Question

Which part of a fatty acid is hydrophilic?

Answer 1

The carboxylic acid part. The COOH or `COO^-` part

Explanation:

Fatty Acids have 2 parts.
1st is the Carboxylic acid part. This group has hydrogen bond donors and acceptors, and is very polar. This part loves water.

2nd part is the "fat" part. It is really just a long carbon chain `-CH_2CH_2CH_2CH_2CH_2-------CH_3`. This part is very hydrophobic because it only contains nonpolar bonds.

Acetic Acid is not a fatty acid, but it is useful to think about it.
`CH_3COOH` - it has the same 2 parts. The COOH group and the carbon chain group (only 1 carbon long). This freely dissolves in water.

If we make this longer, it starts to not dissolve in water.
`CH_3CH_2CH_2CH_2COOH` - this is pentanoic acid and it really doesn't like water because the long chain is too hydrophobic. The COOH group still loves water, but it loses the 'war' over water loving and hating.

IF that carbon chain group gets really long, then you've got a fatty acid.