Problem 52 - Chemistry The Molecular Science

Question

Explain why water will not remove tar from your shoe, but kerosene will.

Step-by-Step Solution

Answer

Water can't remove tar because they're not similar in polarity, but kerosene can because it's a non-polar solvent like tar.

Step 1: Understanding Tar and Water Interaction

Tar is a hydrocarbon-based substance that is non-polar, meaning it does not have a charge polarity in its molecular structure. Water, on the other hand, is a polar molecule, having a charge distribution which allows it to dissolve other polar substances. Since like dissolves like, a polar solvent like water cannot effectively dissolve non-polar substances like tar.

Step 2: Understanding Kerosene and Tar Interaction

Kerosene is a non-polar solvent, similar in terms of polarity to tar. Non-polar solvents are capable of dissolving other non-polar substances because they can mix on a molecular level, breaking the intermolecular forces holding the tar together. Thus, kerosene is effective at removing tar.

Key Concepts

Polar and Non-Polar Substances

In chemistry, understanding the distinction between polar and non-polar substances is key to solving many real-world problems. A polar substance, like water, has an uneven distribution of electrons. This means there is a positive charge on one side and a negative charge on the other. Such molecules can interact strongly with other polar substances.

Conversely, non-polar substances have an even distribution of electrons, resulting in no distinct charge regions. Examples include oils, fats, and hydrocarbons like tar. The electrons in these molecules are spread out evenly, preventing them from having significant electrostatic interactions with polar molecules. This fundamental difference in molecular structure explains why water, a polar molecule, cannot dissolve tar, a non-polar substance.

Molecular Polarity

Molecular polarity is determined by both the polarity of individual bonds within a molecule and the shape of the molecule itself. When a molecule has atoms with different electronegativities, the electrons may be shared unequally, resulting in polar bonds. If these polar bonds are arranged asymmetrically, the entire molecule becomes polar.

Water is a classic example of a polar molecule due to its bent shape and polar O-H bonds. This allows it to dissolve other polar substances effectively. However, in the case of non-polar molecules like tar, the bonds are either non-polar due to equal electronegativity or the symmetry of the molecule cancels out any polar bond effects. As a result, these molecules do not interact well with polar solvents.

Like Dissolves Like

The principle of "like dissolves like" is a simple way to predict solubility. It means that polar substances are more likely to dissolve in polar solvents, while non-polar substances are more likely to dissolve in non-polar solvents. This rule stems from the nature of intermolecular forces.

Polar solvents can create strong hydrogen bonds or dipole interactions with polar solutes, leading to effective dissolution. On the other hand, non-polar solvents like kerosene can only induce weak Van der Waals forces. However, these are sufficient to break down intermolecular interactions in other non-polar substances like tar, allowing the solvent to dissolve the tar efficiently.

• Kerosene dissolves tar because both are non-polar and can mix thoroughly.
• Water cannot dissolve tar due to their polarity difference.