Chemical compounds can be broadly divided into three categories - acids, bases and salts. Acids and bases are strong chemicals and have opposite chemical properties. Acids have a sour taste while bases have a bitter taste. When acids and bases react with each other, they form another class of compounds called salts. Thus acid, bases and salts form the three main classes of chemicals.
The word acid is derived from the Latin word acidus meaning sour to taste. In our daily life we come across many acidic substances such as citrus fruits, tomato, vinegar, tamarind, black coffee, yoghurt etc. The word alkali is derived from the Arabic word meaning calcined ashes of plants. Acids can destroy the properties of bases and vice versa the process being called neutralization.
DefinitionBack to Top
The term acid was originally used to describe any substance that had a sour taste. Many fruits are sour because they contain acids. This property gave acids their name. Vinegar which is used to preserve food and sulfuric acid which is found in car batteries are examples of acidic substances.
Bases are substances that taste bitter and feel slippery to the skin. All metal oxides and hydroxides are bases. (except ammonium hydroxide, which is not a metallic hydroxide.
Salts are neutral compounds. Salt is ionic compounds composed of a metal and a non metal or polyatomic ion. Solutions of salts in water are called electrolytes. Electrolytes as well as molten salts conduct electricity.
PropertiesBack to Top
Properties of acids
- Acids release a hydrogen ion into a water solution.
- Acids neutralize base in a neutralization reaction.
- Acids turn blue litmus to red.
Properties of bases
- Base taste bitter. Tasting of bases is more dangerous than tasting acids due to the property of stronger bases to denature protein.
- Bases release a hydroxide ion into a water solution.
- Base neutralizes acids in a neutralization reaction. Acid plus base makes water plus salt.
- Bases denature protein. This accounts for the "slippery" feeling on hands when exposed to base.
- Bases turn red litmus blue.
Properties of salts
- Salts are usually solid crystals with a relatively high melting point. Inorganic salts usually have a low hardness and low compressibility similar to edible salt.
- Salts can be clear and transparent, opaque or metallic and lustrous.
TypesBack to Top
- Organic acids - Acids derived from plants. For example, citric, oxalic, tartaric, acetic acid.
- Inorganic acids - Acids derived from minerals (mineral acids). For example, HCl, H2SO4, HNO3.
- Hydracids - Acids containing hydrogen and non-metallic element other than oxygen. For example, HCl, HBr, HI.
- Oxyacids - Acids containing hydrogen, another element and oxygen. For example, HNO3 and H2SO4.
- Normal salt - A normal salt is formed when all the ionizable hydrogen atoms in the acid are replaced by the metal.
- Acid salt - An acid salt is formed when only a part of the ionizable hydrogen atoms in the acid are replaced by the metal atom.
- Strong base - Strong bases which dissociate completely in solution. For example, NaOH, KOH etc.
- Weak base - Weak bases do not dissolve 100% completely and so they exist in equilibrium. For example ammonia and amines are weak bases.
Is Salt an Acid or Base?Back to Top
When a salt is dissolved in water ions of the salt interact with water to cause acidity or basicity in aqueous solution. A solution of the salt of strong acid and weak base is acidic and for it pH < 7 or [H+] > 10-7, for example FeCl3. Here the solution is acidic and involves cationic hydrolysis.
A solution of the salt of strong base and weak acid in basic and for it, pH > 7 or [H+] < 10-7, for example, KCN (salt of a strong base + weak acid). Here the solution is basic and involves anionic hydrolysis.
UsesBack to Top
- Sulfuric acid is used in the manufacture of paints, plastics, drugs, detergents and in petrol refining, in lead accumulators and in cleaning of metal surfaces before they are coated with other metals.
- Nitric acid is used to prepare dyes, drugs, explosives, fertilizers and quick drying paints. It is also used in the purification of gold and silver.
- Hydrochloric acid is used to remove the impurities from the surface of the articles made of iron and steel before galvanization with zinc.
- Sodium hydroxide is used in the manufacture of soap, paper, artificial silk, medicines and organic dyes in the purification of oil products. It is also used as an electrolyte in alkaline batteries.
- Calcium hydroxide is used as a material for dressing of acid burns. It is also used as a manufacture of bleaching powder and as an antidote for acid poisoning.
- Ammonium hydroxide is used as a reagent in the laboratory, and in removing grease from glass panes, ink spots from clothes.
- Ammonium chloride is used in dry cell, battery and in the manufacture of medicine.
- Potassium nitrate as meat preservative and in the manufacture of fertilizers and fireworks.
- Sliver nitrate in medicine and photography.
- Potassium carbonate in the manufacture of glass and soap.
- Sodium bicarbonate in baking and in fire extinguishers.
- Sodium hypochlorite in bleaching.
- Potash alum in the purification of water.
- Calcium sulfate in making moulds and casts.