Notes On Bases - CBSE Class 10 Science
According Arrhenius theory any substance that can produce hydroxide ions when dissolved in water is called as a base.         Substance + Water → Metal ion + OH– Example:         NaOH (aq)  → Na+ (aq) + OH– (aq) A base is said to be an alkali if it is soluble in water. In general hydroxides of alkali metals and alkaline earthmetals are considered as alkalies. Example:         KOH  (aq) → K+(aq) + OH– (aq)          Ca(OH)2(aq) → Ca+2 (aq) + OH– (aq) It is not a necessary that a base should contain hydroxide ion. There are some bases even they does not contain hydroxide ion, can be considered as bases. Example: Ammonia (NH3) Ammonia when dissoled in water forms ammonium hydroxide which is a weak base. NH3 + H2O → NH4OH (aq) Oxides of alkali metals and alkaline earthmetals are also considered as basic in nature. Example: CaO, MgO, Na2O, K2O...etc Classification of bases: Classification based on the strength: Based on the extent of ionisation bases are classified into strong bases and weak bases. Strong bases: The bases which undergoes complete ionisation in aquesous solution are called as strong bases. Example: NaOH, KOH...etc Weak bases: The bases which undergoes partial ionisation in aqueous solution are called weak bases. Example: NH4OH, NH3...etc Classification of based on acidity: Based on acidity bases can be classified into different types. They are: Mono acidic base Di acidic base Tri acidic base Mono acidic bases: Bases which produces only one hydroxide (OH-) ion in aqueous solutions are called mono acidic bases. Example: NaOH, KOH...etc Di acidic bases: Bases which produces two hydroxide ions in aqueous solutions are called di acidic bases. Example: Ca(OH)2, Mg(OH)2...etc Tri acidic bases: Bases which produces three hydroxide ions in aqueous solutions are called tri acidic bases. Example: Al(OH)3, Fe(OH)3...etc Physical properties of bases:         • Bases are bitter to taste, soapy to touch.         • Bases are good conductors of electricity in aqueous solution. In aqueous solution, they release ions, which conduct electricity.         • Bases liberates heat on dilution. Indicators in pressence of bases: Bases turns red litmus to blue. Phenolphthalein turns pink in pressence of bases. Methyl orange turns to yellow in pressence of bases. Chemical properties: Reaction with active metals: Bases react with metals to liberate hydrogen gas Example: Sodium hydroxide react with zinc and liberate hydrogen and sodium zincate.            NaOH + Zn → Na2ZnO2 + H2 Reaction with non-metal oxides: Bases react with non-metallic oxides to form salt and water. This is similar to a neutralization reaction between an acid and a base.             Example: Calcium hydroxide reacts with carbon dioxide to form calcium carbonate and water               Ca(OH)2 + CO2 → CaCO3 + H2O From this reaction, it can be concluded that non-metallic oxides are acidic in nature. Reaction with acids: Bases reacts with acids to form salts and water. Example: Potassium hydroxide reacts with hydrochloric acid to form potassium chloride and water. KOH + HCl → KCl + H2O Uses of Bases:         •  Mild bases neutralise the acidity in the stomach.         • Sodium hydroxide is used in the manufacture of soaps, paper and synthetic fibres like rayon.         • Calcium hydroxide is used in the manufacture of bleaching powder. Bleaching powder is used as a disinfectant.         • Magnesium hydroxide is used as an antacid to neutralize the acid in the stomach.         • Ammonium hydroxide is used in the preparation of fertilizers like ammonium phosphate and ammonium sulphate.

#### Summary

According Arrhenius theory any substance that can produce hydroxide ions when dissolved in water is called as a base.         Substance + Water → Metal ion + OH– Example:         NaOH (aq)  → Na+ (aq) + OH– (aq) A base is said to be an alkali if it is soluble in water. In general hydroxides of alkali metals and alkaline earthmetals are considered as alkalies. Example:         KOH  (aq) → K+(aq) + OH– (aq)          Ca(OH)2(aq) → Ca+2 (aq) + OH– (aq) It is not a necessary that a base should contain hydroxide ion. There are some bases even they does not contain hydroxide ion, can be considered as bases. Example: Ammonia (NH3) Ammonia when dissoled in water forms ammonium hydroxide which is a weak base. NH3 + H2O → NH4OH (aq) Oxides of alkali metals and alkaline earthmetals are also considered as basic in nature. Example: CaO, MgO, Na2O, K2O...etc Classification of bases: Classification based on the strength: Based on the extent of ionisation bases are classified into strong bases and weak bases. Strong bases: The bases which undergoes complete ionisation in aquesous solution are called as strong bases. Example: NaOH, KOH...etc Weak bases: The bases which undergoes partial ionisation in aqueous solution are called weak bases. Example: NH4OH, NH3...etc Classification of based on acidity: Based on acidity bases can be classified into different types. They are: Mono acidic base Di acidic base Tri acidic base Mono acidic bases: Bases which produces only one hydroxide (OH-) ion in aqueous solutions are called mono acidic bases. Example: NaOH, KOH...etc Di acidic bases: Bases which produces two hydroxide ions in aqueous solutions are called di acidic bases. Example: Ca(OH)2, Mg(OH)2...etc Tri acidic bases: Bases which produces three hydroxide ions in aqueous solutions are called tri acidic bases. Example: Al(OH)3, Fe(OH)3...etc Physical properties of bases:         • Bases are bitter to taste, soapy to touch.         • Bases are good conductors of electricity in aqueous solution. In aqueous solution, they release ions, which conduct electricity.         • Bases liberates heat on dilution. Indicators in pressence of bases: Bases turns red litmus to blue. Phenolphthalein turns pink in pressence of bases. Methyl orange turns to yellow in pressence of bases. Chemical properties: Reaction with active metals: Bases react with metals to liberate hydrogen gas Example: Sodium hydroxide react with zinc and liberate hydrogen and sodium zincate.            NaOH + Zn → Na2ZnO2 + H2 Reaction with non-metal oxides: Bases react with non-metallic oxides to form salt and water. This is similar to a neutralization reaction between an acid and a base.             Example: Calcium hydroxide reacts with carbon dioxide to form calcium carbonate and water               Ca(OH)2 + CO2 → CaCO3 + H2O From this reaction, it can be concluded that non-metallic oxides are acidic in nature. Reaction with acids: Bases reacts with acids to form salts and water. Example: Potassium hydroxide reacts with hydrochloric acid to form potassium chloride and water. KOH + HCl → KCl + H2O Uses of Bases:         •  Mild bases neutralise the acidity in the stomach.         • Sodium hydroxide is used in the manufacture of soaps, paper and synthetic fibres like rayon.         • Calcium hydroxide is used in the manufacture of bleaching powder. Bleaching powder is used as a disinfectant.         • Magnesium hydroxide is used as an antacid to neutralize the acid in the stomach.         • Ammonium hydroxide is used in the preparation of fertilizers like ammonium phosphate and ammonium sulphate.

#### Activities

Activity 1 Scootle.edu has created an animated video on "Bases". In this activity student will know the properties of acids bases and neutral substances by testing them with red, blue litmus papers and phenolphthalein solution. Later student will use this information to classify substances into acidic, basic and neutral.  Go To Activity Activity 2 Kidsknowit.com has created an interactive educational video on Acids and Bases. This video begins with an introduction of topics to the students which they are going to be learn. At the end of the each topic student will get online quiz. In order to successfully complete each video student should answer the quiz with 80% accuracy. If student do not obtain 80% marks of then he/she will be instructed to watch the video again. Go To Activity

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