Acids and Bases:
Formation of Acids:
The word acid comes from the Latin word ‘acidus‘ which means ‘sour’ in taste. Examples of substances that are sour in taste are lemons, mangoes, tomatoes, grapes, tamarind, green apples etc. Non-metals such as sulfur, carbon and phosphorous burn in oxygen (air) and form oxides. These oxides are called acidic oxides or non-metallic oxides.
Some examples of acidic oxides are-
|S (sulfur) + O2 (oxygen) ——-> SO2 (sulfur dioxide)|
C (Carbon) + O2 (oxygen) ——-> CO2 (carbon dioxide)
4P (phosphorous) + 5O2 (oxygen) ——-> 2P2O5 (phosphorous pentoxide)
2N2 (nitrogen) + 5O2 (oxygen) ——-> 2N2O5 (nitrogen pentoxide)
Non-metallic oxides react with water to form acids-
|SO2 + H2O ——-> H2SO3 (sulfurous acid)|
CO2 + H2O ——-> H2CO3 (carbonic acid)
P2O5 + 3H2O ——-> 2H3PO4 (phosphoric acid)
SO3 + H2O ——-> H2SO4 (sulfuric acid)
The above-mentioned acids are inorganic acids. They should never be tasted to find their sourness. They corrode skin, clothes and metals.
There are acids that we consume daily in our food. They are called organic acids. They are weak and hence are not corrosive. Examples of these acids are citric acid present in lemons and acetic acid present in vinegar. All the above examples of acids contain hydrogen.
Testing For Acids:
It is dangerous to taste unknown solutions in the laboratory. Some chemicals may burn our tongue, and others may be poisonous. So for testing, we use a substance called litmus to indicate the presence of a certain substance (acid). Litmus is a plant (vegetable) dye that is blue in colour. It changes colour when it comes in contact with certain substances. When a piece of blue litmus paper is dipped in any acid, it turns red. A base turns red litmus blue.
General Properties of Acids:
- All the acids contain hydrogen as an essential constituent. For example- H2SO3, H2SO4, H2CO3, HNO3, HCl, CH3COOH (acetic acid), H3PO4.
- All acids have sour taste.
- All acids turn blue litmus red.
- All acids (except nitric acid) react with metals (zinc, magnesium, iron, etc.) to produce salts and liberate hydrogen gas.
|Zn (Zinc) + H2SO4 (sulfuric acid) ——-> ZnSO4 (zinc sulfate)+ H2 (hydrogen gas)|
Zn (Zinc) + 2HCl (hydrochloric acid) ——-> ZnCl2 (zinc chloride)+ H2 (hydrogen gas)
- Acids are corrosive. They can cause serious burns on the skin and can eat away paper, metal or cloth on contact.
- Acids react with carbonates to release carbon dioxide gas with effervescence and produce the corresponding salt and water. This property of acid is used in fire extinguishers to release carbon dioxide.
|Zinc carbonate + Sulfuric acid ——-> Zinc sulfate + Water + Carbon dioxide|
In symbols, the chemical equation is,
|ZnCO3 (zinc carbonate) + H2SO4 (sulfuric acid) ——-> ZnSO4 (zinc sulfate) + H2O + CO2|
Uses of Acids:
- Vinegar used in the kitchen is a liquid containing 3-6% acetic acid. It is used in pickles and in many food preparartions.
- Lemon and orange juice contain citric acid. It is used in the preparation of effervescent salts and as a food preservative.
- Acids have been put to many uses in industry. Nitric acid and sulfuric acid are used in the manufacture of fertilizers, dyes, paints, drugs and explosives.
- Sulfuric acid is used in batteries which are used in cars etc. Tannic acid is used in the manufacture of ink and leather.
Formation of Bases:
The oxides and hydroxides of metals are called bases. The most important bases are sodium hydroxide (NaOH), potassium hydroxide (KOH) and calcium hydroxide Ca(OH)2. The symbol for the metal is written first in the formula, followed by the word hydroxide. The valency of the hydroxyl group (-OH) is one.
When magnesium ribbon burns in air (oxygen) it forms magnesium oxide (white ash) which is soluble in water, giving rise to magnesium hydroxide.
The reaction is as follows-
|2Mg + O2 ——-> 2MgO (magnesium oxide)|
MgO + H2O ——-> Mg(OH)2 (magnesium hydroxide)
Calcium hydroxide is prepared by adding water to calcium oxide (quick lime). A large amount of heat is produced during the reaction-
|CaO + H2O ——-> Ca(OH)2 + heat|
Alkalis- All hydroxide bases soluble in water are called alkalis. Therefore all bases are not alkalis but all alkalis are bases. Alkalis are the chemical opposites of acids. Some common alkalis are NaOH (Caustic Soda), KOH (Caustic Potash), Ca(OH)2 etc.
General Properties of Bases (and Alkalis):
Bases are usually oxides and hydroxides of metals. Bases that dissolve in water are called alkalis. They have the following properties-
- Bases are bitter to taste and soapy to touch.
- Bases turn red litmus paper blue.
- Bases are characterized by the hydroxyl (-OH) group.
- Bases react with acids to form salt (a neutral substance) and water.
- Baes are corrosive to skin; they should not be touched by hand.
Uses of Bases:
- Sodium hydroxide (caustic soda) is used in the manufacture of soap. It is used in petroleum refining; in making medicines, paper, pulp etc. It is also used in making rayon.
- Calcium hydroxide is also known as slaked lime. It is used to neutralize acid in water supplies; in the manufacture of bleaching powder; as a dressing material for acid burns; as an antidote for food poisoning; in the preparation of fungicide and in the mixture of whitewash. It is mixed with sand and water to make mortar which is used in the construction of buildings. It is also used by farmers on the fields to neutralize the harmful effects of acid rain.
- Ammonium hydroxide is used to remove ink spots from clothes and to remove grease from window panes.
- Alkalis are used in alkaline batteries. Generally, potassium hydroxide is used in such batteries.