Chapter - 2 Acids, Bases And Salts NCERT

Chapter - 2 Acids, Bases And Salts

IN–TEXT QUESTIONS SOLVED

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Q1 - 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 solution in each test tube. Red colour become light in the test tube containing water. Colour will tun blue in test tube containing basic solution. Red colour will become dark n the test tube containing acidic solution

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Q1 - Why should curd and sour substances not be kept in brass and copper vessels?

Ans - Curd and sour substances contain acids which can react with copper vessels and brass to form toxic compounds.

Q2 - 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 - Usually hydrogen gas is liberated when an acid reacts with a metal

Example - Zn(s) + 2HCl(aq) → ZnCl2 (aq) + H2(g)

To test the hydrogen gas bring a candle or match stick near to it it burns with pop sound.

Q3 - 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 - Metal compound A is CaCO3

Candle is extinguishes then it is CO2

Balanced equation:

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

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Q1 - 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 - Compounds like HCl, HNO3 release H+ ions in aqueous solution therefore they show acidic character.

While compounds like alcohol and glucose do not release hydrogen ions. therefore, they do not show acidic properties.

Q2 - Why does an aqueous solution of an acid conduct electricity?

Ans - Electricity is conducted in a solution by ions. Acid release H+ ions in a solution so, it conducts electricity.

Q3 - Why does dry HCl gas not change the colour of the dry litmus paper?

Ans - Dry HCl gas does not release H+ ions and hence, the acidic property of gas is not imparted.

Q4 - While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?

Ans - Addition of water to acid is an exothermic reaction. If we add water to acid lot of heat is produced that may breaks the glass container or sprout to burns the person adding it.

But when acid is added to water with constant stirring the heat produced is absorbed by water and no harm occurs.

Q5 - How is the concentration of hydronium ions (H30+) affected when a solution of an acid is diluted?

Ans - Concentration of hydronium ions decreased when the solution of an acid is diluted.

Q6 - How is the concentration of hydroxide ions (OH– ) affected when excess base is dissolved in a solution of sodium hydroxide?

Ans - The concentration of hydroxide ions (OH-) would increase when ecess base is dissolved in a solution of sodium hydroxide.

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Q1 - 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 - A with pH = 6 is acidic

B with pH = 8 is basic

‘A’ has more hydrogen ion concentration as it is acidic in nature.

Q2 - What effect does the concentration of H+ (aq) ions, have on the nature of the solution?

Ans - If a solution has higher concentration of H+ ions it is more acidic in nature.

Q3 - Do basic solutions also have H+ (aq) ions? If yes, then why are these basic?

Ans - Yes basic solutions also have H+ ions, but they are basic in nature due to more number of OH– ions.

Q4 - 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 - When the soil is acidic in nature, the farmer would add quick lime (CaO) or slaked lime (Ca(OH)2) or chalk (CaCO3)to make it neutral.

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Q1 - What is the common name of the compound CaOCl2?

Ans - The common name of CaOCl2 is bleaching powder.

Q2 - Name the substance which on treatment with chlorine yields bleaching powder.

Ans - Calcium hydroxide (Ca(OH)2) when treated with chlorine yields bleaching powder.

Ca(OH)2 + Cl2 → CaOCl2 + H2O

Q3 - Name the sodium compound which is used for softening hard water.

Ans - Washing soda (Na₂CO₃.10H₂O)

Q4 - What will happen if a solution of sodium hydrocarbonate is heated? Give the equation of the reaction involved.

Ans - When sodium hydrocarbonate is heated, sodium carbonate and water along with the evolution of carbon dioxide .

Q5 - Write an equation to show the reaction between Plaster of Paris and water.

Ans - CaSO₄.H₂O + 1 ½H₂O → CaSO₄.2H₂O

Exercise

Q1 - A solution turns red litmus blue, its pH is likely to be

(a) 1

(b) 4

(c) 5

(d) 10

Ans - (d) 10

Q2 - 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) HCl

Q3 - 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

Q4 - Which one of the following types of medicines is used for treating indigestion?

(a) Antibiotic

(b) Analgesic

(c) Antacid

(d) Antiseptic

Ans - (c) Antacid.

Q5 - Write word equations and then balance 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 ions filings.

Ans -

(a)

Zinc granule + dil. Hydrochloric acid → Zinc chloride + Hydrogen gas

Zn(s) + 2HC1 (aq) → ZnCl2(aq) + H2(g)

(b)

Magnesium+dil. Hydrochloric acid → Magnesium chloride + Hydrogen gas

Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)

(c)

Aluminium + dil. Sulphuric acid → Aluminium sulphate + Hydrogen gas

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

(d)

Iron + dil. Hydrochloric acid → Iron chloride + Hydrogen

2Fe(s) + 3HCl(aq) → Fe2Cl3(aq) + 3H2(g)

Q6 - Compounds such as alcohols and glucose also contain hydrogen but are not categorized as acids. Describe an activity to prove it.

Ans - The activity as follows :-

• Take a cork with two nails fixed on it.

• Keep this cork inside the beaker.

• Connect the nails to the battery, bulb and key as shown in the figure.

First test the circuit with dilute acid solution in which bulb glow.
Now add ethanol in it and record your observation repeat the same set up for glucose and record your observations

Observation: The bulb will not glow because charge is not flowing through it.

Conclusion: The experiment shows that glucose and ethanol do not ionise.

Q7 - Why does distilled water not conduct electricity, whereas rain water does?

Ans - Distilled water is pure foam of water and is dvoid of any onic specis. Therefore, it does not conduct electricity. Rain water, being an impure form of water contains many ioni species such as acids and therefore it conducts electricity.

Q8 - Why do acids not show acidic behaviour in the absence of water?

Ans - Acids cannot release H+ ions in absence of water, only on dissolving in water acids release H+ ions. Acids show acidic behaviour only due to H+ ions released in presence of water.

Q9. 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.

Ans -

(a) Neutral - Solution D with pH 7

(b) Strongly alkaline - Solution C with pH 11

(d) Weakly acidic - Solution A with pH 4

(e) Weakly alkaline - Solution E with pH 9

pH in the increasing order of the concentration of hydrogen ions as:-

11 < 9 < 7 < 4 < 1.

Q10 - 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 - In test tube A hydrochloric acid is present which is a strong acid as compared to acetic acid present in test tube B.

The fizzing occurs more vigorously in test tube A as HCl is strong and dissociates completely into H+ and Cl ions for the reaction.

Q11 - Fresh milk has pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.

Ans - pH of milk will reduce when it turns into curd due to the formation of lactic acid.

Q12 - 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) pH is shifted to alkaline so that milk does not spoil by becoming sour by releasing lactic acid.

(b) Since this milk is slightly basic than usual milk, acids produced to set the curd are neutralized by the base. Therefore, it take a longer time for the curd to set.

Q13 - Plaster of Paris should be stored in a moisture-proof container. Explain why?

Ans - Plaster of Paris absorb water (moisture) to form hard solid gypsum.

Q14 - What is a neutralisation reaction? Give two examples.

Ans - The reaction in which acid reacts with base to form salt and water is called neutralisation reaction.

Examples -

NaOH + HCl → NaCl +H₂O

Mg(OH)₂ + 2HCl → MgCl₂ +2H₂O

Q15 - Give two important uses of washing soda and baking soda.

Ans -

Washing soda

(i) It is used in glass, soap and paper industries.

(ii) It is used to remove permanent hardness of water.

Baking Soda

(i) It is used as an antacid.

(ii) It is used in soda, acid fire extinguisher.

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Acids, Bases and Salts

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