CBSE Class 10 Science Acids Bases and Salts VBQs Set 03

Question. Why should curd and sour substances not be kept in brass and copper vessels? 
Answer: Curd and the sour substances if kept in brass or copper vessels react with them and form hydrogen and other harmful substances due to presence of acid in them. These toxic substance can cause food poisoning or other damage to health. Due to this reason, curd and the sour substances should not be kept in brass and copper vessels.

Question. 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: Calcium carbonate (A), when reacts with hydrochloric acid, produces carbon dioxide gas with effervescence. Carbon dioxide gas is used as a fire extinguisher. Therefore, it extinguishes a burning candle. Hence, the metal compound A is calcium carbonate.
\( CaCO_3(s) + 2HCl(aq) \rightarrow CaCl_2(s) + CO_2(g) + H_2O(l) \)

Question. Why do \( HCl, HNO_3 \), etc. show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character?
Answer: \( HCl, HNO_3 \), etc. dissociate into their ions in the presence of water. They form hydrogen ions \( (H^+) \). These hydrogen ions combine with \( H_2O \) to form hydronium ions \( (H_3O^+) \). The reaction can be given as follows:
\( HCl \rightarrow H^+ + Cl^- \)
\( H^+ + H_2O \rightarrow H_3O^+ \)
\( HCl + H_2O \rightarrow H_3O^+ + Cl^- \) 
Similarly, \( HNO_3 \rightarrow H^+ + NO_3^- \)
\( H^+ + H_2O \rightarrow H_3O^+ \)
\( HNO_3 + H_2O \rightarrow H_3O^+ + NO_3^- \)
Due to this property, \( HCl \) and \( HNO_3 \) show acidic character in aqueous solutions. On the other hand, alcohol and glucose cannot dissociate in water to form hydrogen ions. Hence, they do not show acidic character.

Question. Why does an aqueous solution of an acid conduct electricity?
Answer: An aqueous solution of an acid conduct electricity because of the presence of charged particles called ions in it. When dissolved in water, acids dissociate to form ions, e.g.
\( HCl + H_2O \rightarrow Cl^- + H_3O^+ \)
These ions are responsible for conducting electricity.

Question. Why does dry \( HCl \) gas not change the colour of the dry litmus paper? 
Answer: We know that the colour of the litmus is changed by \( H^+ \) ions of an acid. Dry \( HCl \) does not dissociate to give \( H^+ \) ions. Acids dissociate to give ions only in the aqueous medium. Since in this case, neither \( HCl \) is in aqueous form nor is the litmus paper wet, so the colour of litmus paper does not change.

Question. 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 dissolving an acid in water is a highly exothermic reaction. 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 severe burns. The glass container may also break due to excessive heating.

Question. How is the concentration of hydronium ions \( (H_3O^+) \) affected when a solution of an acid is diluted?
Answer: Concentration of hydronium ions \( (H_3O^+) \) decreases when a solution of an acid is diluted.

Question. 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 \( (OH^-) \) increases when excess base is dissolved in a solution of sodium hydroxide.

Question. 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 more \( H^+ \) ion concentration. A pH value of less than 7 indicates the acidic nature while greater than 7 indicates the basic nature of a solution. So, solution A is acidic and solution B is basic.

Question. What effect does the concentration of \( H^+ \) (aq) ions have on the nature of the solution?
Answer: As the concentration of \( H^+ \) ions increases, the solution becomes more acidic while a decrease of \( H^+ \) ion causes an increase in the basicity of the solution.

Question. Do basic solutions also have \( H^+ \) (aq) ions? If yes, then why are these basic? 
Answer: Yes, basic solutions also have \( H^+ \) ions. However, their concentration is less as compared to the \( OH^- \) ions that makes the solution basic.

Question. 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: If the farmer finds his soil to be more acidic, then to increase the basicity of the soil, he should treat the soil of his field with quick lime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate).

Question. What is the common name of the compound \( CaOCl_2 \)? 
Answer: Bleaching powder.

Question. Name the substance which on treatment with chlorine yields bleaching powder.
Answer: Dry slaked lime \( [Ca(OH)_2] \).

Question. Name the sodium compound which is used for softening hard water.
Answer: Washing soda, i.e. sodium carbonate \( (Na_2CO_3 \cdot 10H_2O) \).

Question. What will happen if a solution of sodium hydrocarbonate is heated? Give the equation of the reaction involved. 
Answer: When sodium hydrogen carbonate is heated, sodium carbonate and water are formed along with the liberation of carbon dioxide gas, e.g.
\( 2NaHCO_3 \xrightarrow{Heat} Na_2CO_3 + H_2O + CO_2 \uparrow \)

Question. Write an equation to show the reaction between Plaster of Paris and water.
Answer: \( CaSO_4 \cdot \frac{1}{2} H_2O + 1 \frac{1}{2} H_2O \rightarrow CaSO_4 \cdot 2H_2O \)

NCERT EXERCISES

Question. A solution turns red litmus blue, its pH is likely to be
(a) 1
(b) 4
(c) 5
(d) 10
Answer: (d) 10

Question. 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) \( HCl \)

Question. 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
(a) 4 mL
(b) 8 mL
(c) 12 mL
(d) 16 mL
Answer: (d) 16 mL

Question. Which one of the following types of medicines is used for treating indigestion?
(a) Antibiotic
(b) Analgesic
(c) Antacid
(d) Antiseptic
Answer: (c) Antacid

