Pouring oil on troubled waters   

Water and oil do not mix easily and there are a number of activities which show this. Fill a plastic cup one third full of water and another one third full of cooking oil. Let a few drops of food colouring fall on separate areas of the liquid surface in the two cups. Observe that the drops fall to the bottom of the water and start to spread out (Fig. 16). The drops on the oil also fall to the bottom of the cup but stay as discrete little balls (Fig. 17). When the children have seen what happens, fill a another cup one third full of water and add enough oil to make a thin layer on the top of the water.

  
Figures 16 & 17 - Food colouring into water (left) and oil (right)

Ask the pupils to predict what might happen if you drop on food colouring as before. You will see that the coloured droplets come to rest at the base of the oil layer (Fig. 18). However, if you poke them down with a pencil point, into the water below, they lose the tight ball shape and spread out in the water (Fig. 19).

 
Figures 18 & 19 - Food colouring balls sit in the oil layer then sink through and disperse into the water layer after a while.

Left long enough, they will eventually fall down by themselves. Food colouring is mostly made of water, so it mixes easily in the cup with water but does not mix with the oil. This can be relevant to classes studying properties of materials.

Acid_rain