrogen gas (H2), which is responsible for the fizzing.

(a) Magnesium + hydrochloric acid → magnesium chloride + hydrogen gas Mg + 2HCl → MgCl2 + H2

(b) Magnesium + acetic acid → magnesium acetate + hydrogen gas Mg + 2CH3COOH → (CH3COO)2Mg + H2

Question 11: Fresh milk has a pH of 6. How does the pH change as it turns into curd? Explain your answer. Answer: During the curdling of milk, the fermentation of lactose (milk sugar) to lactic acid occurs. This formation of lactic acid lowers the pH of the curd. As a result, curd becomes acidic with a pH lower than 6, which is the pH of fresh milk.

Question 12: A milkman adds a very small amount of baking soda to fresh milk. (a) Why does he shift the pH of fresh milk from 6 to slightly alkaline? (b) Why does this milk take a longer time to set as curd? Answer: (a) The milkman shifts the pH of fresh milk from 6 to slightly alkaline by adding a very small amount of baking soda. This is done to prevent the spoilage of milk because milk sours easily at lower pH levels. The slight alkalinity helps in maintaining the freshness of the milk. (b) The presence of baking soda makes the milk alkaline. For the curdling of milk, the fermentation of lactose to lactic acid is required, which happens under acidic conditions. Therefore, when the milk is slightly alkaline, it takes a longer time to set as curd because the required acidic pH is not achieved quickly.

Question 13: Why should Plaster of Paris be stored in airtight moisture-proof containers? Answer: Plaster of Paris (POP) is chemically calcium sulfate hemihydrate (CaSO4·1/2H2O). When POP comes in contact with water, it sets into a hard solid mass called gypsum (CaSO4·2H2O). To prevent the premature setting of POP due to moisture in the air, it should be stored in airtight moisture-proof containers.

Question 14: What is a neutralization reaction? Give two examples. Answer: A neutralization reaction is the reaction between an acid and a base, resulting in the formation of water and a salt. Examples:

NaOH + HCl → NaCl + H2O (Sodium hydroxide + Hydrochloric acid → Sodium chloride + Water)
2KOH + H2SO4 → K2SO4 + 2H2O (Potassium hydroxide + Sulfuric acid → Potassium sulfate + Water)

Question 15: Give two important uses of washing soda and baking soda. Answer: Washing soda (Na2CO3·10H2O):

Used for removing permanent hardness of water.
Used as a cleaning agent for domestic purposes.

Baking soda (NaHCO3):

Used in soda-acid fire extinguishers.
Used as an ingredient in antacids to neutralize excess acid in the stomach.

Selected NCERT Exemplar Problems

Multiple Choice Questions

Question 1: What happens when a solution of an acid is mixed with a solution of a base in a test tube? (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. Answer: (d) Because when an acid reacts with a base, a neutralization reaction takes place, forming salt and water. This reaction is exothermic, resulting in an increase in temperature.

Question 2: An aqueous solution turns red litmus solution blue. Excess addition of which of the following solutions would reverse the change? (a) Baking powder (b) Lime (c) Ammonium hydroxide solution (d) Hydrochloric acid Answer: (d) Because the aqueous solution that turns red litmus solution blue is a base, and hydrochloric acid is an acid. Adding hydrochloric acid to the solution would neutralize the base and reverse the color change.

Question 3: During the preparation of hydrogen chloride gas on a humid day, the gas is usually passed through a guard tube containing calcium chloride. The role of calcium chloride taken in the guard tube is to: (a) Adsorb the evolved gas. (b) Moisten the gas. (c) Adsorb moisture from the gas. (d) Adsorb Cl- ions from the evolved gas. Answer: (c) Because calcium chloride is a good dehydrating agent, it helps in adsorbing or removing moisture from the gas during the preparation of hydrogen chloride.

Question 4: Which of the following salts does not contain water of crystallization? (a) Blue vitriol (b) Baking soda (c) Washing soda (d) Gypsum Answer: (b) Baking soda (NaHCO3) does not contain water of crystallization. Blue vitriol has 5 molecules of water (CuSO4·5H2O), washing soda has 10 molecules of water (Na2CO3·10H2O), and gypsum has 2 molecules of water (CaSO4·2H2O).

