Class 10 Science Chapter 2: Acids, Bases and Salts - NCERT Solutions

Litmus papers are indicators that differentiate between acidic, basic and neutral solutions. Red litmus changes to blue with basic solution If the colour of red litmus paper gets changed to blue, then it indicates that the given solution is a base and if there is no colour change, then it is either acidic or neutral. Whereas blue litmus turns to red with acidic solution. If the colour of blue litmus paper gets changed to red, then it indicates that the given solution is a acid and if there is no colour change, then it is either basic or neutral. This can be best explained by a simple experiment. Let's take 3 test tubes and labelled as A, B, and C which contain water, acidic and basic solutions respectively by using litmus paper test. We confirm that test tube A does not show any change in that indicates it is not acid and base. Test tube B also does not show any change in litmus paper. Only test tube C changes red litmus to blue indicating the solution is base.

Curd and other sour substances should not be kept in metal vessels like copper and brass because this substance contains acids that react with the acid to liberate hydrogen gas and harmful products or poisonous substances, therefore spoiling the food.

An acid when reacts with a metal Hydrogen gas is liberated

Let's take a few pieces of zinc metals and add a few drops of dilute H₂SO₄ mouth of the test tube is fitted with a cork and the delivery tube is fitted so that the gas evolved reaches the soap solution. Hydrogen gas forms bubbles on the soap solution.

H₂SO₄ + Zn → ZnSO₄ + H₂ ↑

When we bring a burning candle near the gas, a pop sound will produce. It indicates the presence of Hydrogen gas.

Suppose Metal compound A is calcium carbonate when reacted with HCl it produces a compound CaCl₂ with CO₂ and water.

HCl or HNO₃  shows acidic character on aqueous solution because hydrogen ions  dissociation always occurs in the presence of water. Hydrogen ions (H₊) when combined  with H₂O to form hydronium ions (H₃O₊).

The reaction is as follows:

HCL_(aq)  →  H⁺ + CL^-

H⁺ + H₂O → H₃O⁺

Whereas aqueous solutions of glucose and alcohol contain hydrogen, these cannot dissociate in water to form hydrogen ions. Therefore they do not show acidic character.

In aqueous solutions acids dissociate to form ions. These ions conduct electricity. In dry condition acid does not dissociate to form ions.

Dry HCl gas does not contain H₊ ions in the aqueous condition only an acid dissociates to give ions. Hence, their is no change in the colour of the litmus paper.

Dissolving an acid in water is an exothermic reaction, it is always recommended that acid should be added to water, in this case heat produced is not sufficient to burn out, if we add acid to water then it is possible that because of the large amount of heat generated, the mixture splashes out and causes burns.

When an acid gets diluted, the concentration of hydronium ions (H₃O⁺) per unit volume decreases. This means that the strength of the acid decreases when it gets diluted.

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

When pH value is less than 7 it indicates an acidic solution, while greater than 7 it indicates a basic solution. Therefore, the solution A with pH = 6 is acidic and has more hydrogen ion concentration than the solution B of pH=8 which is basic.

Concentration of H⁺ can have a varied effect on the nature of the solution. H⁺ ion concentration increases, the solution becomes more acidic, while a decrease of H⁺ ion causes an increase in the basicity of the solution.

Yes, the basic solution also has H⁺ ions. However, H⁺ concentration is less as compared to the concentration of OH⁻ ions so the solution is basic.

When the soil is acidic and improper for agriculture, then to increase the basicity of soil, the farmer would treat the soil with quick lime or slaked lime or chalk.

CaOCl₂ is commonly known as bleaching powder.

[Ca (OH)₂] Calcium hydroxide commonly known as slaked lime, on treatment with chlorine, yields bleaching powder.

Washing soda (Na₂CO₃.10H₂O) chemically known as sodium carbonate deca hydrate  is used for softening hard water.

When a solution of sodium hydrocarbonate (sodium hydrogen carbonate) is heated, sodium carbonate water and carbon dioxide gas evolved.

The chemical equation when Plaster of Paris and water reacts

(d) 10

Because, Bases turn red litmus to blue and acids turn blue litmus to red. pH value of basic solution greater than 7. Therefore the solution turns red litmus blue, its pH is likely to be 10.

(b) The solution contains HCl.

(d) 16 mL of HCl solution will be required to neutralise.

(c) Antacid is used for treating indigestion.because antacid neutralises the acidity produced in the stomach.

