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

Surface tension: aniline dripping in water

Demonstration

In this experiment aniline drops grow in water. Apparently weightless (because aniline density is almost the same as water), the drops are perfect spheres.

Apparatus and materials

Aniline, bottle of

Separating funnel

Retort stand and clamp

Glass beaker, 500 ml

Translucent screen and lamp or projector

Health & Safety and Technical notes


Wear eye protection when handling aniline (phenylamine). If aniline gets on the skin, wash it off with soap and cool water. 
 
Do not attempt to pour directly from the stock bottle to the separating funnel or burette. Pour into a small beaker with a good pouring lip, and then from the beaker into the separating funnel which, is standing in a sink. 
 
Alternatively, use a larger beaker and a small funnel in the top of a burette, which is in its stand on the floor. After the demonstration, safe disposal is essential. Pour off the water to leave a puddle of aniline at the bottom of the large beaker. 
 
Add about six times its volume of 2 M hydrochloric acid, stir and leave it in a safe place for 24 hours. Pour carefully down the foul-water drain (toilet) and flush it away.

A burette can be used in place of the separating funnel.  

Aniline darkens with age so old stock is best. Aniline damages most plastics. If you use a plastic beaker, place a small glass beaker inside it to catch the aniline drops. 
 
An alternative would be olive oil drops in alcohol, but aniline drops are more clearly visible. 

 

Procedure


Beaker with drops forming

a Put about 10 ml of aniline in a glass separating funnel with a tap. Clamp it so that the end of the funnel dips into a tall beaker of water. 

b Release aniline to form a drop at the end of the funnel. Students should observe that a narrow neck forms before the drop breaks away.


Teaching notes


1 Students will need to be nearby to see the drops forming, so ask them to come forward in small groups. 

Alternatives: Either project the experiment, using a bright lamp or projector behind the beaker and a translucent screen in front of it. Or take a series of photos of a drop forming with a digital camera or, even better, make a short video clip. Project the result onto a screen. 
 
2 Surface tension is caused by short range molecular attractions and very short range repulsions. The energy and force fields of surface molecules are very different from those of molecules deep inside the liquid. Therefore, their behaviours are different too. Both sets of molecules are in equilibrium. 
 
This experiment was safety-checked in December 2004