Question 5: Sodium carbonate is a basic salt because it is a salt of: (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) Sodium carbonate (Na2CO3) is a salt of a weak acid (carbonic acid) and a strong base (sodium hydroxide), making it a basic salt.

Question 6: Calcium phosphate is present in tooth enamel. Its nature is: (a) Basic. (b) Acidic. (c) Neutral. (d) Amphoteric. Answer: (c) Calcium phosphate, present in tooth enamel, is a salt that has a neutral nature.

Question 7: 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 color of this pH paper to greenish-blue? (a) Lemon juice. (b) Vinegar. (c) Common salt. (d) An antacid. Answer: (d) An antacid would change the color of the pH paper to greenish-blue. The pH paper turns greenish-blue for weakly basic compounds, and antacids contain weak bases like sodium bicarbonate (NaHCO3).

Question 8: Which of the following gives the correct increasing order of acid strength? (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) Hydrochloric acid is a mineral acid that completely ionizes in water, making it a stronger acid. Acetic acid is an organic acid that only partially ionizes in water, making it a weaker acid. Water itself has a relatively neutral nature. Thus, the correct order of increasing acid strength is water < acetic acid < hydrochloric acid.

Question 9: If a few drops of concentrated acid accidentally spill over the hand of a student, what should be done? (a) Wash the hand with saline solution. (b) Wash the hand immediately with plenty of water and apply a paste of sodium hydrogen carbonate. (c) After washing with plenty of water, apply a solution of sodium hydroxide on the hand. (d) Neutralize the acid with a strong alkali. Answer: (b) In case of accidental spillage of concentrated acid on the hand, it should be immediately washed with plenty of water to dilute and remove the acid. Applying a paste of sodium hydrogen carbonate (baking soda), which is a mild base, can help neutralize any remaining acid on the hand.

Question 10: Sodium hydrogen carbonate, when added to acetic acid, evolves a gas. Which of the following statements are true about the gas evolved? (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 odor. (a) (i) and (ii). (b) (i), (ii), and (iii). (c) (ii), (iii), and (iv). (d) (i) and (iv).

Answer: (b) When sodium hydrogen carbonate reacts with acetic acid, carbon dioxide gas is evolved. The evolved gas turns lime water milky (indicating the presence of carbon dioxide), extinguishes a burning splinter (indicating the absence of oxygen), and dissolves in a solution of sodium hydroxide. However, it does not have a pungent odor.

Question 11. Common salt besides being used in kitchen can also be used as the raw material for making

(i) washing soda (ii) bleaching powder (iii) baking soda (iv)slaked lime

(a) (i) and (ii) (b) (i), (ii) and (iv)

Answer (c) Baking powder contains NaHCO₃and tartaric acid.

Question 12. One of the constituents of baking powder is sodium hydrogen carbonate, the other constituent is

(a) hydrochloric acid (b) tartaric acid

Answer (b) Baking power contains NaHCO₃and tartaric acid.

Question 13. To protect tooth decay we are advised to brush our teeth regularly. The nature of the tooth paste commonly used is (a) acidic (b) neutral (c) basic (d) corrosive

Answer (c) The tooth paste commonly used is basic so that the extra acid formed during tooth decay is neutralised.

Question 14. Which of the following statements is correct about an aqueous solution of an acid and 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

(a) (i) and (iii) (b) (ii) and (iii) (c) (i) and (iv) (d) (ii) and (iv)

Answer (d) i.e., higher the pH, weaker the acid and lower the pH, weaker the base.

Question 15. The pH of the gastric juices released during digestion is (a) less than 7 (b) more than 7

Answer (a) Because of the presence of strong acid, HCl.

Question 16. 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 (a) (i) and (ii) (b) (i) and (iii) (c) (ii) and (iii) (d) (ii) and (iv)

Answer (b) When acid is added to water, it forms ions and the concentration of acid also decreases.

Question 17. Which one of the following can be used as an acid-base indicator by a visually impared student?

(a) Litmus (b) Turmeric

Answer (c) Vanilla essence because its smell is different in acid and basic media which can be detected easily by a visually impared student..

Question 18. Which of the following substances will not give carbon dioxide on treatment with dilute acid?

(a) Marble (b) Limestone (c) Baking soda (d) Lime

Answer (d) Lime, CaO (calcium oxide) does not evolve CO₂when reacted with dilute acid.Other given compounds are carbonates and hydrogen carbonates, so evolve CO₂with dilute acid.

