Filter paper soaked in potassium iodide solution which also contains starch and phenolphthalein is placed on an aluminium sheet which forms one electrode of an electric circuit. The other electrode is used as a ‘pen nib’ to ‘write’ on the filter paper. When this electrode is made positive and the aluminium sheet negative, the writing is blue/black and when the polarity is reversed, the writing is pink.
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This practical is part of our Chemistry for non-specialists collection.