Milk Magic – exploring surface tension

We did this coloring changing milk experiment a little while ago, and it was easy to do with materials from your own cupboards.

Materials
Shallow plate
Milk (the higher the fat content the better, buttermilk would work too)
food coloring (we used red, blue, yellow and green)
liquid soap (Dawn works well)

We poured milk onto the plate then added our four colors of food dye.

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You can see the colors are next to each other but not mixed. The food colors are less dense than the milk, so they just sit there on top of the milk. The surface tension of the milk is undisturbed so they don’t mix. Surface tension is a property of liquids that causes it to behave like an elastic sheet. It is the property that allows such insects like the pond skater to walk on water.

Next, add a drop of liquid soap and observe what happens. Instantly, the milk mixes.

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Why does this happen? The fat molecules of the milk are dissolved by the soap, breaking the surface tension so that the molecules are more free to move around, moving the molecules of food coloring with them.

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