Question

3) The polarity of a molecule is a consequence of its structure and is one of the determinants of the physical and chemical characteristics exhibited by the molecule. If you have ever observed a bottle of oil and vinegar salad dressing before shaking it you have observed one of the consequences of the polarity of molecules, “like dissolves like” and consequently “oil and water don’t mix”. (It is important to point out here that vinegar is 95% water and 5% acetic acid.) The oil floats on top of the vinegar because it is less dense than the vinegar but also because oil molecules are non-polar and water is a polar molecule.

a) Which liquid is least likely to mix with water, CH3F or CF4?

b) Why?

Answer 1

The explanation of polarity of molecules is clearly given in the question itself. I would further add that a molecule with ionic bonds doesn't mix with any molecule with covalent bonds.

For example, if we take CH₄. This molecule doesn't dissociate and is practically insoluble in water.

In CCl₄ however, electronegativity dfference between C and Cl is 0.6, compared to C and H = 0.5. Therefore each C - Cl bond is polar, but because of the symmetry the molecule itself is non polar.

Now we apply the same explanation in the given question.

a) CF₄ is least likely to mix with water.

b) This is because of the symmetry in the CF₄ molecule the polarity is cancelled and the molecule becomes non polar. Whereas in CH₃F, the molecule is slightly assymetric because of the varied electronegativity difference between C-F and C-H. It is more likely to be soluble than the former.