The hydrophobic tail is the long, non-polar hydrocarbon end of a soap or detergent particle that dissolves in grease but not in water.
The hydrophobic tail is the long, non-polar hydrocarbon end of a soap or detergent particle that dissolves in grease but not in water. Malay: ekor hidrofobik · Chinese: 疏水尾.
The word hydrophobic means “water-fearing”. The tail is a long chain of carbon and hydrogen atoms with no charge and no polarity, so it is not attracted to polar water molecules but mixes freely with oil and grease, which are also non-polar. This tail is one half of the cleaning particle; the other half is the charged hydrophilic head that dissolves in water. Because the two ends prefer different liquids, a single soap particle can bridge water and grease at the same time, which is exactly what cleaning requires.
Example. When a greasy plate is washed, the hydrophobic tails push into the grease and anchor themselves there while the heads stay out in the water. As the plate is rubbed, the grease is broken into small droplets, each one studded with tails on the inside and heads on the outside, so the droplet floats away in the water and is rinsed off.
Confusion to avoid. Do not swap the tail with the head. The hydrophobic tail dissolves in grease and avoids water; the hydrophilic head dissolves in water and stays out of the grease. If you attach the wrong liquid to each end, the whole cleaning explanation collapses, so always link “grease” to the tail and “water” to the head.
The hydrophobic tail and the hydrophilic head together are the reason soap and detergent can lift grease, which is one of the most heavily marked explanations in this chapter. Drawing and labelling both ends correctly secures the SPM Chemistry explanation marks. A useful memory aid is that “phobic” sounds like “fearful”, so the hydrophobic tail fears the water and hides in the grease, while the hydrophilic head loves the water and reaches out into it. When you write the cleaning explanation, describe the tail entering the grease first, then the head staying in the water, and finally the droplet being carried away, so the examiner can follow the sequence step by step.
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