In Class 10 Science, Chapter 2: Acid, Bases, and Salts (NCERT, NBSE, SEBA, TBSE etc.), we learn about acids, bases, and salts – three important categories of chemicals that play a crucial role in various chemical reactions and processes. Acids are substances that release hydrogen ions (H+) when dissolved in water and have a sour taste. They have a pH value less than 7 and turn litmus paper red. Bases, on the other hand, are substances that release hydroxyl ions (OH-) when dissolved in water and have a bitter taste. They have a pH value greater than 7 and turn litmus paper blue. Salts are compounds that are formed when an acid reacts with a base, resulting in the neutralization of both. In this chapter, we will learn about the properties, reactions, and uses of acids, bases, and salts, and how they can be identified and measured using various techniques.
Below are questions, answers, MCQs, and textbook solutions for the chapter. If you are a registered user, you can also download it in PDF by clicking on the PDF icon.
Intext / Page 18
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: If the colour of red litmus does not change then it is acid. If the colour of red litmus changes to blue then it is base. If there is a slight change in the colour of red litmus (such as purple) then it is distilled water.
Intext / Page 22
1. Why should curd and sour substances not be kept in brass and copper vessels?
Answer: Curd and other sour substances contain acids. Therefore, when they are kept in brass and copper vessels, the metal reacts with the acid to liberate hydrogen gas and harmful products, thereby spoiling the food.
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 metal. Take a few pieces of zinc granules and add 5 ml of dilute H2SO4. Shake it and pass the gas produced into a soap solution. The bubbles of the soap solution are formed. These soap bubbles contain hydrogen gas. H2SO4 + Zn → ZnSO4 + H2 ↑ We can test the evolved hydrogen gas by its burning with a pop sound when a candle is brought near the soap bubbles.
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: CaCO3 (s) + 2HCl (aq) → CaCl2 (aq) + CO2 (g) + H2O (l)
Calcium Carbonate + Hydrochloric acid → Calcium Chloride + Carbon dioxide + Water
Intext / page 26
1. 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: When HCl or HNO3 are mixed with water then they dissolve in water to form H+ or H3O+ ions which shows their acidic character. For example: HCl (aq) → H+ + Cl- H+ + H2O → H3O+ When alcohols and glucose are mixed with water then they do not dissolve to form ions. Hence they do not show acidic character.
2. Why does an aqueous solution of an acid conduct electricity?
Answer: The presence of hydrogen (H+) or hydronium (H3O+) ions in the aqueous solution of acid are responsible for conducting electricity.
3. Why does dry HCl gas not change the colour of the dry litmus paper?
Answer: Dry HCl gas not change the colour of the dry litmus paper because it has no Hydrogen ions (H+) in it.
4. While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?
Answer: Since the process of dissolving an acid in water is exothermic, it is always recommended that acid should be added to water. If it is done the other way, then it is possible that because of the large amount of heat generated, the mixture splashes out and causes burns.
5. How is the concentration of hydronium ions (H3O+) affected when a solution of an acid is diluted?
Answer: When an acid is diluted, the concentration of hydronium ions (H3O+) per unit volume decreases. This means that the strength of the acid decreases.
6. How is the concentration of hydroxide ions (OH−) affected when excess base is dissolved in a solution of sodium hydroxide?
Answer: When excess base, in this case sodium hydroxide, is dissolved in a solution, the concentration of hydroxide ions (OH−) will increase.
Intext / Page 29
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, which has a pH of 6, is more acidic and has a higher concentration of hydrogen ions than solution B, which has a pH of 8 and is more basic.
2. What effect does the concentration of H+ (aq) ions have on the nature of the solution?
Answer: The concentration of hydrogen ions (H+) in a solution affects the nature of the solution. If the concentration of H+ ions is higher than 10^-7, the solution becomes acidic. If the concentration of H+ ions is lower than 10^-7, the solution becomes basic.
3. Do basic solutions also have H+ (aq) ions? If yes, then why are these basic?
Answer: Basic solutions do contain H+ ions, but they also have a higher concentration of hydroxide ions (OH-). The higher concentration of OH- ions makes the solution basic.
