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Saturday Science: Invisible Ink


Invisible ink: staple of classic spy stories everywhere. Or is it? I typed that, and then immediately thought “I’ve never actually seen invisible ink in a spy story, I’ve just seen people claim that it’s in all of the spy stories.” Whatever, it’s still cool. Invisible ink!

There are a couple of ways to create and reveal invisible writing, and today’s involves a chemical interaction between starch and iodine. The materials and process are clearly laid out in our fancy schmancy graphic, so I won’t repeat all that here. Instead, go do the experiment, then come back for your…


  • Baking soda (5 tbsp.)
  • White paper
  • Water (1 cup)
  • Paint brushes
  • Spoon
  • Grape juice concentrate


  1. Add 5 tablespoons of baking soda into 1 cup of warm H20. Stir for 1 minute to dissolve (not all of it will), and then allow the baking soda to settle at the bottom of the cup. 
  2. Use your paint brush as your pen. Write a message or two, and remember that you must let your ink dry completely (for about 10 minutes) before you try to reveal your secret letters with the "visible-izer."
  3. Use a new paintbrush to paint over your paper with grape juice concentrate. Watch as a different color appears wherever the baking soda mixture is.


The bright colors of berries come from a natural plant dye called anthocyanin, which changes color! When the baking soda dries, the H20 evaporates, but the baking soda stays on the paper as tiny white crystals. When we spread the grape juice across the paper, the natural fruit dye changes color as it touches the streaks of baking soda crystals, but remains the original color in the other parts of the paper.


Welcome back! I hope you did the experiment first and didn’t just keep reading. If you haven’t done the experiment, 50 points from Gryffindor (or whatever house you’re in). If you did, great! But no point, because you don’t get points for doing the thing you were supposed to do. So there.

Anyway…Here’s what’s going on, in two parts. 

Part 1: Why did the paper turn blue? Well, paper contains molecules of starch. Starch is found in all sorts of plants, and has a particular reaction to iodine: when they combine, they turn blue. Why? Well, honestly, that’s still kind of a mystery. Scientists think that the reason has to do with the shape of some of the starch molecules. Some of them are long branching chains, and some are straight chains of round coils. These ones are called amylose. What seems to be happening is that the iodine molecules fit perfectly inside the coils of amylose, which changes how the amylose absorbs and reflects light, giving it a blue or black color instead of whatever color it was before. It’s kind of crazy that something you can do so easily in your kitchen is still kind of a mystery, but if there weren’t any mysteries left then we wouldn’t need science anymore!

Part 2: Why didn’t your lemon juice writing turn blue? Well, it has to do with the particular chemistry of the Vitamin C in the lemon juice. The chemical name for Vitamin C is ascorbic acid, and ascorbic acid reacts to iodine much differently than starch does. Here’s the short version: Vitamin C changes iodine in such a way that it can no longer turn starch blue. The long version: Vitamin C gives one of its electrons to each atom of iodine in the solution, which changes it from a proper atom into a negatively charged ion called iodide. The ions can no longer fill those amylose coils, so the sections of paper with Vitamin C soaked into them can’t turn blue.

If you want a different invisible ink formulation, try writing or drawing with some water with a bit of baking soda dissolved in it. Letting it dry, and then covering it with grape juice concentrate. It’s a lot stickier, and it’s a totally different chemical reaction!