Coloured Milk Pump

Psychedelia rules!

 

 

 

Category - Solids, Liquids & Gases

Key Idea - Surface Tension

 

Purpose - A great introduction to the effect of detergent on surface tension
Nature -  Demonstration
Materials -  a glass bowl, milk, food colours, a dropper of detergent
Method - 
  1. Half fill the bowl with milk.
  2. Place three or four different coloured drops of food colouring evenly spaced on the milk surface.
  3. Onto the centre of each food colour spot drip a spot detergent and watch the colours swirl.
  4. If you watch carefully at the edge of the bowl you can see that the colours slide down the side of the bowl as well as across the milk surface. 
Safety - Nil.
Explanation - The milk molecules are pulling against each other tightly to form surface tension.  Detergent breaks these bonds causing a recoil effect away from the detergent drop.  The food colouring is simply there to help us see the molecular action. 
Notes -

Of course this ability of detergent to break the bond between fats is what makes it useful in cleaning dishes, clothes etc.  

This can be done as a class practical by giving each group a Petri dish, a droppers of food colouring, a dropper of detergent and some milk.