An oily problem

Follow these instructions to make your own oil activities and recipes while learning about emulsions.

• 26 June 2006 | Updated 14 October 2011

• What you need
• What to do
• What's happening
• Mix it up
• Mayonnaise
• Salad dressing

When oil tankers spill oil into the ocean, the oil floats on top of the water, causing environmental damage and killing fish and seabirds. Oil floats on water because it's less dense. Oil and water don’t mix.

Here's a great activity that will turn you into a star chef as well as a scientist.

What you need

To do this activity you will need:

• four clear glasses, each half full of water
• cooking oil
• food colouring – red or blue show up best
• liquid detergent.

What to do

1. Add a few drops of food colour to the first glass and stir. Notice how well the colour mixes with the water.
2. Pour some cooking oil into the second glass. Does it mix in like the food colour did? Try stirring the oil and observe what happens. The oil will eventually rise to form a layer on top of the water.
3. Pour enough cooking oil into the third glass to form a layer on top of the water approximately five centimetres thick. When the oil layer has settled, add a few drops of food colour. Don’t stir. Watch how each drop behaves as it hits the oil layer.
4. Now mix everything in together. What happens to the drops of colour? Some tiny drops of colour will probably stay in the oil layer. Watch what happens to them after a while.
5. Create the same oil and water mixture in the fourth glass, with several drops of food colour. Add a teaspoon of dishwashing detergent and stir vigorously. What is different this time? Notice the colour of the oil layer. Is it the same as the water layer?

What's happening

Oil and water don’t mix. Even when you stir them together, they will soon separate. The oil, which is lighter (or less dense), rises to the top. Food colouring is water-based, so it mixes easily with water, but can’t mix with the oil. Did you notice how the drops of food colour behaved when they travelled through the oil? Sometimes the food colour forms perfect little beads, which slowly drop through the oil layer.

Mix it up

You can make oil and water mix by breaking them down into tiny drops that won’t re-form. The homogenised milk that we buy from the shop is made this way. Milk from the cow naturally separates into cream (oil based) on top and skim milk (water based) on the bottom. Before milk is bottled, it is squirted through a sieve at high speed. This breaks down the two liquids into tiny drops that won’t separate.

Any mix of two liquids similar to this is called an emulsion. An emulsion consists of millions of tiny droplets of one liquid suspended inside another liquid.

Oil based paint is an emulsion. Many medicines and cosmetics are as well. They are mixtures of oils and water-based substances, held together by an emulsifier.

Egg yolk is a natural emulsifier. Artists use it in tempera paint to hold the various ingredients together. It is also used in making mayonnaise.

Now for that recipe:

Mayonnaise

Mayonnaise is a bit trickier. It uses an egg yolk to emulsify vinegar or lemon juice with olive oil.

You will need to gather these items to make it:

• 1 cup of light olive oil
• 1 egg
• 1 lemon
• a pinch of salt and pepper
• water.

Open the egg and separate the yolk from the white (only the egg yolk is used in this recipe). The egg yolk contains an emulsifier called lecithin which helps form the mayonnaise emulsion.

Mix the juice of one lemon, the egg yolk, salt and pepper with a whisk. Continue to whisk the mixture and slowly add the olive oil. Do not add the oil too quickly, or the mixture will not form an emulsion.

If the emulsion is too thick you can add a small amount of water to thin it. If the emulsion doesn’t begin to thicken after adding a quarter of the oil, you will need to start again.

Once you have finished adding the oil, you can adjust its taste with extra lemon, salt, pepper, or even mustard.

Salad dressing

To make your own salad dressing you will need to gather the following items:

• 1/4 cup fresh lemon juice
• 1/3 cup olive oil
• salt and pepper to taste
• 1 teaspoon seeded mustard.

Place all ingredients in a screw top jar and shake until well mixed. Taste it.

Try adding other flavourings, such as chopped chives or garlic, if you like. You can also use vinegar instead of lemon juice. Do what all good scientists and chefs do – experiment!

Pour over salad vegetables and toss.

This activity and others like it are featured in The Helix: a science magazine for ages 10+.