Colours of amphoteric metal hydroxides are noted with the compound. Ferrous hydroxide Fe OH 2 and ferric hydroxide Fe OH 3 are insoluble in water and form green colour and brown colour precipitates in water. These precipitates do not dissolve in excess aqueous NaOH.
Ammonium hydroxide is very well soluble in water. But its dissociation in water is partial incompleteness. Magnesium hydroxide is not soluble when aqueous NaOH is added. Therefore Mg OH 2 cannot be an amphoteric metal hydroxide.
You know, zinc hydroxide is an amphoteric hydroxide. But, magnesium hydroxide is not a amphoteric hydroxide. So we can see one white precipitate will dissolve when adding aqueous NaOH.
Some metal hydroxides are soluble and some are not. To decide solubility, we have to look solubility product or solubility data from books or any other resource. Some metal hydroxide form precipitates and some are not. Also, it is not soluble in excess aqueous ammonia solution. This trend is easily seen if you compare the electronegativity of the group II metal to the electronegativity of oxygen.
As you can see the electronegativities of the metals decrease down the column making the change in electronegativities increases down the group.
The greater the difference in electronegativity the more ionic the metal-oxygen bond becomes. The more ionic the metal-oxygen bond the more basic the oxide is. Group II metal hydroxides become more soluble in water as you go down the column. This trend can be explained by the decrease in the lattice energy of the hydroxide salt and by the increase in the coordination number of the metal ion as you go down the column.
The larger the lattice energy the more energy it takes to break the lattice apart into metal and hydroxide ions. Since the atomic radii increase down the group it makes sense that the coordination numbers also increases because the larger the metal ion the more room there is for water molecules to coordinate to it. This simple trend is true provided hydrated beryllium sulfate is considered, but not anhydrous beryllium sulfate.
Solubility figures for magnesium sulfate and calcium sulfate also vary depending on whether the salt is hydrated or not, but the variations are less dramatic. The carbonates become less soluble down the group. All the Group 2 carbonates are very sparingly soluble. Learning Objectives Estimate the pH of the solution due to precipitate of a metal hydroxide.
Calculate the maximum metal ion concentration when the pH is known. Explain behavior of amphoteric metal hydroxides. Solution As usual, we write the equilibrium equation so that we can write the concentration below the formula. Amphoteric Hydroxides Not all metal hydroxides behave the same way - that is precipitate as hydroxide solids. Questions Assume the pH of gastric juice to be 2.
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