Question 19. Which of the following is acidic in nature? (a) Lime juice (b) Human blood

Answer (a) Because it contains citric acid.

Question 20. In an attempt to demonstrate electrical conductivity through an electrolyte, the following apparatus (figure) was set up.

Which among the following statement(s) is (are) correct? (i) Bulb will not glow because 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 electolytic solution.

(a) (i) and (iii) (b) (ii) and (iv)

Answer (c) NaOH being a strong base, provides Na⁺and OH⁻ions, which are responsible for conduction and hence, bulb glows.

Question 21. Which of the following is used for dissolution of gold? (a) Hydrochloric acid

(b) Sulphuric acid

(d) Aqua-regia

Answer (d) Aqua-regia is used for the dissolution of gold.

Question 22. Which of the following is not a mineral acid? (a) Hydrochloric acid (b) Citric acid

Answer (b) Because citric acid it is an example of an organic acid.

Question 23. Which among the following is not a base? (a) NaOH (b) KOH

Answer (d) C H OH _{2 5}is not a base, it is an example of an organic compound known as alcohol (ethyl alcohol) which has somewhat acidic nature.

Question 24. Which of the following statements is not correct? (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) Most metal oxides are insoluble in water but some of these dissolve in water to form alkalies e.g.,

Na2O(s) + H2O(l) -> 2NaOH(aq)

In this reaction, solid sodium oxide (Na2O) reacts with liquid water (H2O) to form aqueous sodium hydroxide (NaOH).

Question 25. Match the chemical substances given in Column (I) with their appropriate application given in Column (II).

Column (I) Column (II)

A. Bleaching powder (i) Preparation of glass

B. Baking soda (ii) Production of H₂andCl₂

C. Washing soda (iii) Decolourisation

D. Sodium chloride (iv) Antacid

(a) A-(ii), B-(i), C-(iv), D-(iii)

(b) A-(iii), B-(ii), C-(iv), D-(i)

(d) A-(ii), B-(iv), C-(i), D-(iii)

Answer Bleaching powder bleaches the clothes and other coloured substances, baking soda is a constituent of antacid, washing soda is used in the preparation of glass and sodium chloride when subjected to electrolyses gives H₂and Cl₂gases.

Question 26. Equal volumes of hydrochloric acid and sodium hydroxide solutions of same concentration are mixed and the pH of the resulting solution is checked with a pH paper. What would be the colour obtained? (You may use colour guide given in figure 2.7 of NCERT Book (Science Class X) on page 26).

(a) Red (b) Yellow

Answer (c) Because the resulting solution is obtained as a result of neutralisation reaction.

Question 27. 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 ion in the solution. (iv) It forms hydronium ion in the solution due to the combination of hydrogen ion with water molecule.

(a) (i) only (b) (iii) only

Question 28. Which of the following statements is true for acids?
(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) Acids are sour in taste and change blue litmus to red.
Question 29. Which of the following are present in a dilute aqueous
solution of hydrochloric acid?

Question 30. Identify the correct representation of reaction occurring
during chloralkali process

Short Answer Type Questions
Question 31. Match the acids given in Column (I) with their correct
source given in Column (II).
Column (I) Column (II)
(a) Lactic acid (i) Tomato
(b) Acetic acid (ii) Lemon
(c) Citric acid (iii) Vinegar
(d) Oxalic acid (iv) Curd
Answer
Column (I) Column (II)
(a) Lactic acid (iv) Curd
(b) Acetic acid (iii) Vinegar
(c) Citric acid (ii) Lemon
(d) Oxalic acid (i) Tomato

Question 32. Match the important chemicals given in Column (I) with
the chemical formulae given in Column (II).
Column (I) Column (II)
(a) Plaster of Paris (i) Ca(OH)2
(b) Gypsum (ii) CaSO
(c) Bleaching powder (iii) CaSO4 2H2O
(d) Slaked lime (iv) CaOCl2
Answer
Column (I) Column (II)
(a) Plaster of Paris (ii) CaSO
(b) Gypsum (iii) CaSO4 2H2O ×
(c) Bleaching powder (iv) CaOCl2
(d) Slaked lime (i) Ca(OH)2
Question 33. 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.
Answer

