Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds

Illustration of oil spreading


This helps students to remember that olive oil molecules stick up out of water like grass on a lawn.

Apparatus and materials

Transparent trough

Drinking straws


Health & Safety and Technical notes

Read our standard health & safety guidance

The drinking straws are cut into similar lengths of about 2.5 cm. Roll a plug of Plasticine and force it into one end of each of the small lengths, closing that end and making a loaded cylinder. The amount of Plasticine used determines whether the straws float or sink. Once you find the right amount, the straws will float upright and bob up and down in the water if disturbed.



Drop a handful of about fifty of the loaded straws into water, from about 10 cm above the water surface. Observe the behaviour.

Apparatus set-up

Photo courtesy of Mike Vetterlein

Teaching notes

The straws should float upright and congregate together just like an oil film. 

The patch is one straw deep - that is significant. 
Olive oil has a long molecule. One end attaches to water and the other end is inert. 
This experiment was safety-checked in July 2006


Related experiments

Estimating the size of a molecule using an oil film


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