Problem 91 - Chemistry Matter and Change

Question

Recognize Cause and Effect Explain why alkanes, such as hexane and cyclohexane, are effective at dissolving grease, whereas water is not.

Step-by-Step Solution

Answer

Alkanes dissolve grease because both are non-polar; water cannot because it is polar.

Step 1: Understand the Nature of Alkanes

Alkanes, including hexane and cyclohexane, are hydrocarbons, meaning they consist only of carbon and hydrogen atoms. They are non-polar molecules, which means they do not have a significant charge difference across their structure.

Step 2: Understand the Nature of Water

Water is a polar molecule due to its bent shape and the significant difference in electronegativity between hydrogen and oxygen. This creates a partial positive charge on the hydrogen side and a partial negative charge on the oxygen side.

Step 3: Understanding Grease Properties

Grease is primarily composed of long hydrocarbon chains similar to alkanes. These chains are non-polar, which means they do not have charged regions.

Step 4: Principle of "Like Dissolves Like"

The principle of "like dissolves like" states that solvents can dissolve solutes that have similar polarities. Non-polar solvents are better at dissolving non-polar substances, while polar solvents are better for polar substances.

Step 5: Application of the Principle

Alkanes such as hexane and cyclohexane, being non-polar, effectively dissolve grease since grease is also non-polar. Water, being polar, does not interact well with non-polar grease, resulting in poor solubility.

Key Concepts

Alkanes

Alkanes are fascinating organic compounds and a type of hydrocarbon. They are made entirely of carbon and hydrogen atoms, connected in a way that forms a chain or ring without any double or triple bonds. This structure makes alkanes saturated hydrocarbons.
Because of their structure, alkanes are non-polar molecules. This means they have no significant difference in charge across their structure. Their non-polarity is a crucial factor in their solubility behavior. By understanding the nature of alkanes, we can better grasp why substances like hexane and cyclohexane dissolve certain materials better than others.

Polar and Non-Polar Molecules

In chemistry, molecules are classified based on their polarity, which refers to how electrons are distributed across the molecule.
Polar molecules have a partial positive charge on one side and a partial negative charge on another. This happens because the atoms in the molecule have different electronegativities, pulling electrons unevenly. Water is a classic example of a polar molecule due to its bent shape and the oxygen-hydrogen electronegativity difference.

• Non-Polar Molecules: Molecules like alkanes have an even distribution of charge, meaning there's no part of the molecule that is significantly more positive or negative than another.
• Importance in Solubility: Understanding the nature of polar and non-polar molecules is key to predicting how substances will interact, especially in solutions.

Like Dissolves Like Principle

The "like dissolves like" principle is fundamental in predicting solubility. It states that solvents typically dissolve solutes that have similar polarities.
This principle explains that:

• Non-polar solvents are excellent at dissolving non-polar substances due to their similar electron distributions.
• Polar solvents, like water, are adept at dissolving other polar solutes.

This principle is why alkanes, which are non-polar, excel at dissolving grease — also non-polar — whereas polar substances like water are not as effective in this regard. It brings insight into why certain cleaning agents work better than water for removing grease.

Hydrocarbons

Hydrocarbons play an integral role in both chemistry and everyday life. These compounds consist solely of hydrogen and carbon atoms, forming simple chains, branched chains, or rings.
They can be divided into several types, including alkanes, alkenes, and alkynes.

• Alkanes: As the simplest form of hydrocarbons, alkanes are non-polar and make vital components in fuels and solvents.
• Role in Solubility: Their non-polar nature makes them perfect for dissolving other non-polar substances, as seen with grease.

The pure hydrocarbon chains in oils and greases mirror those in alkanes, leading to their mutual solubility. Understanding hydrocarbons and their characteristics can clarify why certain substances interact as they do in nature.