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 might treat their soil with quick lime (calcium oxide), slaked lime (calcium hydroxide), or chalk (calcium carbonate) if the soil is acidic and not suitable for cultivation. These substances can increase the basicity of the soil.
Intext / Page 34
1. Name the common name of the compound CaOCl2.
Answer: The common name of the compound CaOCl2 is bleaching powder.
2. Name the substance which on treatment with chlorine yields bleaching powder.
Answer: Calcium hydroxide (Ca(OH)2) yields bleaching powder when treated with chlorine.
3. Name the sodium compound which is used for softening hard water.
Answer: Washing soda (Na2CO3. 10H2O) is a sodium compound used for softening hard water.
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 is heated, it forms sodium carbonate, water, and releases carbon dioxide gas. The equation for this reaction is: NaHCO3 -> Na2CO3 + H2O + CO2
5. Write an equation to show the reaction between Plaster of Paris and water.
Answer: When plaster of Paris (CaSO4. 1/2H2O) is mixed with water, it forms gypsum (CaSO4.2H2O). The equation for this reaction is: CaSO4. 1/2H2O + 3/2H2O -> CaSO4.2H2O
Exercise
1. A solution turns red litmus blue, its pH is likely to be
Answer: (d) 10
2. A solution reacts with crushed egg-shells to give a gas that turns lime-water milky. The solution contains
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 of HCl solution (the same solution as before) required to neutralise it will be
Answer: (d) 16 mL
4. Which one of the following types of medicines is used for treating indigestion?
Answer: (c) Antacid
5. Write word equations and then balanced equations for the reaction taking place when−
Answer: (a) H2SO4 (aq) + Zn (s) → ZnSO4 (aq) + H2 (g)
(b) 2HCl (aq) + Mg (s) → MgCl2 (aq) + H2 (g)
(c) 3H2SO4 (aq) + 2Al (s) → Al2(SO4)3 (aq) + 3H2 (g)
(d) 6HCl (aq) + 2Fe (s) → 2FeCl3 (aq) + 3H2 (g)
6. Compounds such as alcohols and glucose also contain hydrogen but are not categorized as acids. Describe an activity to prove it.
Answer: Two nails are fitted on a cork and are kept it in a 100 mL beaker. The nails are then connected to the two terminals of a 6-volt battery through a bulb and a switch. Some dilute HCl is poured in the beaker and the current is switched on. The same experiment is then performed with a glucose solution and an alcohol solution. Observations: It will be observed that the bulb glows in the HCl solution and does not glow in the glucose solution. Result: HCl dissociates into H+ and Cl− ions. These ions conduct electricity in the solution resulting in the glowing of the bulb. On the other hand, the glucose solution does not dissociate into ions. Therefore, it does not conduct electricity. Conclusion: From this activity, it can be concluded that all acids contain hydrogen but not all compounds containing hydrogen are acids. That is why, though alcohols and glucose contain hydrogen, they are not categorised as acids.
7. Why does distilled water not conduct electricity, whereas rain water does?
Answer: Distilled water does not conduct electricity because it does not contain any ions, while rain water conducts electricity because it contains ions due to the presence of dissolved salts.
8. Why do acids not show acidic behaviour in the absence of water?
Answer: Acids do not exhibit acidic behavior in the absence of water because the dissociation of hydrogen ions from an acid only occurs in the presence of water.
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?
Answer: Arrange the pH in increasing order of hydrogen-ion concentration.
(a) Solution D with a pH of 7 is neutral.
(b) Solution C with a pH of 11 is strongly alkaline.
(c) Solution B with a pH of 1 is strongly acidic.
(d) Solution A with a pH of 4 is weakly acidic.
(e) Solution E with a pH of 9 is weakly alkaline.
The pH can be arranged in increasing order of hydrogen ion concentration as follows: pH 1 < pH 4 < pH 7 < pH 9 < pH 11.
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: When magnesium ribbons are placed in test tube A, which contains hydrochloric acid (HCl), the fizzing will occur more vigorously compared to test tube B, which contains acetic acid (CH3COOH). This is because HCl is a stronger acid than CH3COOH and therefore produces hydrogen gas at a faster rate, leading to more intense fizzing.
