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Introduction |
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Acids, bases and salts are three main categories of chemical compounds. These have certain definite properties that distinguish one class from the other. Scientists like Lavoisier and Davy studied acids and bases in a scientific manner. Initially, Lavoisier believed oxygen to be a constituent of all acids. It led him to frame the word "oxygen" which means "acid producer". Later, Davy was able to disprove this wrong notion, by preparing hydrochloric acid, which does not have oxygen. |
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What is an Acid? |
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The word 'acid' is derived from a Latin word, which means "sour". The sour taste of most of the fruits and vegetables is due to various types of acids present in them. The digestive fluids of most of the animals and humans also contain acids. |
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Classification of Acids |
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Acids can be classified in various ways, depending on the factors mentioned below:
a) Classification Based on the Strength of the acid.
b) Classification Based on the Concentration of the acid.
c) Classification Based on the presence of Oxygen.
d) Classification Based on the Basicity of the Acid. |
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Preparation of Acids |
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Some acids can be prepared by the direct combination of a non-metal with hydrogen. For e.g.,
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Properties of Acids |
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Taste
Acids are generally sour in taste.
Change of colours with indicators
Indicators are substances that undergo a change of colour with a change of acidic, neutral or basic medium. Acids change the colour of indicators as given in the table.
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What is a Base? |
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A base is a compound, which on dissolving in water yields hydroxyl ions (OH-) as the only negative ions. The characteristic property of a base is due to the presence of these negative hydroxyl ions. A base may be an oxide or a hydroxide of a metal. If a base is soluble in water, it dissociates to form a metal ion and the only negative hydroxyl ion. |
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Alkali |
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All alkalis are bases that dissociate in water to yield hydroxyl ion (OH-) as the only negative ions. Sodium hydroxide, potassium hydroxide, calcium hydroxide and ammonium hydroxide are the common alkalis.
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Classification of Bases |
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The strength of a base depends on the concentration of the hydroxyl ions when it is dissolved in water. |
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Methods of Preparation of Bases |
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Some bases can be prepared by the direct combination of a metal with oxygen. For e.g.,
Metal + Oxygen  Basic oxide
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Properties of Bases |
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Corrosive Action : Strong alkalis like sodium hydroxide and potassium hydroxide are highly corrosive or caustic in nature. Sodium hydroxide and potassium hydroxide are commonly called caustic soda and caustic potash respectively. Organic tissues like skin, etc. get completely corroded by these two alkalis. However, the other alkalis are only mildly corrosive. |
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Amphoteric Behavior of Some Hydroxides |
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Some hydroxides behave both as bases and acids under different conditions, e.g., hydroxides of zinc, lead and aluminium. These hydroxides are only weakly basic, but like other bases, they still combine with acids to form salt and water. |
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Modern Concept of Acid and Base |
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When acids combine with bases, acids give hydrogen ions to the base, and the base accepts them. We have already seen that a hydrogen ion is a proton. So when acid gives away a hydrogen ion, it is in fact giving away a proton. Thus, the base while accepting a hydrogen ion, is accepting a proton. So an acid may be defined as a substance that gives away protons, and a base, as a substance that receives protons. |
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Neutralization |
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The acidic property of an acid is due to the presence of hydrogen ions (H+) while that of a base or alkali, is due to the presence of hydroxyl (OH-) ions in them. When an acid and base (alkali) combine, the positively charged hydrogen ion of the acid combines with the negatively charged hydroxyl ion of the base to form a molecule of water. Hence, the water molecule formed does not have any charge because the positive and negative charges of the hydrogen ions and hydroxyl ions get neutralized.
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The pH Scale |
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The strength of acids and bases can be expressed in terms of the concentration of the hydronium ions or hydroxyl ions in a solution. The concentration of hydronium ions in a solution can be expressed as moles of hydronium ions per litre of solution. This is expressed by a term called 'pH' which represents the 'power of hydrogen ion concentration' in solution. |
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Indicators |
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Indicators are the chemicals that when added to solutions show whether the solution is acidic or basic, by a sudden change of their colour. The common indicators used and the colour changes observed are mentioned below:
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Salt |
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Generally, salts are obtained by treating an acid with a base. Salts consist of both positive ions or cations, and negative ions or anions. The cations are called basic radicals and are obtained from metallic ions or ammonium ion, while the anions are called acidic radicals and are obtained from acids.
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Classification of Salts |
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Normal salt : A salt that does not contain any replaceable hydrogen ions is a normal salt. It is obtained by replacing all the hydrogen ions of an acid by metal ions or ammonium ions. Examples: NaCl, Na2SO4, Na3PO4, NH4Cl, K2CO3, CuSO4 etc. |
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General Properties of Salts |
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Water of Crystallisation : When crystals of certain salts are formed, they do so with a definite number of molecules of water, chemically combined in a definite proportion. Water of crystallisation is the number of water molecules, chemically combined in a definite molecular proportion, with the salt in the crystalline state. This water is responsible for the geometric shape and colour of the crystals. |
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Summary |
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Strength of an acid depends on the concentration of hydronium ions present in a solution. Greater the numbers of hydronium ions present, greater is the strength of the acid. |
