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#1711 |
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The other Group 2 hydroxides
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#1712 |
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The other hydroxides of the Group 2 metals are all basic. They react with acids to form salts. For example:
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#1713 |
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Calcium hydroxide reacts with dilute hydrochloric acid to give calcium chloride and water.
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#1714 |
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Beryllium hydroxide
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#1715 |
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Beryllium hydroxide reacts with acids, forming solutions of beryllium salts. For example:
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#1716 |
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But it also reacts with bases such as sodium hydroxide solution. Beryllium hydroxide reacts with the sodium hydroxide to give a colourless solution of sodium tetrahydroxoberyllate.
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#1717 |
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This contains the complex ion, [Be(OH)4]2-. The name describes this ion. Tetra means four; hydroxo refers to the OH groups; beryllate shows that the beryllium is present in a negative ion. The "ate" ending always shows that the ion is negative.
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#1718 |
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A simple explanation of what is happening
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#1719 |
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You need to think about where the beryllium hydroxide came from in the first place. It would probably have been made by adding sodium hydroxide solution to a solution of a beryllium salt like beryllium sulphate.
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#1720 |
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Remember that beryllium ions in solution exist as the hydrated ion, [Be(H2O)4]2 . The beryllium has such a strongly polarising effect on the water molecules that hydrogen ions are very easily removed from them.
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#1721 |
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The sodium hydroxide solution contains hydroxide ions which are powerful bases. If you add just the right amount of sodium hydroxide solution, you get a precipitate of what is normally called "beryllium hydroxide" - but which is a shade more complicated than that!
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#1722 |
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The product (other than water) is a neutral complex, and it is covalently bonded. All that has happened to the original complex ion is that two hydrogen ions have been removed from the water molecules.
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#1723 |
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You get a precipitate of the neutral complex because of the lack of charge on it. There isn't enough attraction between this neutral complex and water molecules to bring it into solution.
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#1724 |
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What happens if you add an acid to this?
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#1725 |
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The hydrogen ions that were originally removed are simply replaced. The precipitate dissolves as the original hydrated beryllium ion is re-formed.
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