Materials
- Cocoa powder
- Milk
- Spoon
- Bowl or cup
- Wooden stick or plastic knife
Instructions
- Pour milk into a bowl or cup so it is a few centimeters deep.
- Fill a spoon with dry cocoa powder, keeping the powder loose and fluffy.
- Hold the spoon level and slowly dip it straight down into the milk, fully submerging the spoon and cocoa powder.
- Hold the spoon still under the milk for a few seconds.
- Slowly lift the spoon back out of the milk.
- While holding the spoon above the bowl, gently tap the cocoa powder on the spoon with a wooden stick or plastic knife.
- Observe that when the outer milk layer is disturbed, it drains away, revealing the remaining cocoa powder on the spoon that stays dry.
What’s going on?
- Cocoa powder has hydrophobic, or water-repelling, properties. When it is dipped into milk, air trapped between the tiny powder particles and the surface tension of the milk prevent the liquid from fully wetting the cocoa. When the outer milk layer is disturbed, it drains away, leaving some of the cocoa powder on the spoon dry.
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