Experiment 4 - Water of Hydration

Objective - Determine the percent of water in a hydrate.

Introduction

Salts are compounds composed of a metal ion plus a non metal (or polyatomic) ion, e.g., sodium chloride (NaCl, and sodium phosphate (Na₃PO₄).

Hydrated salts (or Hydrates) are salts which have a definite amount of water chemically combined.Some common hydrates are:

copper (II) sulphate pentahydrate

magnesium sulphate heptahydrate

cobalt (II) chloride hexahydrate

tin (II) chloride dihydrate

The dot indicates an attractive force between the polar water molecules and the positively charged metal ion. On heating, the attractive forces are overcome and the water molecules are released leaving behind the anhydrous salt. e.g.

The water released on heating is called the water of hydration. Since heat is absorbed during this process, the reaction is "endothermic". (In an "exothermic" reaction heat is liberated.)

decomposes at temperatures greater than 560^oC. Therefore, avoid letting the flame of the Bunsen burner rest on one spot of the crucible. Heat gently by holding the bunsen burner in a sweeping motion. When decomposes a black solid, CuS, is formed.