Lesson Hook: "The Cat's Meow"

Materials

• Milk (whole or 2%)
• Dinner plate or Petri dish
• Food colouring (red, yellow, green, blue)
• Dishwashing soap (Dawn or Dove brands work well)
• Tooth Pick

Procedure

1. Pour enough milk into a dish to completely cover the bottom and allow it to settle.

2. Add one drop of each of the four colours of food coloring - red, yellow, blue, and green –
   to the milk.

3. (a) Predict what will happen if a clean tooth pick touches the centre of the milk.
(b) Using a clean tooth pick, touch the tip of the tooth pick to the centre of the milk. It's
important not to stir the milk, just gently touch the milk with the tip of the tooth pick. Observe what happens and compare observations with prediction.

4. (a) Predict what will happen if a tooth pick with a dab of dish soap touches the centre of
the milk.
(b) Place a drop of liquid dish soap on the tip of the tooth pick. Place the soapy end of the tooth pick in the middle of the milk and hold it there for 1 or 2 seconds. Observe what happens and compare observations with prediction. Also, compare observations in this part of the activity to observations in Part 3.

Explanation

Milk is mostly water but it also contains vitamins, minerals, proteins, and tiny droplets of fat suspended in solution. Fats and proteins are sensitive to changes in the surrounding solution (the milk).

When you add soap, the weak chemical bonds that hold the proteins in solution are altered. The molecules of protein and fat bend, roll, twist, and contort in all directions. The food colour molecules are bumped and shoved everywhere, providing an easy way to observe all the invisible activity. At the same time, soap molecules combine to form a micelle, or cluster of soap molecules. These micelles distribute the fat in the milk.

This rapidly mixing fat and soap causes swirling and churning where a micelle meets a fat droplet. When there are micelles and fat droplets everywhere the motion stops.

There's another reason the colours explode the way they do. Since milk is mostly water, it has surface tension like water. The drops of food colouring floating on the surface tend to stay put. Liquid soap disrupts the surface tension by breaking the cohesive bonds between water molecules and allowing the colours to zing throughout the milk.

Detergent, because of its bipolar characteristics (hydrophilic on one end and hydrophobic on the other), weakens the milk's bonds by attaching to its fat molecules. The detergent's hydrophilic end dissolves in water and its water-fearing end attaches to a fat globule in the milk.

Why is this a worthwhile Lesson Hook Activity?

This is a very interesting hands-on, minds-on discrepant event that elicits questions requiring further study and research. It is a visually appealing lesson hook that can be done in a short period of time with inexpensive, common household materials.

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Course: SBI4U
Unit B: Biochemistry
Expectations addressed:
B3.2 describe the structure of important biochemical compounds, including carbohydrates, proteins, lipids, and nucleic acids, and explain their function within cells.
B3.3 identify common functional groups within biological molecules (e.g., hydroxyl, carbonyl, carboxyl, amino, phosphate), and explain how they contribute to the function of each molecule.