Chapter 2 : Acids , Bases and Salts

Internal Question and Answer :

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: Dip the litmus paper in the first test tube. If it remains red, it is distilled water.

Dip the litmus paper in the second test tube. If it turns blue, it is a basic solution.

Dip the litmus paper in the third test tube. If it remains red, it is an acidic solution.

Internal Question and Answer :

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

Answer: Curd and sour substances should not be kept in brass and copper vessels because they contain acids that can react with the metal, leading to the formation of harmful compounds that can be toxic if consumed. 

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: Hydrogen gas is usually liberated when an acid reacts with a metal. For example, when hydrochloric acid reacts with zinc, it produces zinc chloride and hydrogen gas. The presence of hydrogen gas can be tested by bringing a lit splint near the mouth of the test tube, where it will produce a 'pop' sound.

3. 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: Metal compound A (calcium,) reacts with dilute hydrochloric acid to produce effervescence () and hydrogen gas () .

 

Internal Question and Answer :

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

Answer:    , , etc., show acidic character in aqueous solutions because they ionize completely, releasing hydrogen ions (H+). Alcohol and glucose, being covalent compounds, do not ionize in water and therefore do not exhibit acidic character.

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

Answer: An aqueous solution of an acid conducts electricity because acids ionize in water, producing positively charged hydrogen ions () and negatively charged ions. These ions allow for the flow of electric current through the solution.

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

Answer : Dry HCl gas does not change the color of dry litmus paper because litmus paper requires the presence of water (moisture) to exhibit a color change. In the absence of water, the dry litmus paper remains unaffected by dry HCl gas.

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

Answer:  When diluting an acid, it is recommended to add the acid to water slowly because the reaction is exothermic. Adding water to concentrated acid can cause a rapid release of heat, leading to splattering and potential hazards.

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

Answer: The concentration of hydronium ions () decreases when a solution of an acid is diluted because dilution increases the volume of the solution while keeping the amount of acid constant.

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

Answer:  The concentration of hydroxide ions (OH-) increases when excess base is dissolved in a solution of sodium hydroxide. The additional base contributes more hydroxide ions, raising their concentration in the solution.

Internal Question and Answer :

1. 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 :  Solution A has a higher concentration of hydrogen ions than solution B.

Solution A (pH = 6): Since 6 is less than 7, solution A is acidic.

Solution B (pH = 8): Since 8 is greater than 7, solution B is basic.

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

Answer: The concentration of  (aq) ions determines the acidity of the solution. Higher concentrations of  ions make the solution more acidic, while lower concentrations result in a less acidic or even basic solution.

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

Answer: Yes, even basic solutions contain a small concentration of (aq) ions. However, their concentration is significantly lower compared to acidic solutions. Basic solutions are characterized by a higher concentration of  ions, which make the solution alkaline or basic in nature.

4. 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:  A farmer would typically treat the soil with quicklime (calcium oxide), slaked lime (calcium hydroxide), or chalk (calcium carbonate) in acidic soil conditions. These substances help neutralize the acidity and raise the pH level of the soil, making it more suitable for cultivation.

Internal Question and Answer :

1. What is the common name of the compound  ?

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

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

Answer: The substance that, when treated with chlorine, yields bleaching powder is calcium hydroxide [  ].

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

Answer: The sodium compound commonly used for softening hard water is sodium carbonate ( ), also known as soda ash or washing soda.

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

Answer:  When a solution of sodium hydrogen carbonate (sodium bicarbonate, ) is heated, it undergoes a decomposition reaction, producing water, carbon dioxide gas, and sodium carbonate () as products. The equation for the reaction is as follows:

 

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

Answer : The reaction between Plaster of Paris and water :

 

Exercises of Chapter 2: Acids , Bases and Salts:

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) 16mL

[ Hint: 10 mL of a solution of NaOH is found to be completely neutralised by 8 mL of a solution HCl .

So, 20 mL of a solution of NaOH is found to be completely neutralised by = 2 × 8 mL = 16 mL of a solution HCl . ]

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) 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.
 

6. Compounds such as alcohols and glucose also contain hydrogen but are not categorised as acids. Describe an Activity to prove it.

Answer: Set up a circuit with two nails fixed in a cork, placed in a beaker, and connected to a 6-volt battery through a bulb and switch. Pour dilute HCl and turn on the current; the bulb lights up, indicating conductivity due to ionization (acid). Repeat with dilute H₂SO₄, and the bulb also lights up. Now, repeat with glucose and alcohol solutions; the bulb remains off, proving they don't ionize like acids. Because, glucose and alcohols do not ionize in solution, hence they don't conduct electricity, unlike acids that release ions.

7. Why does distilled water not conduct electricity, whereas rain water does?
Answer: Distilled water, with minimal impurities, lacks ions necessary for conducting electricity, thus remaining a poor conductor. Rainwater, however, picks up ions (e.g., from atmospheric gases and dust) as it falls, enhancing its conductivity. These ions enable the flow of electric current, rendering rainwater a better conductor compared to distilled water.

8. Why do acids not show acidic behaviour in the absence of water?
Answer: Acids require water to exhibit acidic behavior because they ionize in aqueous solutions, releasing hydrogen ions ( ). In the absence of water, acids cannot dissociate into ions and, therefore, do not display their characteristic acidic properties.

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 :  The  value of B is 1 .

The  value of C is 11.

The  value of D is 7 .

The  value of E is 9 .

(a) The solution D is neutral .

(b) The solution C is strongly alkaline .

(c) The solution B is strongly acidic .

(d) The solution E is week acid .

(e) The solution A is week alkaline .

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 () is added to test tube B. In which test tube will the fizzing occur more vigorously and why?
Answer: The fizzing will occur more vigorously in test tube A, where hydrochloric acid () is added to the magnesium ribbon. This is because  is a stronger acid compared to acetic acid (). Strong acids, like , ionize completely in water, producing a higher concentration of hydrogen ions (). The higher concentration of  ions leads to a more vigorous reaction with magnesium, resulting in increased fizzing.

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:  As milk turns into curd, the pH typically decreases. This is because bacteria ferment lactose, the sugar in milk, into lactic acid. The accumulation of lactic acid lowers the pH, making the curd more acidic compared to fresh milk.

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) The milkman shifts the pH of fresh milk to slightly alkaline (above pH 7) by adding baking soda (sodium bicarbonate) to neutralize any acidic components present. This prevents souring and prolongs the milk's shelf life.

(b) The milk takes longer to set as curd because the alkaline environment created by the baking soda slows down the bacterial activity responsible for converting lactose into lactic acid. As a result, the acidification process necessary for curd formation is delayed, leading to a longer setting time.

13. Plaster of Paris should be stored in a moisture-proof container. Explain why?
Answer:  Plaster of Paris is a white powder and on mixing with water, it changes to gypsum once again giving a hard solid mass.

So, Plaster of Paris should be stored in a moisture-proof container .

14. What is a neutralisation reaction? Give two examples.
Answer: The reaction between an acid and a base to give a salt and water is called a neutralisation reaction.

Two examples are :

(i)

(ii) 

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

Answer: Uses of washing soda :

(i) Washing soda is used for removing permanent hardness of water.

(ii) Washing soda  is used in glass, soap and paper industries.

 Uses of baking soda :

(i) Baking soda is used in soda-acid fire extinguishers.

(ii) For making baking powder, which is a mixture of baking soda and a mild edible acid such as tartaric acid.