Question

Which of the following is water soluble? A. a saturated fatty acid B. an unsaturated fatty acid C. the side chain on valine D. glucose

Short answer

Glucose is water soluble.

Step-by-step solution

Identify the compounds

Understand the nature of each compound mentioned: saturated fatty acid, unsaturated fatty acid, the side chain on valine, and glucose.

Determine solubility characteristics

Recall that fatty acids (both saturated and unsaturated) and the side chain on valine are hydrophobic due to their long hydrocarbon chains, meaning they do not dissolve well in water.

Analyze glucose

Glucose is a carbohydrate with multiple hydroxyl (OH) groups, which are hydrophilic and interact well with water, making glucose water soluble.

Compare and conclude

Comparing all options, the saturated fatty acid, unsaturated fatty acid, and side chain on valine are not water soluble, whereas glucose is water soluble.

Key concepts

Hydrophilic vs Hydrophobic

Understanding the difference between hydrophilic and hydrophobic substances is essential in chemistry.
Hydrophilic literally means 'water-loving'.
These substances can form hydrogen bonds with water molecules, making them easily soluble in water.
Examples of hydrophilic groups include hydroxyl (OH) groups, which are common in compounds like glucose.

On the other hand, hydrophobic means 'water-fearing'.
These molecules have non-polar properties, meaning they do not interact well with water.
Fatty acids, for example, have long hydrocarbon chains that are hydrophobic.
These chains repel water and do not dissolve in it.
Understanding these properties helps explain why certain compounds are soluble in water while others are not.

Solubility of Fatty Acids

Fatty acids are split into saturated and unsaturated types.
Saturated fatty acids have single bonds between carbon atoms, while unsaturated fatty acids have one or more double bonds.

Despite this difference, both types of fatty acids are generally hydrophobic because of their long hydrocarbon chains.
These chains are non-polar and do not form hydrogen bonds with water.
This lack of interaction with water makes fatty acids insoluble in water.

Additionally, fatty acids tend to aggregate in aqueous environments, forming micelles where the hydrophobic tails point inward and the hydrophilic heads point outward.
This arrangement minimizes the exposure of hydrophobic tails to water, further highlighting their water-insoluble nature.

Glucose Solubility in Water

Glucose is a simple sugar and an excellent example of a hydrophilic molecule.
It contains multiple hydroxyl (OH) groups that can form hydrogen bonds with water molecules.
These interactions make glucose highly soluble in water.

When glucose dissolves, the hydrogen bonds between glucose molecules and water break, allowing glucose molecules to disperse evenly in the solution.
This solubility is crucial for biological processes, as it allows glucose to be quickly transported through the bloodstream to cells where it can be used for energy.

Unlike hydrophobic molecules, glucose does not repel water; instead, it forms strong interactions with it.
This is why, among the given compounds, glucose stands out as being water-soluble.