Video Transcript
Why is sugar, a polar substance, soluble in water? (A) Because water molecules form a hydrogen bond with sugar molecules. (B) Because water molecules form a metallic bond with sugar molecules. (C) Because water molecules form a covalent bond with sugar molecules. (D) Because water molecules form an ionic bond with sugar molecules. (E) Because water molecules can act as a nonpolar solvent as well as a polar
solvent.
Sugar, more formally known as sucrose, has the chemical formula C12H22O11. It can be drawn in the following way. We have a hexagonal and pentagonal arrangement of the 12 carbon atoms, with some
hydrogen and oxygen atoms forming eight hydroxy or OH groups. We are told in the question that sugar is a polar substance.
A polar substance is a substance that contains polar bonds, which create an
asymmetric distribution of electron density across the molecule as a whole. A polar bond is created when two atoms which are chemically bonded have significantly
different electronegativity values. Electronegativity is a measure of an atom’s ability or tendency to attract the
bonding pair of electrons in a chemical bond. We know that sugar contains carbon, hydrogen, and oxygen. And of those elements, oxygen has a significantly higher electronegativity than
either carbon or hydrogen.
Now let’s look at the hydroxy groups a bit closer. Since oxygen’s electronegativity is higher than that of hydrogen’s, the pair of
bonding electrons in each hydroxy, or OH group, is pulled closer to the oxygen
atom. This gives the oxygen atom a partial negative charge, which we represent in chemistry
with the Greek letter 𝛿. And as a result of this electron withdrawal, hydrogen atoms become partially
positively charged, or 𝛿 positive.
So, this difference between oxygen and hydrogen in terms of electronegativity within
each molecule of sugar creates a number of polar bonds, where each end or pole of
the covalent bond has a slightly different charge. In other words, each of the hydroxy groups in a molecule of sugar are polar. When oxygen and hydrogen are bonded together within an organic molecule such as
sugar, special intermolecular forces known as hydrogen bonds can be formed between
sugar molecules and between sugar molecules and water molecules.
The diagram shows a hydrogen bond being formed between one of the hydroxy groups in
the sugar molecule and a molecule of water. This strong electrostatic attraction exists between a highly electronegative atom on
one molecule and a 𝛿 positive atom on an adjacent molecule. The squiggly line represents the rest of the sugar molecule. Or we can use capital R to represent the rest of the sugar molecule. Sugar is soluble in water because the sugar–water intermolecular forces are
approximately equivalent to the intermolecular forces between water molecules.
In conclusion, why is sugar, a polar substance, soluble in water? The correct answer is (A), because water molecules form a hydrogen bond with sugar
molecules.
