Why are permanent markers permanent? They’re scared of water.

Everyone knows not to put a sharpie in the hands of a 2-year-old. When he goes running down the halls, drawing on the walls (and his clothes), those marks are going to be much harder to clean up than if you gave the kid a crayola.

Why is a crayola “washable,” and a sharpie permanent? When you put the crayola through the wash, the water can dissolve the ink out of the shirt, but this doesn’t happen with sharpies. The answer has to do with the chemistry of the dyes.

Some love water, some fear it.

Water is a big part of our lives: it makes up a great proportion of the world we live in. Because of this, we think of most chemicals in two different categories. The ones who like water, hydrophillic (meaning water-loving) and hydrophobic (meaning water-fearing).

It gets worse. Its not enough that they either hate or love water, but the ones who hate water stick together and exclude the ones who love it. It turns out that it takes less energy for them to interact with their water-hating buddies than it would to try to mingle with the water-lovers.

These categories show up all the time in our lives, and we mostly don’t notice it. Take, for example a vinaigrette salad dressing:

Leave it in the refrigerator too long, and it will separate out into fractions. The water-hating hydrophobic oil floats up to the top of the bottle and sticks together. This leaves the water-loving hydrophillic vinegar by itself.

To clean, you must dissolve

The different dyes in different markers work just like the oil and vinegar in the vinaigrette. Crayola markers’ dye molecules are very hydrophillic. They love to hang out with water, and other water-lovers. When the dye is put onto a surface, it will stay there until it gets dissolved in something else. Because the dye loves water so much, you can just rinse it in water and it will dissolve away.

So when little Johnny decides that his shirt would look better in red, you can just throw the shirt in the wash. The water will dissolve away  the marker and it will go down the drain.

Despite how much scientist know about dissolving, we can’t solve the mysteries of the dissolving card:

httpv://www.youtube.com/watch?v=yFmBwffoZJI

Yes, the video is a bit off topic. But still very cool.

( Kid Drawing photo credit: juhansonin;vinaigrette photo credit: surekat)