The property of lipids causes this to occur is known as Non polarity.
What is Non polarity?
Non polar molecules are those which don't dissolve easily in water and are also referred to as being hydrophobic. On the other hand, polar molecules are those which can easily dissolve in water.
Lipids are non polar and so do not mix with polar water molecules which is the reason one of the test tubes showed a layer of oil sitting on top of the water.
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