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
Ans :
Add a few drops of red litmus paper to each test tube.
In the water test tube, the red litmus paper will stay red.
In the basic solution test tube, the red litmus paper will turn blue.
In the acidic solution test tube, the red litmus paper will become darker red.
2. Why should curd and sour substances not be kept in brass and copper vessels?
Ans :
Curd and sour substances should not be kept in brass and copper vessels because these metals (copper and zinc) react with the acids in the curd and sour substances, forming soluble salts. These salts are poisonous and can make the curd or food unfit for consumption
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?
Ans :
When an acid reacts with a metal, hydrogen gas is usually liberated.
Example:
Zn (s)+2HCl (aq)→ZnCl2 (aq)+ H2(g)
Here, zinc reacts with hydrochloric acid to form zinc chloride and hydrogen gas.
Test for the presence of hydrogen gas:
To test for hydrogen gas, bring a burning matchstick or candle near the gas. Hydrogen gas will burn with a pop sound, confirming its presence.
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?
Ans :
The metal compound A is calcium carbonate (CaCO3), as it reacts with hydrochloric acid to produce carbon dioxide gas, which extinguishes a burning candle. The reaction produces calcium chloride and carbon dioxide.
The balanced chemical equation is:
CaCO3 + HCI ⟶ CaCl2 + CO2 + H2O
In this reaction, calcium carbonate reacts with hydrochloric acid to form calcium chloride, carbon dioxide, and water.
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?
Ans :
HCl, HNO3, and similar acids show acidic character in aqueous solutions because they dissociate in water to produce hydrogen ions (H⁺). These hydrogen ions then combine with water molecules to form hydronium ions (H₃O⁺), which are responsible for the acidic behavior.
The reaction is as follows:
HCl(aq) ⟶ H++Cl−
H++H2O ⟶ H₃O⁺
In contrast, alcohol and glucose do not dissociate into hydrogen ions in water. Therefore, they do not exhibit the same acidic behavior because they do not release H⁺ ions.
6. Why does an aqueous solution of an acid conduct electricity?
Ans :
An aqueous solution of an acid conducts electricity because acids dissociate in water to form ions (such as hydrogen ions, H⁺, and anions like Cl⁻). These free-moving ions allow the flow of electric current, making the solution capable of conducting electricity.
7. Why does dry HCl gas not change the colour of the dry litmus paper?
Ans :
Dry HCl gas does not change the color of dry litmus paper because it does not contain hydrogen ions (H⁺) in its gaseous form. The color change of litmus paper occurs due to the presence of hydrogen ions, which are released when an acid dissociates in water. Since dry HCl gas is not in an aqueous form and the litmus paper is dry (not wet), no dissociation occurs, and therefore, the color of the litmus paper does not change.
8. While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?
Ans :
When an acid is dissolved in water, the process is exothermic, meaning it releases heat. If water is added to the acid, the heat generated can cause the mixture to splash and potentially cause burns. By adding the acid to water, the heat is spread out more evenly, reducing the risk of splashing and ensuring a safer dilution process.
9. How is the concentration of hydronium ions H3O+affected when a solution of an acid is diluted?
Ans :
When an acid is diluted, the concentration of hydronium ions (H₃O⁺) decreases per unit volume. This is because dilution increases the volume of the solution without increasing the number of hydronium ions, which reduces the acid’s strength.
10. How is the concentration of hydroxide ions OH− affected when excess base is dissolved in a solution of sodium hydroxide?
How is the concentration of hydroxide ions (OH⁻) affected when excess base is dissolved in a solution of sodium hydroxide?
Ans :
When excess base is dissolved in a solution of sodium hydroxide, the concentration of hydroxide ions (OH⁻) increases. This is because the excess base contributes additional hydroxide ions to the solution, making it more alkaline.
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?
Ans :
Solution A with a pH of 6 is acidic and has more hydrogen ion concentration because a pH value less than 7 indicates an acidic solution.
Solution B with a pH of 8 is basic and has fewer hydrogen ions, as a pH value greater than 7 indicates a basic solution.
12. What effect does the concentration of H+(aq) ions have on the nature of the solution?
Ans :
The concentration of H⁺ ions directly affects the nature of the solution:
Increase in H⁺ concentration makes the solution more acidic.
Decrease in H⁺ concentration leads to an increase in basicity, making the solution more basic.
