ICSE Class 10 Chemistry Chapter 04 Analytical Chemistry

Analytical Chemistry: Units Of Ammonium Hydroxide And Sodium Hydroxide

Syllabus

Analytical Chemistry - Use of Ammonium Hydroxide and Sodium Hydroxide.

(i) On solution of salts: colour of salt and its solution; formation and colour of hydroxide precipitated for solutions of salts of Ca, Fe, Cu, Zn and Pb; special action of ammonium hydroxide on solutions of copper salt and sodium hydroxide on ammonium salts.

On solution of salts:

Colour of salt and its solution.

Action on addition of Sodium Hydroxide to solution of Ca, Fe, Cu, Zn and Pb salts drop by drop in excess. Formation and colour of hydroxide precipitate to be highlighted with the help of equations.

Action on addition of Ammonium Hydroxide to solution of Ca, Fe, Cu, Zn and Pb salts drop by drop in excess. Formation and colour of hydroxide precipitated to be highlighted with the help of equations.

Special action of Ammonium Hydroxide on solutions of copper salts and sodium hydroxide on ammonium salts.

(ii) On certain metals and their oxides (relevant laboratory work is essential).

The metals must include zinc and aluminium, their oxides and their hydroxides, which react with caustic alkalis (NaOH, KOH), showing the amphoteric nature of these substances.

Introduction

Determination of the chemical components in a given sample is called Analysis. In chemistry, we study two types of analysis, Qualitative analysis which involves the identification of the unknown substances, and Quantitative analysis which involves the determination of composition of a mixture.

Qualitative analysis, i.e., identification of an unknown substance is done by carrying out chemical tests with the help of reagents. A reagent is a substance that reacts with another substance.

Alkalis are important laboratory reagents. When they are added to certain salt solution characteristic coloured precipitates of metallic hydroxides are formed. Thus a metal ion is identified.

Colours Of The Salts And Their Solutions

The salts of representative elements of the periodic table, i.e., the elements of the Groups 1, 2 and 13 to 17 are generally colourless, while those of the transition elements, i.e., salts of elements of Groups 3 to 12 are generally coloured.

Different colours of coloured salts help in their identification during qualitative analysis. Some examples of colourless and coloured ions are given below:

Action of alkali on metal cations results in the formation of their hydroxides which often appear as precipitates.

Teacher's Note

When you observe rust forming on iron or tarnishing on copper items at home, you are seeing metal oxidation - the same chemistry behind identifying metal ions in qualitative analysis.

Action Of Sodium Hydroxide Solution On Certain Metallic Salt Solutions

When the sodium hydroxide (caustic soda) solution is added drop by drop to the solution of metallic salts, the metal hydroxide formed gets precipitated. Colour of the precipitate identifies the metal ion. Some precipitated metallic hydroxides dissolve in excess of sodium hydroxide solution to give soluble complexes.

Note: Potassium hydroxide (caustic potash) solution also shows similar behaviour.

Teacher's Note

When you clean your kitchen sink with baking soda and vinegar, you are observing acid-base chemistry - the same reactions that occur when testing for different metal ions in the lab.

Precipitation is the process of forming an insoluble solid when solutions are mixed. The solid thus formed is called precipitate. Only those compounds form precipitates which are insoluble in water.

Action Of Ammonium Hydroxide On Certain Salt Solutions

When ammonium hydroxide solution is added dropwise to the solutions of metallic salts, precipitates of their hydroxides are formed, which are identified by their distinct colours. Some precipitated metallic hydroxides are soluble in excess of ammonium hydroxide, because of the formation of soluble amino complexes on further reaction with excess of NH4OH.

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