Hydrophobic hot cocoa

This experiment can be an exciting way to dive deeper into some of the chemistry and physics principles you learned in Combining and Separating … and it’s tasty too!

Materials

Take a guess (hypothesis)

Do cocoa powder and milk like each other? Can you combine cocoa powder and milk? Would their combination be a mixture, a suspension, or a solution?

Directions

Milk and Cocoa Powder
Dunk Spoon in Milk
Poke With Toothpick
  1. Fill your bowl or cup with milk so that it is deep enough to fully dunk your spoon in.
  2. Get a heaping spoonful of cocoa powder.
  3. Carefully dunk your spoonful of cocoa powder into the milk. Does the cocoa powder mix in, or stay on the spoon?
  4. Take the spoon out. What do you notice? Is the cocoa powder wet or dry?
  5. Poke the pile of cocoa powder with your toothpick. What happened? Can you explain it?

Conclusion & further research

  1. Extend the Experiment: We know particles have Different Properties at Different Temperatures. How would heat affect the outcome of this experiment? Try heating up your milk and trying it again. Try mixing cocoa powder with cold milk, and then warm milk. Does cocoa powder mix more easily with cold milk or warm milk?
  2. Research Project: How does cocoa powder get from the cacao tree into our grocery stores?

How it works

In Combining and Separating, we talk about how some particles like each other and stay close together, some particles might touch each other if they are close but can be easily pulled apart, and some particles will do anything they can to make sure they stay away from each other.

When a substance WANTS to be near water molecules, we call it hydrophilic, meaning water-loving. When a substance does NOT WANT to be near water molecules, we call it hydrophobic, meaning water-repelling.

Can you think of a substance that is hydrophobic? You may have seen one in The First Great Lesson.

Oil! Oil and water do not mix. Oil repels water. Oil is hydrophobic. The fat molecules inside cocoa powder are hydrophobic too. They repel water.

On the other hand, the starch molecules are hydrophilic. They love water.

Milk is mostly water, so the same rules are true for milk.

When you dunk the cocoa powder in milk, the starch molecules quickly absorb the milk. At the same time, the fat molecules stop the milk from getting any further than the surface. This creates a shell of milk around the outside of the cocoa powder.

When you poke it with a toothpick, you break the shell’s surface tension. The milk rolls off the cocoa powder, and back down into the cup.

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