Question Video: Recognizing Why It Is Difficult for Water to Remove Fatty, Oily Stains | Nagwa Question Video: Recognizing Why It Is Difficult for Water to Remove Fatty, Oily Stains | Nagwa

Question Video: Recognizing Why It Is Difficult for Water to Remove Fatty, Oily Stains Chemistry • Third Year of Secondary School

Why are stains that are made up of oils and fats difficult for water molecules to remove? [A] Because both the stains and water molecules are nonpolar molecules [B] Because the stains are nonpolar molecules, while water molecules are polar [C] Because both the stains and water molecules are polar molecules [D] Because the stains are polar molecules, while water molecules are nonpolar

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Video Transcript

Why are stains that are made up of oils and fats difficult for water molecules to remove? (A) Because both the stains and water molecules are nonpolar molecules. (B) Because the stains are nonpolar molecules, while water molecules are polar. (C) Because both the stains and water molecules are polar molecules. Or (D) because the stains are polar molecules, while water molecules are nonpolar.

To answer this question, we need to understand why stains made up of oils and fats are difficult for water molecules to remove. Looking at the answer choices, we can see that this must have something to do with the polar and/or nonpolar characteristics of these molecules. So let’s take a closer look at water molecules and oils and fats.

A water molecule consists of one oxygen atom and two hydrogen atoms. Oxygen atoms are significantly more electronegative than hydrogen atoms. This means that the shared electrons of an oxygen-hydrogen single bond lie closer to the oxygen atom than the hydrogen atom. This means that the electron density in a water molecule is unequally distributed.

The electron density is greater around the oxygen atom. So the oxygen atom has a partial negative charge, while the hydrogen atoms have a partial positive charge. As water molecules have a nonuniform electron density with a partial negatively charged region and a partial positively charged region, water molecules are considered polar. With this information, we can eliminate answer choices (A) and (D) because both of these answer choices state that water molecules are nonpolar.

Now, let’s take a look at oils and fats. Plant- and animal-based oils and fats consist of a variety of triglycerides. A triglyceride contains three ester groups bonded to long hydrocarbon chains. Triglycerides contain six electronegative oxygen atoms, so we might expect a triglyceride molecule to be polar. But the majority of the molecule consists of long hydrocarbon chains. The carbon and hydrogen atoms that make up the hydrocarbon chains do not have a significant difference in electronegativity.

This means that the electron density is evenly distributed and the hydrocarbon chains are nonpolar. Since these nonpolar hydrocarbon chains make up a significant portion of fats and oils, these compounds are typically considered nonpolar. We can now see that the answer choice which correctly identifies the stains as nonpolar and water as polar is answer choice (B).

While we have arrived at the correct answer, it’s worth briefly mentioning why it is difficult for polar water molecules to remove nonpolar fats and oils. Because of the large electronegativity difference between an oxygen atom and a hydrogen atom in a water molecule, water molecules can form strong hydrogen bonds with one another. Triglyceride molecules are attracted to one another, but as these molecules are nonpolar, they are only attracted via weak dispersion forces. There is a force of attraction that exists between triglyceride molecules and water molecules. However, this force of attraction is not strong enough to overcome the strong hydrogen bonds between water molecules.

As such, the two substances don’t tend to mix, and this is why it’s difficult for water molecules to remove stains that are made up of oils and fats. So the reason why it is difficult for water molecules to remove stains that are made up of oils and fats is because the stains are nonpolar molecules, while water molecules are polar, or answer choice (B).

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