ACIDS, BASES AND SALTS (QUESTION ANSWER)

 ACIDS, BASES AND SALTS 

(QUESTION ANSWER)

QUESTIONS

1. You have been provided with three 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: When the red litmus paper was dipped one after another in the solutions in the three test tubes, the liquid that will make the red litmus paper blue will be basic. This blue litmus paper can now be used for acid testing. If this blue litmus paper is dipped one after another in the remaining two samples, the liquid which will change its color to red will be acid and the liquid which can not change the color of the blue litmus paper will be distilled water.

2. Why should curd and sour substances not be kept in brass and copper vessels?

Answer: Curd and sour foods are acidic. Brass and copper are metals. These acids react with brass and copper to form poisonous salts. These toxic salts can cause food poisoning. Therefore, curd and sour food should not be kept in brass and copper vessels.

3. 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: Hydrogen gas is liberated when an acid reacts with a metal. For example: when sodium metal reacts with hydrochloric acid to form sodium chloride and hydrogen gas.

                                Na(s) + HCl(aq)→ NaCl(aq) + H₂(g)

Test for hydrogen gas: If the gas produced from the reaction between metal and acid is passed through a soap solution, bubbles will form in the solution. When a burning candle is placed in the path of these bubbles, Hydrogen gas will burn with pop sound.

4. Metal compound A reacts with dilute hydrochloric acid 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 calcium chloride.  

Answer: The gas evolved that extinguishes a burning candle is carbon di oxide (CO2). The balanced chemical equation for the reaction is-

                            CaCO₃ (s) + 2HCl( l) → CaCl₂(aq) + H₂O (l) + CO₂(g)  

5. Why do HCl, HNO3, etc., show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character? 

Answer: HCl, HNO3, etc. produce hydrogen ions (H⁺) or hydronium ions (H₃O⁺) in aqueous solution. Therefore, they shows acidic character in aqueous solution. But compounds like alcohol and glucose cannot produce hydrogen ions (H⁺) or hydronium ions (H₃O⁺). Therefore, the solution of these compounds do not show acidic character.

6. Why does an aqueous solution of an acid conduct electricity? 

Answer: In aqueous solution an acid produces positive hydrogen ions (H⁺) or hydronium ions (H₃O⁺) and negative ions. As a liquid carries electricity as a result of the movement of ions, therefore, an aqueous solution of an acid carries electricity.

7. Why does dry HCl gas not change the colour of the dry litmus paper? 

Answer: Acids produce hydrogen ions (H⁺) or hydronium ions (H₃O⁺) only in aqueous solution, which is responsible for its acidic behaviour. Therefore, dry HCl does not change the color of dry litmus paper.

8. While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid? 

Answer: The process of mixing an acid or a base in water is a highly exothermic. The acid must always be added slowly to water with constant stirring. If water is added to a 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.

9. How is the concentration of hydronium ions (H3O+) affected when a solution of an acid is diluted? 

Answer: When a solution of an acid is diluted concentration of hydronium ions (H3O+) decreases. Consequently, strength of acid also decreases. 

10. How is the concentration of hydroxide ions (OH^- ) affected when excess base is dissolved in a solution of sodium hydroxide? 

Answer: Concentration of hydroxide ions (OH^-) will increase when excess base is dissolved in a solution of sodium hydroxide. Consequently strength of sodium hydroxide will also increase.

11. You have two solutions, A and B. The pH of solution A is 6 and pH of solution B is 8. Which solution has more hydrogen ion concentration? Which of this is acidic and which one is basic? 

Answer: A has more hydrogen ion concentration. A is acidic B is basic.

12. What effect does the concentration of H+(aq) ions have on the nature of the solution? 

Answer: The nature of any solution depends on the concentration of H⁺ ions in the liquid. As the concentration of H⁺ ions increases, the acidic nature of the solution increases. On the other hand, as the concentration of H⁺ ions decreases, the acidic nature of the solution decreases.

13. Do basic solutions also have H⁺(aq) ions? If yes, then why are these basic? 

Answer: Basic solutions contain H⁺ (aq) ion but its number is less than the number of OH^-ions. this is why basic solutions do not show acidic properties but shows basic properties.

14. Under what soil condition do you think a farmer would treat the soil of his fields with quick lime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate)?

Answer: Soil pH should be within a certain range for healthy growth of the crop. If the pH value of the soil moves towards excessive acidity, then the farmer has to apply quick lime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate) in the soil.

15. What is the common name of the compound CaOCl2? 

Answer: Common name of the compound CaOCl2 is bleaching powder.

16. Name the substance which on treatment with chlorine yields bleaching powder. 

Answer: The substance which on treatment with chlorine yields bleaching powder is slaked lime (calcium hydroxide) [Ca(OH)₂]

17. Name the sodium compound which is used for softening hard water. 

Answer: The sodium compound which is used for softening hard water is washing soda (Na₂CO_3.10 H₂O).

18. What will happen if a solution of sodium hydrocarbonate is heated? Give the equation of the reaction involved. 

Answer: If a solution of sodium hydrogencarbonate is heated  sodium carbonate (Na2CO3), carbon dioxide and water are produced.

