Tie Dye Milk
| Chemistry: | Surface Tension |
| Grade Range: | Elementary School |
| Format: | Hands-on |
Tie Dye Milk is a favorite at hands-on events and in classrooms. Be sure to bring enough supplies for everyone, and be prepared to clean up spilled milk!
Materials
- Whole Milk
- Petri Dishes
- Cotton Swabs
- Soap
- Food Dye
- Large Tub
Safety Precautions
Please read the General Safety Precautions section of the Demonstration Safety page before performing this demonstration.
Demonstration
- Set out petri dishes for the students. In two separate petri dishes, pour soap into them and set aside.
- As students come up, give them a petri dish and a cotton swab. Pour milk into their petri dishes, just enough to cover the entire bottom of the dish, and have them add 2-3 drops of food dye into their milk.
- Note: Only let them add 2-3 drops total, not 2-3 drops per color. If they want to add multiple colors, have them add one drop of each color.
- Have the students dip one end of their cotton swab into a petri dish with soap in it, and then dip their cotton swab into their milk dish. They will see the milk start to swirl with the food dye colors they had added!
- After students see the effect and hear the explanation, collect the used dishes and put them in the tub. Depending on the length of the event and popularity of the demo, you may need to take the tub to a sink and rinse the dishes to reuse them. Otherwise, be sure to rinse them afterwards.
Why This Works
Surface Tension is the attraction between atoms and molecules at the surface of a liquid. The surface tension of a liquid can be affected by what is dissolved in it, as well as by what is added to it. Whole milk has some fat in it, as well as protein and sugar. The protein and sugar increase the surface tension of the milk, but the fat in the milk increases the surface tension the most. This is why it can be hard to rinse out all the milk from a container or wipe it all up if it is spilled; the fat is really good at clinging together, and so it will resist being rinsed away or wiped off. Soap, however, is a Surfactant, meaning that it is good at reducing the surface tension of a liquid. This is because soap will bond with the particles in the milk and pull them apart, weakening the surface tension and making it easier to clean.
The food dye, when first dropped into the milk, doesn't move much because the milk has high surface tension. It can't disperse through the milk because the fat molecules are holding it in place, so we just see the dots where it was dropped. When the soap is put in the milk, however, the food dye starts moving! This happens because of the soap breaking the fat and pulling it apart, which causes the rest of the liquid to move and pulls the food dye along as well!
Additional Information
- This demonstration pairs well with the Giant Soap Bubbles demonstration.
- Be sure to tell students that they can do this demonstration at home. This demonstration works the best with whole milk, and has limited success with other types of milk.