Electric charge on ions is used for writing with an electric pen.

Method :

• Take a filter paper soaked in potassium iodide solution to which a pinch of starch is added.
• Place the filter paper on a metal sheet.
• Connect the negative terminal of battery to the metal sheet.
• Write on the paper with the end of the wire connected to the positive terminal.
• The writing appears on the paper. This happens because the when current is passed, the K⁺ ions are attracted to the metal sheet. The iodide ions (I^-) react with starch to turn blue black.