11. Acids do not show acidic behaviour in the absence of water but do so in water. Justify this statement with the help of a chemical equation.
Answer: Acids do not exhibit acidic behavior when they are not dissolved in water because the dissociation of hydrogen ions, which are responsible for the acidic behavior, only occurs in the presence of water. This can be represented by the following chemical equation: HCl + H2O —–> H3O+ + Cl-. Hydrogen ions cannot exist on their own, but they can combine with water molecules to form hydronium ions (H3O+).
12. What are antacids? Explain their role in providing relief from stomach ache.
Answer: Antacids are substances that neutralize stomach acid and are used to alleviate heartburn, indigestion, and stomach discomfort. They are usually basic compounds, such as magnesium hydroxide or calcium carbonate. When an excess of stomach acid is produced due to indigestion or overconsumption, it can cause acidity. Antacids neutralize the excess acid, providing relief from the pain or discomfort.
13. The marble statues are often slowly corroded when kept in open for a long time. Give a reason.
Answer: Marble statues can become slowly corroded when left in the open for an extended period of time because the acidic compounds present in the air react with the calcium carbonate in the marble, leading to corrosion.
14. When soap is scrubbed on a stain of curry on a white cloth, why does it become reddish brown and turns yellow again when it is washed with plenty of water?
Answer: When soap is applied to a stain of curry on a white cloth, the stain turns reddish-brown because curry contains turmeric, which is an indicator and reacts with the basic soap. When the cloth is washed with plenty of water, the stain turns yellow again because water is neutral and the turmeric in the curry returns to its original yellow color.
15. (a) If we take hydrochloric acid and acetic acid of the same concentration, which produces less H+ ion concentration? Out of two which one is a weak acid?
Answer: Acetic acid of the same concentration produces less H+ ion concentration. Acetic acid is a weak acid because hydrochloric acid has more H+ ion concentration compared to acetic acid and this makes hydrochloric a stronger acid.
(b) If someone is suffering from acidity, which of the following would you suggest in order to cure it: vinegar, orange juice, baking soda solution? Give reason for your answer.
Answer: From the given options, we will choose baking soda as it is a basic salt. This basic salt will neutralize the effect of the acidity and heal the person.
16. 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 of milk is 6. As it changes to curd, the pH will reduce because curd is acidic in nature. The acids present in it decrease the pH.
17. 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 the fresh milk from 6 to slightly alkaline because in alkaline conditions, milk does not set as curd easily.
(b) Since this milk is slightly basic than usual milk, acids produced to set the curd are neutralized by the base. Therefore, it takes a longer time for the curd to set.
18. Plaster of Paris should be stored in a moisture-proof container. Explain why?
Answer: Plaster of Paris should be stored in a moisture-proof container because it absorbs water from moisture and turns into a hard substance (gypsum) as shown in the following chemical equation: CaSO4 + 2H2O —–> CaSO4.2H2O
19. What is a neutralization reaction? Give two examples.
Answer: A neutralization reaction is a reaction in which an acid and base react with each other to give salt and water. Two examples of neutralization reactions are:
(i) NaOH + HCl —–> NaCl + H2O
(ii) HNO3 + KOH —–> KNO3 + H2O
20. Give two important uses of washing soda and baking soda.
Answer: Two important uses of washing soda are:
- It is used in the glass, soap, and paper industries.
- It is used to remove permanent hardness of water.
- Two important uses of baking soda are:
- It is used as baking powder. Baking powder is a mixture of baking soda and a mild acid known as tartaric acid. When it is heated or mixed in water, it releases CO2 that makes bread or cake fluffy.
- It is used in soda-acid fire extinguishers.
Tick (✓) the correct option
1. With the increase in the concentration of hydrogen ions, the pH value will:
Answer: (b) decrease
2. The compound which is used in the glass, soap, and paper industries is:
Answer: (a) washing soda
3. Sodium carbonate is a basic salt because it is a salt of:
Answer: (d) weak acid and strong base
4. 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?
Answer: (d) an antacid
5. The compound which is used for removing permanent hardness of water is:
Answer: (d) Na2CO3.10H2O
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