Acids, Bases and Salts

Intext Questions 1

Question 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

We take three pieces of red litmus paper. The solution which turns red litmus paper to blue is basic solution. Distilled water (neutral) and acidic solution will not effect red litmus paper.

Now we divide the so formed blue litmus paper into two pieces. The test tube containing distilled water does not change the colour of the blue litmus paper.

The test tube containing acidic solution changes the colour of the blue litmus paper to red. Hence, we can identify the three solutions.

Intext Questions 2

Question 1

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

Answer

Curd and sour substances are acidic in nature, they react with metals present in brass and copper vessels and results in formation of soluble salts and hydrogen gas. These salts are poisonous in nature and make the food unfit for consumption.

Question 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 released when an acid reacts with a metal along with formation of salt.

Metal + Acid ⟶ Salt + Hydrogen gas ↑

For example : When sodium hydroxide is reacted with Zn, salt of sodium zincate is formed along with hydrogen gas.

2NaOH(aq) + Zn(s) ⟶ Na₂ZnO₂(s) + H₂(g)

In order to test the presence of hydrogen gas, a burning candle is bought near hydrogen gas, the gas burns with a pop sound.

Question 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

Metal carbonates react with dil HCl to produce metals chloride and carbon dioxide gas. As the products formed in the above reaction are carbon dioxide and calcium chloride, hence the reaction is as follows :

CaCO₃ + 2HCl [dil.] ⟶ CaCl₂ + CO₂ ↑ + H₂O

Therefore, the metal A is calcium carbonate [CaCO₃].

Intext Questions 3

Question 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

HCl and HNO₃ show acidic character because they ionise in aqueous solution to form hydronium ions (H₃O⁺).

Ionisation of HCl in water:

HCl ⟶ H⁺ + Cl^-

H⁺ + H₂O ⟶ H₃O⁺

Ionisation of HNO₃ in water:

HNO₃ ⟶ H⁺ + NO₃^-

H⁺ + H₂O ⟶ H₃O⁺

C₂H₅OH (Ethanol) and C₆H₁₂O₆ (Glucose) are covalent compounds. They do not ionise in water to form hydronium ions [H₃O⁺]. Hence, they don't show acidic character.

Question 2

Why does an aqueous solution of an acid conduct electricity?

Answer

Acids ionise in aqueous solution to form hydronium ions (H₃O⁺) as cation and their respective anion such as Cl^- in HCl, NO₃^- in HNO₃ and so on. The electric current is carried through the acidic solution by these ions.

Question 3

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

Answer

The colour of litmus paper changes only in presence of ions like hydrogen [H⁺] or hydronium ions [H₃O⁺]. HCl can produce these ions only in the form of aqueous solution. Hence, dry HCl gas does not change the colour of dry litmus paper.

Question 4

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

Answer

As the process of dissolving an acid or a base in water is a highly exothermic one so the acid must always be added slowly to water with constant stirring. If water is added to a conc. acid, the large amount of heat generated may cause the mixture to splash out and cause burns. The glass container may also break due to excessive local heating.

Question 5

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

Answer

Diluting an acid, results in decrease in the concentration of hydronium ions (H₃O⁺) per unit volume. This means that the strength of the acid decreases.

Question 6

How is the concentration of hydroxide ions OH^- affected when excess base is dissolved in a solution of sodium hydroxide?

Answer

The concentration of hydroxide ions OH^- would increase when excess base is dissolved in a solution of sodium hydroxide as base generates hydroxide (OH^-) ions.

Intext Questions 4

Question 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 is acidic with more hydrogen ion concentration. Solution B is basic.

Explanation

A solution with a pH value below 7 is considered acidic, indicating a higher concentration of hydrogen ions (H⁺). Conversely, a solution with a pH greater than 7 is classified as basic, indicating a higher concentration of hydroxide ions (OH^-).

Question 2

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

Answer

Acids with a higher concentration of H⁺(aq) ions are strong acids and acids with a lower concentration of H⁺(aq) ions are weak acids.

Question 3

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

Answer

Yes, basic solutions also have H⁺(aq) ions. In a basic solution the concentration of H⁺(aq) ions is very less as compared to the concentration of OH^- ions hence the solution is basic in nature.

Question 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

When the soil is acidic and not fit for cultivation, then the farmer would treat the soil with quicklime or slaked lime or chalk in order to increase the basicity of soil and decrease its acidity.

Intext Questions 5

Question 1

What is the common name of the compound CaOCl₂?

Answer

Bleaching powder.

Question 2

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

Answer

Dry slaked lime [Ca(OH)₂]

Question 3

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

Answer

Washing soda or sodium carbonate [Na₂CO₃.10H₂O]

Question 4

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

Answer

When a solution of sodium hydrogen carbonate is heated, sodium carbonate and water are formed along with evolution of carbon dioxide gas.

2NaHCO₃ $\xrightarrow{\text{Heat}}$ Na₂CO₃ + H₂O + CO₂ ↑

Question 5

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

Answer

CaSO₄. $\dfrac{1}{2}$ H₂O + $1\dfrac{1}{2}$ H₂O ⟶ CaSO₄.2H₂O

Exercises

Question 1

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

1. 1
2. 4
3. 5
4. 10

Answer

10

Reason — As the solution turns red litmus blue, hence it is basic in nature. Therefore, its pH should be greater than 7, hence, 10.

