Why are simple hydrocarbons generally insoluble in water even though both are liquids?

Introduction / Context:
Solubility is a fundamental concept in chemistry, and understanding why certain substances dissolve or do not dissolve in water is very important. Hydrocarbons, such as alkanes, alkenes and aromatic hydrocarbons, are commonly used fuels and solvents, yet they are typically insoluble or only very slightly soluble in water. This question asks for the underlying reason, which is linked to the polarity of molecules and the principle that like dissolves like.

Given Data / Assumptions:
- Hydrocarbons are compounds made only of carbon and hydrogen atoms.
- Water is a polar solvent with a bent molecular shape and uneven charge distribution.
- We consider simple, nonpolar hydrocarbons such as methane, hexane or benzene.
- No chemical reaction is assumed between the hydrocarbons and water under normal conditions.

Concept / Approach:
The key concept is molecular polarity and intermolecular forces. Water is a polar molecule with strong hydrogen bonding between its molecules. Hydrocarbons have C-H bonds which are only weakly polar and overall nonpolar molecular structures. When a nonpolar hydrocarbon tries to dissolve in polar water, the water hydrogen bonding network must be disrupted without sufficient new attractive interactions being formed. Because water molecules are more strongly attracted to each other than to hydrocarbon molecules, hydrocarbons tend to separate into a different phase instead of dissolving.

Step-by-Step Solution:
Step 1: Recognise that water is a polar solvent with a partial negative charge near oxygen and partial positive charges near hydrogen atoms. Step 2: Recognise that simple hydrocarbons such as alkanes are nonpolar overall because their C-H bonds have very small electronegativity differences and their structures are symmetrical. Step 3: Apply the principle like dissolves like, which states that polar solutes tend to dissolve in polar solvents and nonpolar solutes dissolve in nonpolar solvents. Step 4: Understand that dissolving a nonpolar hydrocarbon in water would require breaking many strong hydrogen bonds between water molecules, but only weak dispersion forces would form between water and hydrocarbon. Step 5: Conclude that because this is energetically unfavourable, hydrocarbons remain insoluble or only slightly soluble in water, which is why they tend to form a separate layer.

Verification / Alternative check:
An everyday verification is to observe what happens when oil or petrol is mixed with water. The hydrocarbon liquid floats or forms droplets instead of mixing uniformly, demonstrating immiscibility. In contrast, ethanol, which has both polar and nonpolar parts, can mix with water due to its polar hydroxyl group that forms hydrogen bonds with water. This comparison shows that the absence of significant polar groups in hydrocarbons is the main reason for their low solubility in water.

Why Other Options Are Wrong:
Option B incorrectly states that hydrocarbons are ionic compounds; they are actually covalent and mainly nonpolar. Option C claims that being heavier than water prevents dissolution, but density differences only affect whether a substance floats or sinks, not whether it can dissolve. Option D suggests that hydrocarbons always react with water instead of dissolving, which is not generally true under normal conditions; most hydrocarbons are quite unreactive toward water. These explanations ignore the real controlling factor, which is polarity and intermolecular forces.

Common Pitfalls:
Students may mistakenly believe that any liquid can dissolve in any other liquid, or they may focus on physical properties such as density rather than molecular interactions. Another pitfall is to assume that if two substances do not mix, they must be reacting, when in fact they might just be immiscible. Emphasising the principle like dissolves like and thinking about polar versus nonpolar molecules can help avoid these misunderstandings and provide a consistent explanation for solubility behaviour.

Final Answer:
Simple hydrocarbons are generally insoluble in water because hydrocarbons are nonpolar molecules while water is polar, so there is little attraction between them.