Question. 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) \( Zn(s) + H_2SO_4(dil) \rightarrow ZnSO_4(aq) + H_2(g) \)
Zinc + Sulphuric acid \( \rightarrow \) Zinc sulphate + Hydrogen gas
(b) \( Mg(s) + 2HCl(dil) \rightarrow MgCl_2(aq) + H_2(g) \)
Magnesium + Hydrochloric acid \( \rightarrow \) Magnesium chloride + Hydrogen gas
(c) \( 2Al(s) + 3H_2SO_4(dil) \rightarrow Al_2(SO_4)_3(aq) + 3H_2(g) \)
Aluminium + Sulphuric acid \( \rightarrow \) Aluminium sulphate + Hydrogen gas
(d) \( Fe(s) + 2HCl(aq) \rightarrow FeCl_2(aq) + H_2(g) \)
Iron + Hydrochloric acid \( \rightarrow \) Iron(II) chloride + Hydrogen gas

Question. Why does distilled water not conduct electricity, whereas rain water does?
Answer: Distilled water, does not contain any ionic compounds like acids, bases or salts. Therefore, it does not dissociate into ions as it is a weak electrolyte. Salts are present in rain water which help in dissociation of rain water into ions which help in conduction of electricity.

Question. Why do acids not show acidic behaviour in the absence of water?
Answer: It is because acids do not dissociate into ions in absence of water. But when an acid is dissolved in water, it forms hydrogen ions and hence, shows acidic behaviour, e.g.
\( HCl(g) \xrightarrow{dissolve in water} H^+(aq) + Cl^-(aq). \)

Question. 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) ‘D’ with pH = 7 is neutral.
(b) ‘C’ with pH = 11 is strongly alkaline.
(c) ‘B’ with pH = 1 is strongly acidic.
(d) ‘A’ with pH = 4 is weakly acidic.
(e) ‘E’ with pH = 9 is weakly alkaline.
‘C’, ‘E’, ‘D’, ‘A’, ‘B’ is increasing order of \( H^+ \) ion concentration.

Question. 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 (\( CH_3COOH \)) 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?
Answer: Fizzing occurs more vigorously in hydrochloric acid (A) than in acetic acid (B) because \( HCl \) is stronger acid than acetic acid. \( HCl \) dissociates into \( H^+ \) and \( Cl^- \) ions completely whereas acetic acid partially dissociates into its ions.

Question. Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.
Answer: Milk contains a carbohydrate lactose. When milk sets into curd, lactose gets converted into an acid called lactic acid. Due to formation of lactic acid, pH of milk falls below 6.

Question. 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) It is done to increase the shelf life of milk.
(b) The alkaline milk takes a longer time to set into curd because lactic acid being formed has to neutralise the alkali present in it.

Question. Plaster of Paris should be stored in a moisture-proof container. Explain why?
Answer: It will absorb water to form gypsum which set into hard solid mass. This will make Plaster of Paris useless after some time, e.g.
\( CaSO_4 \cdot \frac{1}{2} H_2O + 1 \frac{1}{2} H_2O \rightarrow CaSO_4 \cdot 2H_2O \)
Plaster of Paris \( \rightarrow \) Gypsum

Question. What is a neutralisation reaction? Give two examples.
Answer: The reaction in which a base reacts with an acid is called neutralisation reaction.
Examples:
\( NaOH(aq) + HCl(aq) \rightarrow NaCl(aq) + H_2O(l) \)
\( H_2SO_4(aq) + 2NH_4OH(aq) \rightarrow (NH_4)_2SO_4(aq) + 2H_2O(l) \)
\( HCl(aq) + NH_4OH(aq) \rightarrow NH_4Cl(aq) + H_2O(l) \)

Question. Give two important uses of washing soda and baking soda.
Answer: Uses of washing soda:
(i) It is used in the manufacture of glass, soap, paper and other sodium compounds like borax, etc.
(ii) It is used in softening of hard water.
Uses of baking soda:
(i) It is used as antacid to neutralise excess of acidity (hyper-acidity) in the stomach.
(ii) It is an ingredient of baking powder which contains \( NaHCO_3 \) and tartaric acid.

SELECT NCERT EXEMPLAR PROBLEMS

Question. 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.
(a) (i) only
(b) (i) and (iii)
(c) (ii) and (iii)
(d) (i) and (iv)
Answer: (d) (i) and (iv)

Question. An aqueous solution turns red litmus solution blue. Excess addition of which of the following solution would reverse the change? 
(a) Baking power
(b) Lime
(c) Ammonium hydroxide solution
(d) Hydrochloric acid
Answer: (d) Hydrochloric acid

Question. During the preparation of hydrogen chloride gas on a humid day, the gas is usually passed through the guard tube containing calcium chloride. The role of calcium chloride taken in the guard tube is to
(a) absorb the evolved gas
(b) moisten the gas
(c) absorb moisture from the gas
(d) absorb \( Cl^- \) ions from the evolved gas
Answer: (c) absorb moisture from the gas

Question. Which of the following salts does not contain water of crystallisation?
(a) Blue vitriol
(b) Baking soda
(c) Washing soda
(d) Gypsum
Answer: (b) Baking soda

Question. 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) weak acid and strong base.

Question. Calcium phosphate is present in tooth enamel. Its nature is
(a) basic
(b) acidic
(c) neutral
(d) amphoteric
Answer: (a) basic

Question. 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?
(a) Lemon juice
(b) Vinegar
(c) Common salt
(d) An antacid
Answer: (d) An antacid

Question. Which of the following gives the correct increasing order of acidic 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) Water < Acetic acid < Hydrochloric acid