13. Do basic solutions also have H+(aq) ions? If yes, then why are these basic?
Ans :
Yes, basic solutions do contain H⁺ (aq) ions, but their concentration is much lower compared to the concentration of OH⁻ (aq) ions. A solution is considered basic because the concentration of OH⁻ ions is higher, which gives the solution its basic nature.
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)?
Ans :
If the soil is acidic and unsuitable for cultivation, the farmer would treat the soil with quick lime, slaked lime, or chalk to increase the basicity of the soil, making it more suitable for plant growth.
2NaHCO3 ⟶ Na2CO3 + H2O + CO2
15. What is the common name of the compound CaOCl2?
Ans :
Bleaching powder.
16. Name the substance which on treatment with chlorine yields bleaching powder.
Ans :
Slaked lime or calcium hydroxide.
17. Name the sodium compound which is used for softening hard water.
Ans :
Sodium carbonate is used for softening hard water.
18. What will happen if a solution of sodium hydrocarbonate is heated? Give the equation of the reaction involved.
Ans :
When sodium bicarbonate (sodium hydrogen carbonate) is heated, it decomposes to form sodium carbonate, water, and carbon dioxide gas. The reaction is as follows:
2NaHCO3(s) → Na2CO3(s)+ H2O (l)+CO2(g)
This reaction releases carbon dioxide gas and water, and produces sodium carbonate
19. Write an equation to show the reaction between Plaster of Paris and water.
Ans :
The chemical equation for the reaction of Plaster of Paris and water is
CaSO4.1/2H2O + 3/2H2O → CaSO4.2H2O
20. A solution turns red litmus blue, its pH is likely to be
(a) 1
(b) 4
(c) 5
(d) 10
Ana :
(d) 10
21. 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
Ans :
(b) The solution contains HCI.
22. 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
Ans :
(d) 16 mL
23. Which one of the following types of medicines is used for treating indigestion?
(a) Antibiotic
(b) Analgesic
(c) Antacid
(d) Antiseptic
Ans:
(c) Antacid is used for treating indigestion.
24. 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.
Ans:
(a) Zinc + Sulphuric acid Zinc sulphate +Hydrogen
H2SO4(aq) + Zn(s) ⟶ ZnSO4(aq) + H2 (g)
(b) Magnesium + Sulphuric acid magnesium chloride +Hydrogen gas
Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
(c) Aluminum + Sulphuric acid Aluminum sulphate +Hydrogen gas
3H2SO4 + 2Al → Al2(SO4)3 + 3H2
(d) Iron + Hydrochloric acid Iron chloride +Hydrogen
6HCl(aq) + 3Fe(s) → 3FeCl2(aq) + 3H2(g)
25. Compounds such as alcohols and glucose also contain hydrogen but are not categorised as acids. Describe an Activity to prove it.
Ans :
alcohol and glucose both contain hydrogen but not categorized as acids. This can be proved by following activity.
Material required :-Beaker, nails, battery, connecting wires, bulb, switch and alcohols.
Procedure :
1. Set up the experiment as follows
2. Take ethyl alcohol in the beaker in the beaker.
3. When the switch is turned on, the bulb does not glow.
4. Take glucose solution in place of alcohols but bulb does not glow.
Objective: To show that alcohols and glucose do not exhibit acidic properties like acids (such as HCl or H₂SO₄).
Materials:
Alcohol (e.g., ethanol)
Glucose solution
Red litmus paper
A few drops of dilute hydrochloric acid (HCl)
Beakers or test tubes
Procedure:
Test with Alcohol:
Take a small amount of alcohol (e.g., ethanol) in a test tube.
Dip a strip of red litmus paper into the alcohol.
Observe if there is any change in the color of the litmus paper.
Test with Glucose Solution:Prepare a dilute solution of glucose in water and place it in a test tube.
Dip a strip of red litmus paper into the glucose solution.
Observe if the litmus paper changes color.
Test with Acid (for comparison):Now, take a small amount of dilute hydrochloric acid in another test tube.
Dip a strip of red litmus paper into the acid.
Observe that the red litmus paper turns blue, indicating the presence of hydrogen ions (H⁺).
Observation:
In the first two cases (alcohol and glucose), the red litmus paper does not change color, indicating that these substances do not release hydrogen ions (H⁺) in solution.
In the third case (dilute HCl), the red litmus paper turns blue, indicating the presence of H⁺ ions and the acidic nature of HCl.
