NCERT Solutions for Class 10 Science chapter 2 – Acid, Bases and Salts

Red litmus paper is an indicator for bases and can turn blue in the presence of a base.

Let us mark the given three test tubes as A, B, and C. Put the given red litmus paper in each solution. If the colour of red litmus paper changes to blue (suppose in test tube A), then it is a base and if there is no colour change, then it is either acidic or neutral.

Now, a drop of the solution from test tube A is put on the red litmus paper. Same process is repeated with solution B and C. If either of them changes colour to blue, then it is basic (let’s suppose B) Therefore, out of three, one is eliminated as a base.

Out of the remaining two (A and C), any solution can be acidic or neutral. To test them for acidic or neutral, a drop of basic solution is mixed with a drop of each of the remaining two solutions separately and then the nature of the drops of the mixtures is checked. If the colour of red litmus turns blue, then the second solution is neutral (C) and if there is no change in colour, then the second solution is acidic (A). This is because acidic and basic solutions neutralise each other. In this way the contents of each test tube can be identified. 

 

Calcium carbonate is a metal compound that reacts with dilute hydrochloric acid to produce calcium chloride, carbon dioxide and water. Carbon dioxide produced has the ability to extinguish a burning candle as it will remove the oxygen supply. The balanced chemical equation for the reaction is:

 

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

For Example:

• Take some pieces of zinc granules in a test tube and add H2SO4 to it. 

• Shake it and pass the gas produced into a soap solution. 

• Bubbles are formed in the soap solution. 

• These soap bubbles contain hydrogen gas. 

• The chemical equation of the reaction is: 

H2SO4+Zn→ZnSO4+H2↑

 

Identification Test: 

Hydrogen gas is identified by bringing a burning candle near the soap bubbles. The candle will burn with a pop sound.

 

Curd contains lactic acid and other sour substances also contain some acids. So, when they are kept in brass and copper vessels, the acid present in curd and sour substances react with the metal to liberate hydrogen gas and harmful products which spoil the food. 

 

The acidic character of any compound is due to the presence of the hydronium ions

(H3O+) or hydrogen ions(H+) in any solution.  In an aqueous solution HCl anad HNO3 gets dissociated and forms hydrogen ions. The hydrogen ions combine with H2O to form hydronium ions. The reaction is as follows:

HCl(aq)→H++Cl−

 H++H2O→H3O+

Due to the presence of hydronium ions HCl,HNO3, etc,show acidic characters. However, the aqueous solutions of glucose and alcohol contain hydrogen, but it cannot dissociate in water to form hydrogen ions. Therefore, they do not show acidic character. 

 

When a solution of an acid is diluted, then water is added to that solution. Dilution results in the concentration of hydronium ions (H3O+) per unit volume to get decreased and thus the strength of the acid also decreases.

 

It is always recommended that acid should be added to water in order to dilute the acid; because the process of dissolving an acid in water is exothermic that releases heat. If water is added to the acid, then a large amount of heat is generated due to which the mixture spills out and causes burns and injuries that may cause accidents.

The acidic nature is due to the presence of hydronium (H3O+) or hydrogen (H+) ions in aqueous solution.  Dry HCl gas does not contain hydronium or hydrogen ions as only in aqueous solution an acid can dissociate to give ions. The colour of the litmus paper is changed by the hydrogen ions. So, in this case there will be no colour change due to dryness of the HCl gas and the litmus paper. 

 

Acids have the tendency to dissociate into hydronium ions (H3O+) or hydrogen ions (H+) in an aqueous solution. Due to the movement of these ions , the solution can conduct electricity. Hence, an aqueous solution of an acid can conduct electricity.

When an excess of a base is dissolved in a solution, the solution becomes concentrated. This would result in the increase in the concentration of hydroxide ions (OH−) per unit volume and the strength of the base will increase.

 

Quick lime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate) is considered a good base. If the soil is acidic which is improper for cultivation, then to increase the basicity of soil, the farmer would treat the soil with these compounds.

 

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

 

When the hydrogen ion (H+)concentration increases, the solution becomes more acidic. While a decrease of hydrogen ion in the solution makes the solution less acidic and increases the basicity of the solution. 

 

The pH less than 7 (pH < 7 ) indicates an acidic solution, while pH greater than 7 (pH > 7) indicates a basic solution. Therefore, the solution with pH = 6 will be acidic and it has more hydrogen ion (H+) concentration than the solution of pH = 8 that will be a basic solution. The solution of pH = 8 has more hydroxide ion (OH−) concentration. 

 

The common name of the given compound CaOCl2 is bleaching powder. 

 

The compound called calcium hydroxide Ca(OH)2 on treatment with chlorine, yields bleaching powder.

Washing soda (Na2CO3.10H2O) is the sodium compound that is used for softening hard water. 

 

When a solution of sodium hydrocarbonate or sodium hydrogen carbonate is heated then sodium carbonate and water are formed with the evolution of carbon dioxide gas.

