Experiment: Cold Water Currents

My son checked out the book Awesome Ocean Science by Cindy Littlefield,* and decided to try the experiment on Deep-Water Currents (pg 32).  In this experiment, you fill a baking dish with tap water, put an ice pack in one end, drop one drop of food coloring in the water near the ice pack, and drop another drop near the opposite end of the pan.  The food coloring near the ice pack starts moving in a streak towards the far end, while the opposite drop does not spread, showing how cold water from the ice pack pushes warmer water away.

Dye dropped in water near the ice has been pushed to the side, forming the streak.  

Dye dropped in the corner away from the ice has formed a stationary puddle.

Since I hate using food coloring near small children (think of the stains!) and since I'm always wondering what candy will do in any situation, we decided to try this as a candy experiment.  We set up our pan again, dropping single M&Ms in the water near the ice.  Would the cold water push the candy dye?

M&Ms dropped in water near ice pack.  The cold water has pushed the dye away in a streak.

As you can see, the first part of the experiment worked very well.  The moving cold water pushed the M&M dye away from the ice, forming lovely streaks of color.  Next we tried the experiment as instructed by the book, with M&Ms placed both close to the ice and far away.  We expected the furthest M&M to dissolve into a round pool of color, as had our dye drop.  But it didn't work quite that way.

M&Ms placed in water with ice pack. The color from the M&Ms in the corners has spread towards the ice pack, while the color from the M&Ms near the ice pack has spread away from the ice.

The M&M color in the corners didn't stay put.  Instead, it pushed back towards the ice packs.  Were we doing the experiment wrong, or was there another explanation?

I think there was another force acting upon the M&M color.  As candy dissolves, the denser sugar water sinks to the bottom.  With water continuing to sink, the widening puddle of color is pushed away from the candy.  Since we'd placed our candy in a corner, the color had no place to go except towards the ice pack.

*Littlefield, Cindy A. Awesome Ocean Science.  Nashville, TN: Williamson Books, 2003.