Question

Why does water not dissolve motor oil?

Short answer

Water does not dissolve motor oil because water is polar and motor oil is non-polar, and "like dissolves like."

Step-by-step solution

Understanding Polar and Non-Polar Molecules

Water is a polar molecule, meaning it has a positive and a negative end due to an uneven distribution of electrons. Motor oil, on the other hand, is non-polar, meaning its electrons are distributed more evenly across the molecule, resulting in no distinct positive or negative ends.

Concept of "Like Dissolves Like"

The principle of "like dissolves like" explains solubility based on molecular polarity. Polar solvents tend to dissolve polar substances, while non-polar solvents dissolve non-polar substances. Therefore, since water is polar and motor oil is non-polar, they do not dissolve in each other.

Molecular Attraction Forces

In the case of water and oil, the strong hydrogen bonds between water molecules prevent the incorporation of non-polar oil molecules. Oil molecules do not have the polarity to disrupt or form interactions with these hydrogen bonds, as they do not have the necessary charge interactions.

Observation of Density and Insolubility

When water and motor oil are mixed, the oil does not dissolve but instead forms a separate layer. This is also because oil is less dense than water and does not mix with it. This visible separation further confirms the insolubility due to polarity differences.

Key concepts

Polar and Non-Polar Molecules

Molecules can be classified as either polar or non-polar based on how their electrons are distributed. In polar molecules, there is an unequal sharing of electrons between atoms. This creates a partial positive charge on one end and a partial negative charge on the other end. A prime example of a polar molecule is water. Water molecules have a bent shape, where oxygen holds a slight negative charge and hydrogen holds a slight positive charge.

Non-polar molecules, by contrast, have an equal distribution of electrons. This results in a balanced charge across the molecule, with no distinct positive or negative end. Motor oil is a classic example of a non-polar substance. The symmetrical structure of non-polar molecules like motor oil means they do not have the charged regions found in polar molecules. Hence, polar molecules, such as water, and non-polar molecules, such as motor oil, do not interact strongly with each other.

Like Dissolves Like

The principle "like dissolves like" is a simple yet powerful rule of chemistry that dictates how substances interact, particularly in the context of dissolving. This rule suggests that substances with similar polarities tend to dissolve in each other. Polarity is critical because like dissolves like means:

• Polar solvents (e.g., water) are effective at dissolving polar solutes (e.g., sugar or salt).
• Non-polar solvents (e.g., hexane) are suitable for dissolving non-polar solutes (e.g., grease or oil).

So why does this happen? It's all about molecular interactions. Polar molecules can form various types of bonds or forces with other polar molecules, such as hydrogen bonds or dipole-dipole interactions. Similarly, non-polar molecules interact with one another through dispersive forces. When water (polar) and motor oil (non-polar) are brought together, their different natures mean they won't mix. Water cannot form effective attraction forces with the oil molecules, highlighting the "like dissolves like" concept.

Hydrogen Bonds

Hydrogen bonds are a special type of dipole-dipole interaction. They occur when a hydrogen atom, which is covalently bonded to a more electronegative atom like oxygen or nitrogen, interacts with an electronegative atom on another molecule. These bonds are particularly strong for intermolecular forces. In water, hydrogen bonds hold water molecules together very tightly, giving water its unique properties like high boiling point and surface tension.

Why do hydrogen bonds prevent water from dissolving oil? Oil molecules lack the necessary polar characteristics needed to break or interact with these hydrogen bonds. Imagine trying to mix oil into a web of tightly held water molecules. The non-polar oil can't disturb the strong, directional bonds between water molecules enough to mix. Consequently, oil is forced to separate and form a distinct layer above the water, illustrating the profound impact hydrogen bonds have on solubility.