WBCS Preliminary (Chemistry):Acid, Base and Salts.

1. ACID:
   • 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.
   • An acid is a compound, which on dissolving in water yields hydronium ions (H₃O⁺) as the only positive ions. The characteristic property of an acid is due to the presence of these hydronium ions.
   • Acids are compounds that contain Hydrogen (Hydrochloric, HCl; Sulphuric, H₂SO₄; Nitric, HNO₃). However, not all compounds that contain Hydrogen are acids (Water, H₂O; Methane, CH₄). Acids are usually compounds of non metals with Hydrogen and sometimes Oxygen.
   • Acids can be classified in various ways, depending on the factors mentioned below:
     1. Classification Based on the Strength of the acid.
     2. Classification Based on the Basicity of the Acid.
     3. Classification Based on the Concentration of the acid.
     4. Classification Based on the presence of Oxygen.
   • The strength of an acid depends on the concentration of the hydronium ions present in a solution. Greater the number of hydronium ions present, greater is the strength of acid. However, some acids do not dissociate to any appreciable extent in water such as carbonic acid. Therefore, these acids will have a low concentration of hydronium ions.
   • Strong Acid: An acid, which dissociates completely or almost completely in water, is classified as a strong acid. It must be noted that in these acids all the hydrogen ions (H+) combine with water molecule and exist as hydronium ions (H3O+). Examples of strong acids are: hydrochloric acid, sulphuric acid, nitric acid etc.
   • Weak Acid:An acid that dissociates only partially when dissolved in water, is classified as a weak acid. Most of the molecules remain in solution in molecular form itself in such acid. Examples are: acetic acid, formic acid, carbonic acid etc.
   • Acids are generally sour in taste. Special type of substances are used to test whether a substance is acidic or basic. These substances are known as indicators. The indicators change their colour when added to a solution containing an acidic or a basic substance. Turmeric, litmus, china rose petals (Gudhal), etc., are some of the naturally occurring indicators.
   • The most commonly used natural indicator is litmus. It is extracted from lichens . It has a mauve (purple) colour in distilled water. When added to an acidic solution, it turns red and when added to a basic solution, it turns blue. It is available in the form of a solution, or in the form of strips of paper, known as litmus paper. Generally, it is available as red and blue litmus paper.
   • The solutions which do not change the colour of either red or blue litmus are known as neutral solutions. These substances are neither acidic nor basic.
   • Acids are corrosive and can burn flesh and dissolve metal.
2. Bases and Alkalis:
   • A Base is a substance that gives OH^- ions when dissolved in water. Bases are usually metal hydroxides (MOH). Examples include Sodium Hydroxide, NaOH, Calcium Hydroxide, Ca(OH)₂. The solution of a base in water is called an alkali.
   • Bases and acids neutralize each other, therefore another way to define a base is 'a compound which reacts with an acid to give salt and water only'. Like acids, alkalis can be strong or weak. The more hydroxide ions they produce, the stronger the alkali.
   • 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.
   • The strength of a base depends on the concentration of the hydroxyl ions when it is dissolved in water.
     1. Strong Base : A base that dissociates completely or almost completely in water is classified as a strong base. The greater the number of hydroxyl ions the base produces, the stronger is the base. Examples: Sodium hydroxide: NaOH , Potassium hydroxide: KOH , Calcium hydroxide: Ca(OH) ₂.
     2. Weak Base: A base that dissociates in water only partially is known as a weak base. Examples: Magnesium hydroxide: Mg(OH) ₂, Ammonium hydroxide: NH₄OH.
   • Bases are bitter to taste. They are soapy and slippery to touch. 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.
3. pH:
   • A scale for measuring hydrogen ion concentration in a solution, called pH scale has been developed. The p in pH stands for ‘potenz’ in German, meaning power. On the pH scale we can measure pH from 0 (very acidic) to 14 (very alkaline). pH should be thought of simply as a number which indicates the acidic or basic nature of a solution. Higher the hydronium ion concentration, lower is the pH value. The pH of a neutral solution is 7. Values less than 7 on the pH scale represent an acidic solution. As the pH value increases from 7 to 14, it represents an increase in OH– ion concentration in the solution, that is, increase in the strength of alkali. Generally paper impregnated with the universal indicator is used for measuring pH. One such paper is shown in .
   • There are chemicals that change colour at different pH values. These are called indicators. One of the most famous is Litmus. This substance turns red when the pH is less than 7 (acidic) and turns blue when the pH is greater than 7 (basic).
4. Salts:
   • A Salt results when an acid reacts with a base. Both are neutralised. The H+ and OH- ions combine to form water. The non metalic ions of the acid and the metal ions of the base form the salt.
   • Important salts used in everyday life and industrial applications are Sodium chloride (NaCl), Sodium carbonate, (Na ₂ CO₃), Sodium Bicarbonate, (NaHCO₃), Sodium Hydroxide (NaOH)
   • The salt ions normally stay in solution. The salt crystalizes out when the water is removed. Some salts are insoluble. They will precipitate out when the acid and base are added together.
   • Salts of a strong acid and a strong base are neutral with pH value of 7. On the other hand, salts of a strong acid and weak base are acidic with pH value less than 7 and those of a strong base and weak acid are basic in nature, with pH value more than 7.