Question 2

A solution reacts with crushed egg-shells to give a gas that turns lime-water milky. The solution contains

1. NaCl
2. HCl
3. LiCl
4. KCl

Answer

HCl

Reason — Egg shells are made up of calcium carbonate and when calcium carbonate reacts with HCl it liberates CO₂ gas which turns lime water milky.

CaCO₃ + 2HCl ⟶ CaCl₂ + H₂O + CO₂ ↑

Question 3

10 mL of a solution of NaOH is found to be completely neutralized 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 neutralize it will be

1. 4 mL
2. 8 mL
3. 12 mL
4. 16 mL

Answer

16 mL

Reason — When 10 mL of a solution of NaOH is found to be completely neutralized by 8 mL of a given solution of HCl, then is we take 20 mL of the same solution of NaOH i.e, double the initial solution of NaOH so double amount of HCl will be required i.e. 16 mL.

Question 4

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

1. Antibiotic
2. Analgesic
3. Antacid
4. Antiseptic

Answer

Antacid

Reason — Antacids are alkaline in nature. So it neutralizes the acid produced in the stomach. Hence, antacids are advised for acidity.

Question 5

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) Zinc + Sulphuric acid (dilute) ⟶ Zinc sulphate + Hydrogen gas
Zn (s) + H₂SO₄ (dil.) ⟶ ZnSO₄ (aq) + H₂ (g)

(b) Magnesium + Hydrochloric acid (dilute) ⟶ Magnesium chloride + Hydrogen gas
Mg (s) + 2HCl (dil.) ⟶ MgCl₂ (aq) + H₂ (g)

(c) Aluminium + Sulphuric acid (dilute) ⟶ Aluminium sulphate + Hydrogen gas
2Al + 3H₂SO₄ (dil.) ⟶ Al₂(SO₄)₃ (aq) + 3H₂ (g)

(d) Iron + Hydrochloric acid (dilute) ⟶ Iron chloride + Hydrogen
Fe (s) + 2HCl (dil.) ⟶ FeCl₂ (aq) + H₂ (g)

Question 6

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

Answer

Materials required — Bulb, switch, beaker, nails, battery, connecting wires, glucose, alcohol, hydrochloric acid, sulphuric acid

Procedure —

1. Fix two nails on a cork and place the cork in a 100 ml beaker.
2. Connect the nails to the two terminals of a 6 volt battery through a bulb and a switch, as shown in the figure above.
3. Now pour glucose and alcohol solutions one by one and switch on the current.
4. Repeat the activity using hydrochloric acid and sulphuric acid.

Observation —

1. The bulb does not glow when glucose and alcohol are used.
2. Bulb glows when hydrochloric acid and sulphuric acid are used.

Result — Glowing of the bulb indicates that there is a flow of electric current through the solution. The electric current is carried through the acidic solution by ions.

This activity proves that hydrogen containing compounds alcohols and glucose do not ionise to give H⁺(aq) ions, hence, they are not categorized as acids.

Question 7

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

Answer

Distilled water consists entirely of molecules. Due to the lack of ions, it does not conduct electricity. Rain water on the other hand contains small traces of acids formed due to the dissolutions of atmospheric gases like carbon dioxide, sulphur dioxide, etc. Hence, due to the presence of ions, rain water can conduct electricity.

Question 8

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

Answer

Acids show acidic behaviour by generating hydrogen ions [H⁺(aq)] in aqueous solutions. In the absence of water, acids do not produce hydrogen ions [H⁺(aq)] and, therefore, do not display their acidic characteristics.

Question 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?

Arrange the pH in increasing order of hydrogen-ion concentration

Answer

(a) D

(b) C

(c) B

(d) A

(e) E

Increasing order of hydrogen-ion concentration — C < E < D < A < B

Question 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. Amount and concentration taken for both the acids are same. In which test tube will the fizzing occur more vigorously and why?

Answer

Fizzing will be more vigorous in test tube A due to the higher concentration of H⁺(aq) ions in hydrochloric acid (HCl) as compared to acetic acid (CH₃COOH). Hydrochloric acid (HCl) is the stronger acid and its reactions with magnesium ribbon will result in faster rate of release of hydrogen gas.

Question 11

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

Answer

As milk changes to curd due to action of bacteria, the lactose in milk is converted to lactic acid which makes it more acidic. Hence, pH will decrease from its initial value of 6 to a lower pH value.

Question 12

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) To prevent the milk from turning sour and spoiling due to lactic acid formation, the milkman shifts its pH from 6 to slightly alkaline by adding baking soda.

(b) For this milk to set as curd, the lactic acid formed during curdling process needs to first neutralise the baking soda added. Hence, it takes longer time to set as curd.

Question 13

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

Answer

Plaster of Paris absorbs moisture and gets converted into a hard solid substance known as gypsum. To prevent this change, Plaster of Paris should be stored in a moisture-proof container.

Question 14

What is a neutralisation reaction? Give two examples.

Answer

The reaction between an acid and a base to give a salt and water as the only products is referred to as neutralisation reaction.

Examples :

1. Reaction of ammonium hydroxide and HCl:
   NH₄OH + HCl ⟶ NH₄Cl + H₂O
2. Reaction of sodium hydroxide and H₂SO₄:
   2NaOH + H₂SO₄ ⟶ Na₂SO₄ + 2H₂O

Question 15

Give two important uses of washing soda and baking soda.

Answer

Uses of washing soda —

1. It is used in glass, soap and paper industries.
2. It is used for removing permanent hardness of water.

Uses of baking soda —

1. It is used for making baking powder.
2. It is an ingredient in antacids.