Dry HCl gas No change
Moistened NH3 gas Red litmus will turn blue.
Lemon juice Blue litmus will turn red.
Carbonated soft drinks Blue litmus will turn red.
Curd Blue litmus will turn red.
Soap solution Red litmus will turn blue.
Question 34. 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.
Answer The acid is formic acid (HCOOH). If baking soda (a base) is applied
on the sting (biting area), it gives relief.
Question 35. What happens when nitric acid is added to egg shell?
Answer Egg shells are made up of calcium carbonate, CaCO3.
So, brisk effervescence due to the liberation of CO2 gas is observed. The
reaction is

Question 36. 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?
Answer For distinguishing the two solutions she can taste the solution. A
sour solution represent an acid and the non-sour solution is the base. You can
use indicators because different indicators give different colours in the
presence of acid or base.
So, to distinguish solution (i) an acid and (ii) a base, we use phenolphthalein
solution. Solution (i) will remain colourless while solution (ii) will give pink
colouration.
Hence, we can say solution (i) is an acid and solution (ii) is a base.
Question 37. How would you distinguish between baking powder and
washing soda by heating?
Answer On heating NaHCO3 (baking soda), CO2 (carbon dioxide) gas is
given out that turns lime water milky.

While on heating Na CO 10H O 2 3 2 × (washing soda) water of crystallisation is
given out and the salt becomes anhydrous. The presence of water of
crystallisation given as product can be tested by treating it with anhydrous
CuSO4 (white) which becomes blue in colour in its contact.

Question 38. 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.
Answer Salt A is sodium bicarbonate. (as it is used in bakery products and
gives Na CO 2 3 on heating).
Salt B is sodium carbonate.
Gas C is carbon dioxide. (as it turn lime water milky)
This can be shown as follows :

Question 39. In one of the industrial processes 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.
Answer Chlorine gas reacts with lime water to give bleaching powder, a
bleaching agent. Thus, X is chlorine gas (Cl2 gas).
Y is calcium oxychloride or bleaching powder (CaOCl ) 2 .
The equation for the preparation of sodium hydroxide is

Question 41. 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.
Answer Strong acid The acid that vigorously ionises in aqueous solution
thus producing a high concentration of H O 3

ions is called a strong acid, e.g.,
HCl, H SO 2 4.
Weak acid Weak acid ionise only partially in aqueous solution and thus they
produce ions as well as molecules, e.g., acetic acid, carbonic acid.

Question 42. 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.

Long Answer Type Questions

Question 43. In the following schematic diagram for the preparation
of hydrogen gas as shown in the figure, what would happen if the
following changes are made?
(a) In place of zinc granules, same amount of zinc dust is taken in the
test tube.

(b) Instead of dilute sulphuric acid, dilute hydrochloric acid is taken.
(c) In place of zinc, copper turnings are taken.
(d) Sodium hydroxide is taken in place of dilute sulphuric acid and the
tube is heated.
Answer
(a) The reaction will slow down.
(b) No change will take place, because both are strong acids.
(c) Cu does not displace H2 from dilute acids because it is less reactive.
So, no reaction will take place.
(d) No change because the following reaction will take place

Question 44. For making cake, baking powder is taken. If at home
your mother uses baking soda instead of baking powder in cake.
(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
(a) Baking powder has tasty tartaric acid which reacts with Na CO 2 3
produced during decomposition of NaHCO3 and neutralises it, so if
tartaric acid is not present, the cake will taste bitter due to the presence
of sodium carbonate (Na C ) 2 3 O .
(b) By adding tartaric acid to baking soda we can form baking powder.
(c) Tartaric acid neutralizes the sodium carbonate formed during
decomposition of NaHCO3 hence making the cake tasty and not bitter in
taste.
Question 45. 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.
Answer X is calcium carbonate.

Question 46. 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.
Answer Base ‘B’ is NaOH (sodium hydroxide).
If CO2 is the acidic oxide taken, the following reaction takes place with ‘B’.

Question 47. 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.
Answer The sulphate salt is calcium sulphate hemihydrate,
plaster of Paris.
When left exposed to atmosphere it takes up the moisture from air and changes
to gypsum.

Gypsum is a hard solid mass.

Question 48. Identify the compound X on the basis of the reactions
given below. Also, write the name and chemical formula of A, B and C.

Solutions – Acids, Bases and Salts

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