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RHYS LEWIS & MINH TAN PHAM, AHS, DECD, UNISA / SCIENCE PHOTO LIBRARY RHYS LEWIS & MINH TAN PHAM, AHS, DECD, UNISA / SCIENCE PHOTO LIBRARY
Iodine clock reaction. Footage of a demonstration of the iodine clock reaction. The two liquids in the beakers are added together to produce a mixture of potassium iodide, sodium thiosulphate, starch, hydrogen peroxide, and sulphuric acid. During the clip, the mixture turns brown and continues to go a darker colour. Chemical equilibrium is reached when the solution becomes dark blue/black. There are two competing reactions. One between the iodide ions and the hydrogen peroxide to produce molecular iodine, and one between the thiosulphate ion and the molecular iodine to restore the iodide ions. As the thiosulphate is used up, the molecular iodine starts to dominate and reacts with the starch to produce the dark blue/black colour that is a characteristic test for starch.
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