ChemixaLEARN CHEMISTRY

Soap the emulsifier

Objective: demonstrate emulsification by comparing oil-water mixtures with and without dish soap. You will shake, observe separation versus stability, and explain the result with surfactant structure and micelles.

Safety:

Keep soapy water away from eyes; rinse with clean water if splashed. Wipe up any oil spills immediately to prevent slipping. Adult supervision is required for minors. Do not drink the mixtures.

Materials

  • Two clear jars with lids
  • Water
  • Vegetable oil
  • Dish soap
  • Measuring spoons
  • Timer or clock

Procedure

1

Prepare two oil-water jars

Fill two jars one-third full with water and add two tablespoons of vegetable oil to each.

2

Add soap to one jar

Add a teaspoon of dish soap to only one jar and mark it.

3

Shake both jars

Seal both jars and shake them hard for 15 seconds.

4

Start the timer

Set them down side by side and start your timer.

5

Compare separation times

Watch and record how long each jar takes to separate back into layers.

6

Inspect the emulsion

Look closely at the soapy jar and describe what the suspended droplets look like.

What to observe

  • Oil and water separate into two layers within a minute of shaking.

  • With soap added, the mixture stays cloudy far longer.

  • Tiny droplets are visible suspended in the soapy mixture.

  • Adding more soap makes the emulsion last even longer.

Why it happens

Oil and water do not mix because water molecules cling to each other through hydrogen bonding and squeeze out the nonpolar oil. Dish soap molecules are surfactants: each has a water-loving hydrophilic head and a water-fearing hydrophobic tail. When you shake the mixture, the soap's tails dive into oil droplets while the heads stay in the water, surrounding each droplet in a protective shell called a micelle. These shells stop the droplets from merging back together, so the mixture stays cloudy instead of separating. This emulsification is exactly how soap lifts grease off dishes and how mayonnaise holds oil and water together.

Try variations

  • Test hand soap versus dish soap as the emulsifier.

  • Try cold versus warm water and compare emulsion lifetimes.

  • Add the soap after shaking and see whether it still works.