Conclusion:
Alcohols and glucose do not exhibit acidic properties because they do not release hydrogen ions (H⁺) in solution, unlike acids such as HCl, which release H⁺ ions and change the color of litmus paper. Therefore, alcohols and glucose are not categorized as acids.
26. Why does distilled water not conduct electricity, whereas rain water does?
Ans :
Distilled water is pure and free from dissolved salts, minerals, or any ionic species, which are necessary for conducting electricity. Since it lacks these ions, it does not conduct electricity.
On the other hand, rainwater contains dissolved gases such as carbon dioxide (forming carbonic acid) and other impurities, including minerals and salts, which dissociate into ions. These ions (e.g., H⁺, OH⁻, Na⁺, Cl⁻) allow rainwater to conduct electricity.
27. Why do acids not show acidic behaviour in the absence of water?
Ans :
Acids do not show acidic behavior in the absence of water because the dissociation of hydrogen ions (H⁺) from an acid occurs only when the acid is in aqueous form. In the presence of water, the acid dissociates to release hydrogen ions, which are responsible for the acidic behavior. Without water, there is no dissociation, and thus, no hydrogen ions are available to exhibit acidity.
28. 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
Solution :
(a) D
(b) C
(c) B
(d) A
(e) E
28. 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
Solution :
(a) D
(b) C
(c) B
(d) A
(e) E
29. 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. Amount and concentration taken for both the acids are same. In which test tube will the fizzing occur more vigorously and why?
Ans :
The fizzing will occur more vigorously in test tube A, which contains hydrochloric acid (HCl). This is because HCl is a stronger acid compared to acetic acid (CH₃COOH). Strong acids dissociate more completely in water, releasing a higher concentration of hydrogen ions (H⁺). Therefore, in test tube A, the higher concentration of H⁺ ions will react more quickly with magnesium, producing more hydrogen gas and more vigorous fizzing than in test tube B, where acetic acid dissociates to a lesser extent.
30. Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.
Ans :
As milk turns into curd, the pH will decrease. This is because lactic acid is produced during the fermentation process, which makes the curd acidic in nature. The increase in acid concentration lowers the pH of the milk, making it more acidic compared to its original neutral pH of 6.
31.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?
Ans :
(a) Why does he shift the pH of the fresh milk from 6 to slightly alkaline?
The milkman adds baking soda to make the milk slightly alkaline because in an alkaline environment, the milk proteins (such as casein) remain in a stable form and do not curdle easily. This helps in preventing the premature setting of curd, and the milk remains fresh for a longer time.
(b) Why does this milk take a long time to set as curd?
When baking soda is added, the milk becomes slightly basic. The acids produced during fermentation, which are necessary for curd formation, are neutralized by the base (baking soda). As a result, the production of lactic acid is slowed down, and it takes a longer time for the milk to set into curd.
32. Plaster of Paris should be stored in a moisture-proof container. Explain why?
Ans :
Plaster of Paris reacts with moisture from the air to form gypsum (CaSO₄·2H₂O). This reaction causes the Plaster of Paris to set and harden, making it unusable for its intended purpose. To prevent this, Plaster of Paris should be stored in a moisture-proof container to protect it from the moisture in the air and maintain its effectiveness.
33. What is a neutralisation reaction? Give two examples.
Neutralization Reaction:
Ans :
A neutralization reaction occurs when an acid and a base react with each other to form a salt and water. This process releases energy in the form of heat.
Examples
Example 1: NaOH + HCl ⟶ NaCl+H2O
(Sodium hydroxide + Hydrochloric acid → Sodium chloride + Water)
Example 2: Mg(OH)2+ 2HCl → MgCl2+2H2O
(Magnesium hydroxide + Hydrochloric acid → Magnesium chloride + Water)
The second example is relevant to indigestion where the antacid neutralizes the excess acid in the stomach, providing relief.
34. Give two important uses of washing soda and baking soda.
Ans :
Uses of Washing Soda (Na₂CO₃):
As a cleansing agent, it helps in cleaning and removing stains.
For removing permanent hardness of water by precipitating calcium and magnesium ions.
Used in glass, soap, and paper industries for various processes.
Uses of Baking Soda (NaHCO₃):
Used as an ingredient in baking powder for leavening in baking.
Acts as an antacid to neutralize stomach acidity and relieve heartburn.