Class 10 Science Acid Bases and Salts Exam Notes

Class 10 Science Acid Bases and Salts Exam Notes. Please refer to the examination notes which you can use for preparing and revising for exams. These notes will help you to revise the concepts quickly and get good marks.

Intorduction

A chemical substance is a kind of matter that cannot be separated into any other kind of matter by physical method. Pure substances have uniform composition and always have some properties like taste, smell, texture, etc.

We use several materials in our daily life for various purposes. Some of them are sweet in taste, some are salty, some are sour and some are bitter.

The sweet taste of a substance is due to sugar present in it. Why are certain food materials sour in taste like curd, vinegar, tamarind, lemon juice, green mango, tomatoes, orange, unripened grapes, others are sweet in taste whereas some are salty and some are bitter in taste ? What substance are present in which make them sour, bitter or salty ?

Acids 

The term 'acid' has its origin in the Latin word acidus, meaning sour. In fact, anything that tastes sour contains an acid. For example, lemon juice, tomato, vinegar, etc., all taste sour. So, each of these substances must contain an acid. Some of the naturally occurring substances that contain acids are given in Table

able 

Aqueous solutions of acids are generally sour in taste. Acids turn blue litmus red, conduct electricityand react with bases to form salts and water. [Bases and salts are discussed a little later.]

An acid may be defined in various ways. Here, we shall study the definition given by Liebig in 1838. According to Liebig, an acid is a compound which contains hydrogen that can be replaced partially or wholly by a metal or a group of elements acting like a metal, to produce a salt.

For example, sulphuric acid (H₂SO₄) is an acid because of the following reasons. (i) It contains hydrogen atoms in its molecule.

(ii) The two hydrogen atoms present in its molecule can be replaced partially or wholly by a metal like sodium (Na) to produce sodium hydrogensulphate or sodium sulphate.

The hydrogen atoms in H₂SO₄ can also be partially or wholly replaced by a group of elements, like an ammonium ion (NH₄+ ) to form ammonium hydrogensulphate (NH₄HSO₄) or ammonium sulphate ((NH₄)2SO₄) respectively.

The substances NaHSO₄, Na2SO₄, NH4HSO₄ and (NH₄)2SO₄ are all salts.

(iii) The acid dissolves in water to make a solution that turns blue litmus red. (iv) It is sour in taste.

(v) It reacts vigorously with a base to produce a salt.

The hydrogen atoms present in an acid that can be replaced by a metal or a group of elements are called replaceable hydrogen or acidic hydrogen.

 Classification of Acids :

Depending upon the elements present, acids may be classified as follows.

(i) Oxyacid : Acids that contain both hydrogen and oxygen are called oxyacids. For example, nitric acid

(HNO3), sulphuric acid (H₂SO₄) and phosphoric acid (H₃PO₄) are oxyacids.

(ii) Hydracid : Acids that contain hydrogen and other nonmetallic element(s), except oxygen, are called hydracids. For example, hydrochloric acid (HCl) and hydrocyanic acid (HCN) are hydracids.

Acids may also be classified as follows.

1. Organic and inorganic acids : All sour things that we use in our daily food contain acids. These acids areorganic acids. Some of the common acids that are generally used in the laboratory are hydrochloric acid (HCl), sulphuric acid (H₂SO₄) and nitric acid (HNO₃). These are inorganic acids, also called mineral acids. Hydrochloric acid is also present in the gastric juice in our stomach.

2.Concentrated and dilute acids : An acid solution may be concentrated or dilute depending upon the amount of the acid present in the solution. Concentrated and dilute solutions of acids are generally used in laboratories. Let us see what these acids are.

An acid is generally used as solution in water. When the solution contains a larger amount of the acid, it is said to be concentrated, whereas a dilute solution contains smaller amount of the acid.

Thus, concentrated and dilute solutions of an acid differ from each other only in the proportions of the acid and water in them.

3. Strong and weak acids : The strength of an acid is determined by the amount of hydrogen ions (H+)

that the acid provides when dissolved in water.

Some of the acids, when dissolved in water, get almost completely dissociated to provide hydrogen ions. These acids are called strong acids. For example, hydrochloric acid (HCl), nitric acid (HNO₃) and sulphuric acid (H₂SO₄) are strong acids.

On the other hand, there are some acids which when dissolved in water, are only incompletely dissociated to give hydrogen ions. These are called weak acids. For example, carbonic acid (H₂CO₃) and acetic acid (CH₃COOH) are weak acids.

 Basicity of an acid :

The basicity of an acid is the number of replaceable hydrogen atoms present in a molecule of the acid.

The acid which contains one replaceable hydrogen atom in its molecule is called a monobasic acid and its basicity is 1. The acids containing two or three replaceable hydrogen atoms in their molecules are called dibasic acids or tribasic acids and their basicities are 2 or 3.

Examples of a few acids with their basicities are given in the table below.

 Preparation of Acids :

There are several methods for preparing acids. Some of them are discussed here.

1. Synthetic method : In the synthetic method, acids are prepared by direct combination of elements.

For example, hydrogen and chlorine react together under the action of an electric spark to produce hydrogen chloride gas which is absorbed in water to give hydrochloric acid.

2.By dissolving acidic oxides in water : Some oxides dissolve in water to give acids. These oxides are called acidic oxides. For example, sulphur trioxide (SO3) dissolves in water to give H₂SO₄.

SO₃ + H₂O → H₂SO₄

Silimlarly, carbon dioxide (CO2) dissolves in water to produce carbonic acid (H₂CO₃).

CO2 + H₂O →H₂CO₃

3. By the action of an acid on the salt of another acid : An acid having higher boiling point can react with the salt of an acid of lower boiling point to produce an acid. For example, NaCl is a salt of HCl. The boiling point of HCI is lower than that of H₂SO₄, When NaCl (salt of HCl) reacts with H₂SO₄, HCl is formed.

 General Properties of Acids :

1. They are sour in taste.

2. They turn blue litmus paper red.

H₂SO₄ + NaCl → NaHSO₄+ HCl

3. Acids show acidic properties only in the presence of water. This can be demonstrated by the following activity.

Dry hydrogen chloride gas does not produce H+ ions in the absence of moisture/water. It produces H+

ions only in the presence of moisture/water.

HCl + H₂O → H₃O+ + Cl–

4. Their aqueous solutions conduct electricity.

5.They react with certain metals with the evolution of hydrogen gas.

EXAMPLES :

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