                        NaHCO₃ ^Heat→ Na₂CO₃ + H₂O + CO₂

19. Write an equation to show the reaction between Plaster of Paris and water.

Answer:  Equation of the reaction between Plaster of Paris and water is-

                        CaSO₄.¹ /₂ H₂O + 1¹/₂ H₂0→ CaSO_4. 2H₂O 

EXERCISES 

1. A solution turns red litmus blue, its pH is likely to be 

(a) 1                  (b) 4                    (c) 5                    (d) 10 

Answer:         (d) 10

2. A solution reacts with crushed egg-shells to give a gas that turns lime-water milky. The solution contains 

(a) NaCl                 (b) HCl                 (c) LiCl                 (d) KCl 

Answer:    (b) HCl 

3. 10 mL of a solution of NaOH is found to be completely neutralised by 8 mL of a given solution of HCl. If we take 20 mL of the same solution of NaOH, the amount HCl solution (the same solution as before) required to neutralise it will be 

(a) 4 mL (b) 8 mL (c) 12 mL (d) 16 mL 

Answer:    (d) 16 mL 

4. Which one of the following types of medicines is used for treating indigestion? 

(a) Antibiotic             (b) Analgesic             (c) Antacid             (d) Antiseptic 

Answer:    (c) Antacid 

5. Write word equations and then balanced equations for the reaction taking place when – 

(a) dilute sulphuric acid reacts with zinc granules. 

(b) dilute hydrochloric acid reacts with magnesium ribbon. 

(c) dilute sulphuric acid reacts with aluminium powder. 

(d) dilute hydrochloric acid reacts with iron filings. 

Answer:

  (a)  Word equation-  Sulpheuric acid + Zinc → Zinc sulphate + Hydrogen

        Balanced equation- H₂SO₄(aq)  + Zn(s)  → ZnSO₄ (aq) + H₂(g)

(b)   Word equation- Hydrochloric acid  + Magnesium → Magnesium chloride + Hydrogen

        Balanced equation- 2HCl (aq) + Mg (s) → MgCl₂(aq)  + H₂ (g)

(c) Word equation-  Sulpheuric acid + Aluminium → Aluminium sulphate + Hydrogen

      Balanced equation- 3H₂SO₄(aq) + 2Al(s) → Al₂ (SO₄)₃ (aq) + 3H₂ (g)

(d) Word equation- Hydrochloric acid + Iron → Iron Chloride + Hydrogen

      Balanced equation- HCl(aq) + Fe(s) → FeCl₂ (aq) + H₂ (g)

7. Why does distilled water not conduct electricity, whereas rain water does?

Answer: Electricity is transmitted only through the movement of ions. No compounds like acid, alkali salts etc. are dissolved in the distilled water, so no ions are formed and as a result distilled water cannot conduct electricity. On the other hand, gases like carbon dioxide and sulfur dioxide are dissolved in rainwater. These gases produce carbonic acid, sulfuric acid etc. This acid dissolves in water and decomposes into ions. These ions conduct electricity.

8. Why do acids not show acidic behaviour in the absence of water? 

Answer: Acids produce ions only in the presence of water. In the absence of water, H + ions cannot be separated from acid. Therefore, in the absence of water, the acid does not show acidic behaviour.

9. 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. 

Answer: (a) Neutral- Solution D

                (b) Strongly alkaline-  Solution C

                (c) Strongly acidic- Solution B

                (d) weakly acidic- Solution A

                (e) weakly alkaline- solution E

    

    pH in increasing order of hydrogen-ion concentration is-

            P^H 11< P^H 9< P^H 7< P^H 4< P^H 1

10. Equal lengths of magnesium ribbons are taken in test tubes A and B. Hydrochloric acid (HCl) is added to test tube A, while acetic acid (CH3COOH) is added to test tube B. In which test tube will the fizzing occur more vigorously and why? 

Answer: Fizzing will occur more vigorously in test tube A, because hydrochloric acid is stronger than acetic acid and magnesium will react more vigorously with hydrochloric acid than with acetic acid.

11. Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer. 

Answer: The pH value of raw milk is 6. This means it is mildly acidic. If it turns into curd, its acidity  will increase and its pH will decrease to less than 6.

12. 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? 

Answer: (a) A milkman adds a small amount of baking soda to the raw milk and makes the milk a little more alkaline so that the milk does not curdle quickly.

(b) Lactic acid is produced in milk when it turns into curd. If baking soda is added to milk then the lactic acid will have to neutralize the base before it can turn the milk into curd. So the milk will take a long time to set as a curd.

13. Plaster of Paris should be stored in a moisture-proof container. Explain why? 

Answer: Plaster of Paris absorbs moisture from air and converts back to gypsum and gets harden. So, it should always be stored in a moisture-proof container.

14. What is a neutralization reaction? Give two examples. 

Answer: When acid and base react with each other, they neutralizes each other to form salt and water. This reaction is called neutralization reaction. Examples are-

        NaOH (ক্ষাৰক) + HCl (এছিড)→ NaCl (লৱণ) + H₂O (পানী)

         KOH (ক্ষাৰক) + HCl (এছিড)→ KCl (লৱণ) + H₂O (পানী) 

15. Give two important uses of washing soda and baking soda. 

Answer:  Two important uses of baking soda are-

        (i) Sodium hydrogen carbonate is an ingredient in antacids. Being alkaline, it neutralizes excess acid in the stomach and provides relief.

         (ii) It is also used in soda-acid fire extinguishers.

 Two important uses of washing soda are- 

        (i) Sodium carbonate can be used as a cleaning agent for domestic purposes. 

        (ii) It is used for removing permanent hardness of water.