Tie Dye Milk: Difference between revisions

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== Age ==
{| class="wikitable" style="color:black; background-color:#ddd; margin-left: auto; float:right"
 
| [[Chemistry]]:
Elementary School
| Surface Tension
 
|-
== Format ==
| Grade Range:
 
| [[Elementary School]]
Hands-on
|-
| 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 ==
== Materials ==


    Petri dish
* Whole Milk
    Whole milk
* Petri Dishes
    Toothpick
* Cotton Swabs
    Liquid soap
* Soap
    Food coloring
* Food Dye
    A basin for disposal of the used milk
* Large Tub
    Wash basin


== Safety Precautions ==
== Safety Precautions ==


Science Theatre demonstrators must keep the safety of themselves and their audience in mind at all times. All Science Theatre demonstrators must have read through the Safety Training page. The ST Safety Box with first aid kit, fire extinguisher, etc. should always be available to demonstrators. Always wear safety gloves, glasses, and a labcoat if handling chemicals; always perform potentially dangerous demonstrations at a safe distance from the audience; and always keep a very close eye on any volunteers you call from the audience. If young children are allowed to perform the demonstration, watch them carefully to make sure they do not consume any of the materials. Make sure they do not keep the toothpicks - they are sharp and could be dangerous to small children.
Please read the General Safety Precautions section of the [[Demonstration Safety]] page before performing this demonstration.
 
== Preparation ==
 
Pour a thin layer of milk into a petri dish, making sure that the milk covers the entire bottom of the petri dish.


== Demonstration ==
== Demonstration ==


Add a drop of food coloring to the layer of milk. Wet one end of the toothpick with a small amount of soap. Touch the soap to the milk near the food coloring to disturb the food coloring. It is not necessary to move the toothpick around in the milk! In fact, this will actually prevent you from demonstrating the concept this demonstration is meant to show – the decrease in surface tension should generate all the motion for you.
# 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.
You may have audience members perform the demonstration. Try placing a few different colors around the milk to watch them mix.
#* ''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!
When a demonstration is completed, pour the milk into the disposal basin and rinse off the dish in the wash basin.
# 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.
 
== What to Say ==
 
Ask the audience if they know what milk is made of. Give them hints until someone says that it's mostly water (cow's milk is almost 90% water), then make sure that they know that there are a variety of fats, proteins, vitamins, and other nutrients in milk.
 
Explain the concept of surface tension and show the audience how it keeps the drop of food coloring from dispersing throughout the milk. Describe surfactants. Then add the soap to demonstrate what happens with lower surface tension.
 
Ask the audience what would happen if we used fat free milk or water for the experiment. Since there are no fat molecules to disrupt, there would not be nearly as much of a change in surface tension (although the surfactant in the soap does cause some decrease in surface tension in pure water) – the colors would not move very much!
 
== Why It Is ==
 
Cow's milk is approximately 87% water, 5% lactose (a carbohydrate), 4% protein, and 4% fat (Ref. 1). Fat is in an emulsion in milk – liquid fat is dispersed within water.


Molecules in a liquid are held together by intermolecular “cohesive” forces. Molecules in the center of a volume of liquid are surrounded on all sides by other liquid molecules, but molecules on the surface have a lot of area where there are no neighboring molecules. Surface molecules bond more strongly to the neighbors they do have than do molecules that are surrounded on all sides. Therefore there is a strongly-connected “film” at the surface of liquids that makes it hard for anything (such as food coloring) to move around on the surface.
== Why This Works ==


The primary cleaning agents in common soaps are detergents composed of surfactants that break up grease. Surfactant molecules have hydroophobic (water “fearing”) and hydrophilic (water “loving”) ends. Essentially, the hydrophobic end is attracted to fat and protein molecules very strongly – it sticks itself in between chains of fats and proteins by breaking the bonds between them. The hydrophilic end attaches itself to water, so that the surfactant helps carry away fats and dissolve them into the water.
''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.


When the coloring is first added to the milk, it is held together in a single drop by the high surface tension provided by the water and the tightly-bonded fats and proteins. The soap lowers the surface tension of the water itself and also breaks up the fats and proteins, so the surface tension of the milk is lowered significantly. The area of the milk not near the soap then has a higher surface tension which pulls on the area of the milk with the soap and coloring, which is why we see the coloring radiating away from the soap.
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!


== Real Life Examples ==
== Additional Information ==


Using soap to clean greasy dishes or to clean your body of its naturally secreted oils. Surfactants in soap help to break off grease molecules and dissolve them in water so they may be washed away.
* 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.

Latest revision as of 17:44, 18 July 2016

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

  1. Set out petri dishes for the students. In two separate petri dishes, pour soap into them and set aside.
  2. 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.
  3. 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!
  4. 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.
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