(a) when sulphuric acid reacts with zinc granules, the product formed is zinc sulphate and hydrogen gas.

H₂SO₄ + Zn → ZnSO₄ + H₂

Sulphuric acid + Zinc → Zinc sulphate + Hydrogen

(b) when dilute hydrochloric acid reacts with magnesium it gives magnesium chloride and hydrogen gas evolved.

2HCl + Mg → MgCl₂ + H₂

Hydrochloric acid + Magnesium → Magnesium chloride + Hydrogen

(c) when sulphuric acid reacts with aluminium it gives aluminium sulphate and hydrogen gas evolve.

3H₂SO₄ + 2Al → Al₂(SO₄)₃ + 3H₂

Sulphuric acid + Aluminium → Aluminium sulphate + Hydrogen

(d) when iron metal reacts with hydrochloric acid it forms ferric chloride and hydrogen gas.

2HCl + Fe → FeCl₂ + H₂

Hydrochloric acid + Iron → Ferric chloride + Hydrogen

Because glucose and alcohol do not dissociate into ions. Therefore these solution are not considered as acid -

Experimental verification :

Procedure : Two nails are fitted in a cork and they are kept in a 100 ml beaker. Then the nails are connected to the two terminals of the battery of 6 volt along with a bulb and a switch. Now some dilute HCl is added in the beaker and allows the current to pass through the circuit. The same experiment is carried out with glucose and alcohol solutions separately in place of HCl.

Observations :

The bulb glows only in HCl solution only. Because HCl solution dissociates into H⁺ and Cl^– ions which conduct electricity whereas glucose and alcohol solutions do not dissociate into ions and so, these solutions do not conduct electricity. So the bulb doesn't glow.

Interpretation :

Activity above hereby proved that all acids contain hydrogen but not all compounds that contain hydrogen are acids. Therefore compounds such as alcohols and glucose contain hydrogen, and are not categorised as acids.

Distilled water is a pure form of water that 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.

Acids do not show acidic behaviour in the absence of water because the dissociation of hydrogen ions from an acid occurs only in the presence of water, the hydrogen ions are responsible for the acidic behaviour.

(a) Solution D is neutral having pH 7

(b) Solution C is strong alkaline with pH 11

(c) Solution B is strong acidic with pH 1

(d) Solution A is weak acidic with pH 4

(e) Solution E is weak alkaline with pH 9

11 < 9 < 7 < 4 < 1. The pH can be arranged in the increasing order of the concentration of hydrogen ions.

Fizzing will occur more strongly in test tube A than in test tube B. As test tube A contains hydrochloric acid (HCl) which is a strong acid that reacts vigorously with magnesium whereas acetic acid  CH₃COOH is weak acid. Being a strong acid, therefore hydrogen gas evolved a faster speed that's why it's more fizzing occurs.

Normally the pH of milk is 6,  as cured is acidic in nature pH will reduce.

(a) Milkman shifts the pH of the fresh milk from 6 to slightly alkaline.This is because in an alkaline condition, milk does not coagulate into curd easily.

(b) The milk is slightly more basic than usual milk, as base neutralise the acids produced to set the curd. Therefore, it takes a longer time to convert the curd.

Plaster of Paris (POP) is stored in a moisture-proof container because POP absorbs water vapour from the surrounding and reacts to form a hard mass solid known as gypsum.

A reaction in which an acid and base react with each other to form salt and water is known as neutralization reaction. During this reaction energy is evolved in the form of heat.

For example:

(i) sodium hydroxide a strong base reacts with strong acid hydrochloric acid produces neutral salt sodium chloride and water molecules.

(ii) In case of indigestion which caused due to the excess production of hydrochloric acid in the stomach resulting in acidity, to neutralise the effect of acid an antacid, Mg(OH)₂ which is basic in nature is given, thus gives relief from indigestion.

Two important uses of washing soda and baking soda are:

1) Washing soda:

      (a) Washing soda is used to remove permanent hardness of water.
      (b) It is used for domestic household washing purposes.
      (c) It is used in glass, soap, and paper industries.

2) Baking soda:

      (a) Baking powder is prepared by using baking soda. Baking powder is formed by the mixture of baking soda with a mild acid known as tartaric acid. When it is heated or mixed in water, CO₂ gas is evolved that makes bread or cake fluffy.
      (b) Soda-acid fire extinguishers are prepared by baking soda.
      (c) It is used in making pakora more crispy.