The Equation is as Follows:

 

The chemical equation can be represented as:

 

 

Uses of Washing Soda are: 

a) It is used to remove permanent hardness of water. 

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

Uses of Baking Soda are: 

a) It is used as baking powder. Baking powder is a mixture of baking soda and tartaric acid. Baking powder makes bread or cake fluffy. 

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

 

Neutralisation Reaction: A reaction in which an acid and base react with each other to form a salt and water is known as a neutralization reaction. For example:

Plaster of Paris (POP) should be stored in a moisture-proof container because it is a powdery mass that can absorb water or moisture to form a hard solid mass known as gypsum. The reaction takes place as follows:

 

a) Why does he shift the pH of the fresh milk from 6 to slightly alkaline? 

Ans: In alkaline condition, milk does not set as curd easily due to the formation of lactic acid as in the acidic condition.

b) Why Does This Milk Take a Long Time To Set As Curd?

Ans: Since, this milk is slightly basic than normal milk, acids produced to set the curd are neutralised by the base added to the milk. Thus, it takes a longer time for the curd to set. 

 

The fizzing occurs strongly in test tube A, in which hydrochloric acid (HCl) is added. This is because HCl is a stronger acid than acetic acid (CH3COOH)and generates more hydrogen (H+) ions. Therefore, HCl produces hydrogen gas at a faster rate due to which fizzing occurs.

The pH of fresh milk is 6. As it changes into curd, the pH will decrease because curd contains lactic acid which reduces the pH. 

 

a) Neutral 

Ans: Solution D with pH=7

b) Strongly Alkaline

Ans: Solution C with pH=11

c) Strongly Acidic

Ans: Solution B with pH=1

d) Weakly Acidic

Ans: Solution A with pH=4

e) Weakly Alkaline

Ans: Solution E with pH=9

f) Arrange the pH in Increasing Order of Hydrogen-ion Concentration. 

Ans: The pH can be arranged in the increasing order of the concentration of hydrogen ions as:

11<9<7<4<1

11<9<7<4<1

 

Acids can only dissociate in aqueous solution to liberate hydrogen ions which are responsible for the acidic behaviour. Acids do not show acidic behaviour in the absence of water because the dissociation of hydrogen ions from an acid occurs in the presence of water only.

 

Distilled water is a pure form of water which is free from ionic species. Therefore, it does not conduct electricity. Rain water is an impure form of water that contains many ionic species like acids and thus, it conducts electricity.

 

Activity:

a. Two nails are fitted on a cork kept in a 100 mL beaker. 

b. The nails are then connected to the terminals of a 6-volt battery through a bulb and a switch. 

c. Some dilute HCl is poured in the beaker and the current is switched on. 

d. The same experiment is then performed with glucose solution and alcohol  solution.

Observations: 

The bulb glows in the HCl solution and does not glow in the glucose or alcohol solution.

Result:

In aqueous solution, HCl dissociates into H+ and Cl− ions. These ions conduct electricity in the solution resulting in the glowing of the bulb. On the other hand, the glucose or alcohol solution does not dissociate into ions. Therefore, the bulb does not glow.

 

Conclusion: 

All acids contain hydrogen but not all compounds containing hydrogen are acids.

So, alcohols and glucose contain hydrogen, but they are not categorized as acids.

 

a) Dilute Sulphuric Acid Reacts with Zinc Granules. 

Word Equation: Sulphuric acid + Zinc → Zinc sulphate + Hydrogen

Balanced Equation: H2SO4(aq)+Zn(s)→ZnSO4(aq)+H2(g)↑

b) Dilute Hydrochloric Acid Reacts with Magnesium Ribbon.

Word Equation: Hydrochloric acid + Magnesium → Magnesium chloride + Hydrogen 

Balanced Equation: 2HCl+Mg(s)→MgCl2(aq)+H2(g)↑ 

c) Dilute Sulphuric Acid Reacts with Aluminium Powder.

Word Equation: Sulphuric acid + Aluminium → Aluminium sulphate + Hydrogen

Balanced Equation: 3H2SO4(aq)+2Al(s)→Al2(SO4)3(aq)+3H2(g)↑

d) Dilute Hydrochloric Acid Reacts with iron Filings.

Word Equation: Hydrochloric acid + Iron → Ferric chloride + Hydrogen 

Balanced Equation: 6HCl+2Fe(s)→2FeCl3(aq)+3H2(g)↑

 

 (c) Antacid is commonly used for treating indigestion.

 

(d) 16 mL

10 mL of a solution of NaOH neutralises 8 mL of a solution of HCl. Thus, 20 mL of a solution of NaOH neutralises 10/8×20/? mL =16 mL of HCl solution.

 

(b). The solution contains HCl (acid) which reacts with crushed egg-shells to give a gas (carbon dioxide) that turns lime-water milky. Egg shells contain calcium carbonate (base). 

 

 

(d). Bases turn red litmus blue. Basic solution has a pH > 7. Since, the solution turns red litmus blue, its pH would likely be 10.