Question #8d9e4
2 Answers
For a compound to be solube in water, it needs to be polar so that it is attracted to the water molecules, and it doesn't have to be an ionic coumpound for that. Sugar molecules have lots of oxygen atoms bound to carbon, which cause polarity due to the electronegativity of the oxygen atoms.
Ionic bonding doesn't guarantee high solubility in water!
Explanation:
It is a classic school-student chemistry error to suppose that things that dissolve in water "must be ionic". Whilst it's true that some ionic compounds are highly water soluble (classic example being sodium chloride), there are others that exhibit extremely low solubility in water, for example silver chloride, calcium carbonate.
And it is also true that some covalent bonded compounds are very soluble in water - sugar is one example. Another is ethanol - if ethanol wasn't water soluble, alcoholic drinks could not exist!
The reason that sugar dissolves in water is that the sucrose molecule contains a number of pendant hydroxyl groups -OH, and in these the electronegative oxygen atom causes a partial negative charge deficiency in the H atom, making it partially positive charged, and the oxygen atom partially negatively charged.
Water contains the same dipolar property, so hydrogen bonds can be formed between the partially charged oxygen atoms on the sucrose molecules, and the partially charged hydrogen atoms on the water molecules, and vice versa.
By that means, it is possible for the sucrose to form a stable solution in water, even though, aside from the presence of the hydroxyl groups, much of the bonding in sucrose is relatively non-polar carbon to carbon bonding.
