Materials:

• 1 flat bowl or aluminium pie pan – about 18cm diameter and at least 3cm deep
• Milk (enough to cover the bottom of the container about 2cm deep)
• Several different colours of food colouring (red, yellow, blue, and green work well)
• Liquid dishwashing detergent 
• Toothpicks

Procedure:

1. Pour about 2cm of milk into the bowl. (use one bowl for each 4-6 students, if possible).
2. Keep back from the table so that the milk becomes motionless.
3. Imagine the bowl as the face of a clock, with 12 o'clock at the top, 3 o'clock to the right, 6 o'clock at the bottom, and 9 o'clock to the left.
4. Drop 2 drops of each food colour carefully into the milk near the outside edge of the bowl, one colour at 12 o'clock, the second colour at 3 o'clock, the third colour at 6 o'clock, and the fourth colour at 9 o'clock, in any order.  There should be one spot each of red, yellow, blue, and green.  Do not bump the table or do anything else to mix the colours.
5. Take a toothpick and dip the one end into the dishwashing detergent.
6. Touch the detergent end of the toothpick into the middle of the bowl of milk, and hold it there for at least 30 seconds.  Observe carefully.  Do the colours mix initially, or just swirl in their own section of the bowl?  Do not use the toothpick to stir the milk.
7. Lift the toothpick, and touch it to the milk in the center of one of the colours.  Observe carefully.
8. Touch the toothpick into other areas of the milk, dipping it first into the detergent again if necessary.  Observe carefully.
9. When you are certain that you are finished with the activity, dispose of the milk down the drain.

Observations:

Initially, the food colours swirl in different patterns across and under the surface of the milk for several minutes.  Note that the colours do not actually mix with each other, but continue in separate swirling patterns.  The time of swirling may depend on the temperature of the milk and amount of dishwashing liquid you use.  You can repeat step 6 to reactivate the swirling motion if needed.  Moving the toothpick to a new spot can later result in mixing of the various colours.

How does it work?

The reason for the swirling motion of the food colours is caused by changes in surface tension of the milk after adding the drop of liquid dishwashing detergent. Detergent molecules have a partially charged polar end and an uncharged nonpolar end.  Thus, they can bind weakly to either polar or nonpolar neighbouring molecules.  When detergent is introduced into the center of the bowl, it may bind weakly to the water in the milk, decreasing its surface tension so that it flows more easily, and indirectly setting up the swirling motion. 

Colour Drops

Watch the ways in which food colouring can move through different liquids.

Materials:

• Plastics cups
• Water
• Salt
• Seltzer water
• Food colouring

Procedure:

1. Fill two plastic cups 2/3 full with water.
2. Add 1 drop of food colouring to the first cup and immediately observe what happens.
3. Add the salt to the second cup and stir until the salt dissolves. Add one drop of food colouring to this cup and immediately observe.
4. Fill the third cup 2/3 full with seltzer water, add 1 drop of food colouring and observe.

Observations:

In plain water the drop slowly swirls and moves throughout. In salt water, the drop starts to sink and then rises. In seltzer water, the drop quickly disperses and evenly colours the liquid.

How does it work?

Putting food colouring in plain water does not have a dramatic effect other than that the colour becomes more pale (diluted). The gas bubbles in the seltzer water act to speed things up, like an invisible stirring spoon. The drop of food colouring is quickly broken up and carried to all parts of the liquid. Salt water is more dense than plain water. This means that anything less dense will float on the top, including the food colouring (which is a drop of coloured water).

Note: Activity adapted from multiple sources by NRC scientist Dr. Mike Day.