Two dark solids

Two dark solids, one giant ionic, the other covalent molecular.

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Iodine

Iodine is a molecular solid. It consists of I2 molecules. The molecules are non-polar and are held together by weak intermolecular attractions in the iodine crystals.

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Potassium manganate (VII)

Potassium manganate (VII) is an ionic solid, made up of K+ ions and MnO4 ions in a giant crystal lattice structure.

The differences in bonding and structure between the two materials means they have quite different properties.

Solubility

Water dissolves many polar molecules and ionic solids.

Here water is being added to iodine and potassium manganate (VII).

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Which tube contains the iodine and which the potassium manganate (VII) ?

Hexane is a non-polar organic liquid which dissolves non-polar molecules, but not ionic solids.

Here hexane is being added to iodine and potassium manganate (VII).

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Which tube contains iodine and which potassium manganate (VII) ?

Behaviour in an electric field

What would happen if solid iodine and solid potassium manganate (VII) were placed on some filter paper moistened with tap water and subjected to 10 volts?

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In potassium manganate (VII) the K+ ions are colourless, whilst the MnO4 ions are deep purple in colour.

Which way do the purple ions move and why?

Iodine dissolves in potassium iodide solution

The I2 molecules in iodine do not dissolve very well in pure water, but they do dissolve in a solution of potassium iodide.  I3 ions are formed when iodine dissolves in potassium iodide.

Behaviour in an electric field

What would happen if solid iodine and solid potassium manganate (VII) were placed on some filter paper moistened with potassium iodide solution and subjected to 10 volts?

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Why does the brown colour move to the left at the top of the slide?

This one maybe harder to explain fully. So let’s close with a little rhyme:

“Two dark solids, sometimes purple, sometimes brown. One ionic, one molecular, with behaviour that can make you frown.”