2.Acids, Bases and Salts

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:

To test the contents we should follow the below method:

i) If the colour of red litmus paper gets changed to blue, then it is a base

ii) if there is no colour change, then it is either acidic or neutral. Thus, basic solution can be easily identified.

 

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

Ans:

Curd and other sour substances have acids in them. The brass or copper metal reacts with these acids and liberate hydrogen gas and harmful products, which further spoil the food.

Therefore, they are not kept in brass and copper vessels, 

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:

Hydrogen gas is usually liberated when an acid reacts with a metal.

 

Q4. 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:

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

Calcium Carbonate Hydrochloric acid        Calcium Chloride           Carbon dioxide                water

 

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:

The dissociation of HCl or HNO3 to form hydrogen ions always occurs in the presence of water.

Hydrogen ions (H+) combine with H2O to form hydronium ions (H3O+).

The reaction

HCl         -->       H+   +     Cl-

H+  +  H2O  -->   H3O+

As aqueous solutions of glucose and alcohol contain hydrogen, these cannot dissociate in water to form hydrogen ions.

Hence, they do not show acidic character.

 

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

Ans:

Acids dissociate in aqueous solutions to form ions.Thus ions formed  are responsible for conduction of electricity.

 

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

Ans:

Here HCl and the litmus paper both are dry.Dry HCl gas does not contain H+ ions.

The colour of the litmus paper is changed by the hydrogen ions.An acid dissociates to give ions only in the aqueous solution.Thus dry HCl gas do not change its colour in dry litmus paper. 

 

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

Ans:

The strength of the acid decreases,as the concentration of hydronium ion (H3O+) per unit volume decreases when an acid is diluted.

 

Q5. How is the concentration of hydronium ions (H3O+) affected when a solution of an acid is diluted?

Ans:

The concentration of hydroxide ions (OH) increases when more than the required base is dissolved in a solution of sodium hydroxide.

 

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

Ans:

pH value < 7 indicates an acidic solution,

pH value > 7 indicates an basic solution.

The solution with pH = 6 is acidic means it  has more hydrogen ion concentration than the solution of pH = 8 which is basic.

 

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:

 

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

Ans:

The nature of the solution may vary based on the concentration of H+(aq) present .With an increase in H+ ion concentration, the solution becomes more acidic, while a decrease of H+ ion increases the basicity of the solution.

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

Ans:

Yes, basic solution also has H+(aq)  ions. But, their concentration is less as compared to the concentration of OH ions that makes the solution basic.

 

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:

Due to the acidity of soil it becomes imprper for cultivation.So to increase the basicity of soil, the farmer would treat the soil with quick lime or slaked lime or chalk.

 

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

Ans:

Bleaching powder

 

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

Ans:

Calcium hydroxide [Ca (OH)2] on treatment with chlorine yields bleaching
powder.

 

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

Ans:

Washing soda (Na2CO3.10H2O) is used for softening hard water.

 

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

Ans:

Sodium carbonate and water are formed with the evolution of carbon dioxide gas.

    2NaHCO3         ----Heat--->    Na2CO3     +      H2O   +    CO2

Sodium Hydro                      Sodium Carbonate  water         Carbon dioxide     

Carbonate

Q5. 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 can be represented as

CaSO4.(1/2)H2O     +       1(1/2)H2O      -->        CaSO4.2H2O

Plaster of Paris                Water                           Gypsum

 

Q1. A solution turns red litmus blue, its pH is likely to be (a) 1 (b) 4 (c) 5 (d) 10

Ans:

(d) Bases turn red litmus blue and acids turn blue litmus red. Basic solution has a pH value more than 7. Since the solution turns red litmus blue, its pH is likely to be 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) The solution contains 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 of HCl solution will be required.

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 is used for treating indigestion.

Q5. 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) Sulphuric acid + Zinc → Zinc sulphate + Hydrogen

H2SO4(aq) + Zn(s)  → ZnSO4(aq) + H2(g)

(b) Hydrochloric acid + Magnesium → Magnesium chloride + Hydrogen

2HCl(g) + Mg(s)  → MgCl2(aq) +H2(g)

(c) Sulphuric acid + Aluminium → Aluminium sulphate + Hydrogen

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

(d) Hydrochloric acid + Iron → Ferric chloride + Hydrogen

6HCl(g) + 2Fe(s) → 2FeCl3(aq) + 3H2(g)

 

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

Ans:

Distilled water is a pure form of water and it does not contain any ionic species. Therefore, it does not conduct electricity. Rain water, being an impure form of water, contains many ionic species such as acids and therefore it conducts electricity.

 

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

Ans:

The dissociation of hydrogen ions from an acid takes place in the presence of water only.Thus the hydrogen ions are responsible for the acidic behaviour.Therefore,Acids do not show acidic behaviour in the absence 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
(c) Strongly acidic → Solution B with pH 1
(d) Weakly acidic → Solution A with pH 4
(e) Weakly alkaline → Solution E with pH 9
The pH can be arranged 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. In which test tube will the fizzing occur more vigorously and why?

Ans:

The fizzing will take place strongly in test tube A, in which hydrochloric acid (HCl) is added.

This is because HCl is a stronger acid than CH3COOH and therefore produces hydrogen gas at a faster speed due to which fizzing occurs. 

 

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

Ans:

The pH of milk is 6. As the curd is acidic in nature its pH will reduce because. The acids present in it decrease the pH.

 

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) The milkman shifts the pH of the fresh milk from 6 to slightly alkaline because in alkaline condition, 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.

 

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

Ans:

Plaster of Paris (POP) should be stored in a moisture-proof container because Plaster of Paris, a powdery mass, absorbs water (moisture) to form a hard solid known as gypsum.

 

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

Ans:

A reaction in which an acid and base react with each other to give a salt and water is termed as neutralization reaction. In this reaction, energy is evolved in the form of heat.

NaOH + HCl ->   NaCl   +  H2O

Base     Acid      Salt      Water

 

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

Ans:

(1) Washing soda:

(a) It is used in glass, soap, and paper industries.
(b) It is used to remove permanent hardness of water.

(2) Baking soda:

(a) 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.

(b) It is used in soda-acid fire extinguishers.

 

 

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