Skip to main content

The chemical structure of detergent makes it perfect for attracting grease and grimeMarco Ugarte/The Associated Press

Let’s Talk Science and the Royal Society of Canada have partnered to provide Globe and Mail readers with relevant coverage about issues that affect us all – from education to the impact of leading-edge scientific discoveries. Let’s Talk Science offers a number of fun activities to get youth engaged in STEM.

Become a pro at washing dishes! Learn how the chemical structure of detergent makes it perfect for attracting grease and grime.

What You Need

· Dishwashing detergent

· Food colouring

· Milk or full cream

· Shallow baking pan, baking cups or custard dish

*Safety first! Do not consume any of the food ingredients used during this investigation. Dispose of ingredients when finished.

What to Do

1. Cover the bottom of the baking pan or custard dish with milk.

2. Add several drops of food colouring to the milk and observe what happens to the drops of colour.

3. Add a drop of dishwashing detergent to the centre of the coloured drops and watch what happens. More food colouring and detergent can be added as the reaction slows down.

What’s happening?

Milk contains oil and water. These two substances do not mix. Milk is, in fact, an emulsion of oil and water. (An emulsion is little bubbles of one liquid suspended in another). Food colouring is water-based and so the colour tends to remain in the form of a drop and does not mix with the milk.

Detergents, on the other hand, are substances that can mix with both oil and water and are used to clean oil and grease. The detergent molecule has one end that is attracted to water-like molecules and the other end is attracted to oil-like molecules, which means it can be detached from clothing or dishes and washed away.

In this experiment, one end of the detergent molecule attaches itself to the fat in the milk and the other end attaches itself to the water. This creates a bubbling effect. The effect should be more pronounced the higher the fat content of the milk and may not work at all with skimmed milk.

Why does it matter?

It can be very important to know what will happen when liquids are mixed together. Companies that manufacture dish washing soap or clothing detergent want their product to attract the dirt, grease and oil found on dishes or clothing in order to make them clean.

Investigate further

· Try this experiment with skim milk and 10% cream. What do you think causes the difference in the reaction?

For more information on this topic check out these Let’s Talk Science resources:

· What happens when we mix liquids? (Hands-on Activities) - Mix it up! What happens when you mix different liquids? Learn about the properties of liquids as you get mixing.

· Stupendous Suds (STEM in Context) - Every time you wash your hands with soap, there are complex chemical reactions taking place.

· Why do oil and water not mix? (Hands-on Activities) - What chemistry determines how oil and water behave when mixed? Explore the mixing of oil and water and the density of liquids in this activity.

Discover more free Let’s Talk Science hands-on STEM activities, resources and events online.