ICSE Class 7 Chemistry Chapter 6 Acids Bases and Salts

Acids, Bases And Salts

In This Chapter You Will Learn

Acids: their properties and uses

Bases: their properties and uses

Salts: their properties and uses

Indicators

Soil treatment

Disposal of factory wastes

Sulphuric acid is used in batteries, while vinegar, used in the kitchen, contains acetic acid.

Introduction

Compounds are pure substances. The smallest particle exhibiting all the properties of a compound is a molecule. There are very large number of compounds which contain different types of molecules. Therefore they all are different in their properties.

On the basis of their properties, chemical compounds are mainly divided into three classes.

Acids (e.g.: Hydrochloric acid, acetic acid, etc.)

Bases (e.g.: Sodium hydroxide, calcium hydroxide, etc.)

Salts (e.g.: Common salt (NaCl), copper sulphate, etc.)

6.1 Acids

The word 'acid' is derived from the Latin word acidus, meaning sour. Many fruits and vegetables are sour in taste because they contain acids. Examples: Oranges, lemons, grapes, etc. Different foods contain different acids. Some of them are listed in table 6.1 ahead.

Teacher's Note

You interact with acids daily - from the citric acid in your morning orange juice to the acetic acid in vinegar used for cooking.

6.2 Definition Of An Acid

All acids essentially contain hydrogen atom(s) in their molecule which can be released in water. Therefore, an acid is a compound, which furnishes hydrogen ions (H+) as the only positively charged ions when dissolved in water.

Example:

H2SO4 (sulphuric acid)

2H+ + SO42- (sulphate ion)

HNO3 (nitric acid)

H+ (hydrogen ion) + NO3- (nitrate ion)

The H+ ions so furnished cannot exist freely. Therefore, they combine with water molecules to give positively charged hydronium ions.

H2O + H+ - H3O+ (hydronium ion)

Teacher's Note

Understanding how acids release hydrogen ions is fundamental to explaining why lemon juice tastes sour and can clean surfaces.

6.3 Classification Of Acids

Acids can be classified in different ways.

A. On The Basis Of Their Sources

Naturally occurring (organic) acids

Mineral (inorganic) acids

6.3.1 Naturally Occurring Acids

These acids are mostly derived from plants, therefore, they are called organic acids. Acetic acid, citric acid, tartaric acid, etc. They are present in our food. They can also be prepared artificially.

6.3.2 Mineral Acids

These acids are prepared from minerals present in the earth's crust. They are also called inorganic acids. They are important laboratory reagents. The three most important inorganic acids are hydrochloric acid, sulphuric acid and nitric acid. Hydrochloric acid is also produced in our stomach. It helps in digestion.

Besides these, boric acid is present in our tears, and carbonic acid in aerated drinks.

Table 6.2 Important inorganic acids and their chemical formulae

B. On The Basis Of Molecular Composition

Oxy acids: These acids contain oxygen atoms in their molecule. e.g.: sulphuric acid (H2SO4), nitric acid (HNO3), etc.

Hydracids: These acids do not contain oxygen atoms in their molecules. e.g.: hydrochloric acid (HCl), hydrobromic acid (HBr).

The atmosphere of venus is made up of thick white and yellowish clouds of sulphuric acid.

Teacher's Note

Organic acids from fruits are used in cooking and preservation, while mineral acids are essential in industries and laboratories for various chemical processes.

6.4 Basicity Of An Acid

The basicity of an acid is the number of hydrogen ions (H+) produced by one molecule of that acid when dissolved in water.

Acids can be monobasic, dibasic, tribasic, or more, depending upon the number of hydrogen ions furnished per molecule.

Monobasic acids: Acids that furnish only one hydrogen ion (H+) per molecule in water, are called monobasic acids.

Examples: Hydrochloric acid, nitric acid, acetic acid, etc.

HCl + H2O - H3O+ + Cl- (chloride ion)

Dibasic acids: Acids that furnish two hydrogen ions (H+) per molecule in water, are called dibasic acids.

Examples: Sulphuric acid, carbonic acid, sulphurous acid, oxalic acid, etc.

Tribasic acids: Acids that furnish three hydrogen ions per molecule in water are called tribasic acids.

Examples: Phosphoric acid, phosphorus acid, etc.

Table 6.3 Acids and their basicity

Teacher's Note

Basicity determines how many particles an acid can release, which affects its strength and how much base is needed to neutralize it - important in cooking and cleaning applications.

6.5 On The Basis Of The Strength Of Acids

Acids can be divided into two types, depending upon the amount of hydrogen ions furnished in water.

Strong acids: Acids that produce large amount of hydrogen ions in water are called strong acids. They dissociate in water to a large extent.

Examples: Sulphuric acid, nitric acid and hydrochloric acid.

Weak acids: Acids that dissociate very less and produce only a few hydrogen ions in water are called weak acids.

Examples: Sulphurous acid, carbonic acid, phosphoric acid, all organic acids, etc.

A solution of a strong acid containing only ions.

A solution of a weak acid containing both ions and molecules.

Teacher's Note

Strong acids like those in car batteries are dangerous and require careful handling, while weak acids like vinegar are safe enough to use in cooking.

6.6 On The Basis Of Concentration

An acid can be concentrated or dilute.

Concentration of acids means amount of acid in a definite amount of water.

A concentrated acid is that in which amount of acid is more and the amount of water is less.

A dilute acid is that which contains less amount of acid and more water.

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