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Do simple molecular covalent substances dissolve in water, and why?

I have my GCSE Chemistry on Monday and cramming a bit of revision this weekend. Though I'm getting lots of different answers on the internet for this question, and I'm terrible and chemistry and confused. It's a question with a table and different substances. This one has a low melting and boiling point and doesn't conduct electricity, even as a liquid. That means it's simple molecular covalent right? I also need to suggest why it will or won't dissolve...Please help! Thanks guys. :smile:

Reply 1

Education_1

Correct it is a simple molecular structure because it doesnt conduct electricity and low melting and boiling point. Why do you think it might not or will dissolve. well it all depends on the polarity of the molecule
is there anymore information or you could talk about intermolecular force (although im not sure if you do this for GCSE. Hmm use your information and make a reasonable deduction

(edited 11 years ago)

Reply 2

Stevo F

Original post by Education_1

I agree, I'm not sure you can deduce whether something will dissolve or not using only GCSE knowledge and application

Reply 3

Umackjiggles

Original post by Stevo F

I agree, I'm not sure you can deduce whether something will dissolve or not using only GCSE knowledge and application

Intermolecular forces are on the syllabus but I was always under the impression that dissolution referred to a compound being split into ions when it comes into contact with water yet in past papers they say that hydrogen chloride is dissolved in Methylbenzene, which of course does not split it into ions, but I don't understand what the actual definition of dissolve (or dissolution) is. Could someone please explain this to me?

Reply 4

Education_1

Original post by Stevo F

I agree, I'm not sure you can deduce whether something will dissolve or not using only GCSE knowledge and application

Yup it mostly depend on how polar the molecule is ..Maybe OP; is there any more information that you could provide or something (maybe structure of the molecule). I may have confuse you if you dont do polarity of molecules; OP does this help?
http://www.bbc.co.uk/scotland/learning/bitesize/higher/chemistry/energy/bsp_rev4.shtml

(edited 11 years ago)

Reply 5

Mattywooda

Original post by Umackjiggles

Could someone please explain this to me?

Dissolving is just going into solution, it doesn't really matter how. So some things break into ions (which are charged) so can come associated with water molecules and hence are dissolved. Other substances are charged nonetheless (like chloromethane) and so can associate with water molecules.

The associations occur because of the charges and hydrogen bonds occur between the H on the H2O and the charged molecule - be it ion or polar.

Reply 6

thegodofgod

It depends on the polarity, e.g. C₂H₆, although being simple covalent in structure, is non-polar, and thus will not dissolve in water very well.

It would, however, dissolve in a non-polar solvent, e.g. cyclohexane, C₆H₁₂.

Essentially, polar solutes dissolve in polar solvents and non-polar solutes dissolve in non-polar solvents. This explains why oils and fats cannot dissolve in water, as oils and fats are non-polar, where as water is polar.

Reply 7

WellOiledProf

Educations_1 is right, it all depends on how polar the molecule is.
Dissolution is really to do with the affinity that the solute has for the solvent (liquid medium)

A non polar substance (like alkenes and alkanes) will dissolve well in nonpolar liquids.
Likewise ionic and polar substances will dissolve best in polar liquids (such as Water)

Since this is GCSE we can guess we're talking water as the solvent. This means that ionic substances will dissolve in them as well as polar substances. For GCSE this means assume all salts (except the insoluble exceptions) and the acids dissociate and dissolve in water.

Reply 8

qiaoyu.he

Original post by ScienceConfusion

In general, yes, low melting and boiling point and non-conductance of electricity suggests that the compound is a simple covalent molecule.

As for solubility, you first must remember that it is all relative. Almost all compounds are soluble in water, its just that some are massively more soluble than others. Iron is soluble in water, it's just that salt (sodium chloride) is much more soluble. Glucose is even more soluble than salt etc.

Water (H2O, so 2 hydrogens and 1 oxygen) is a polar molecule. The oxygen atom attracts the electrons in the O-H covalent bond towards itself, and so is slightly negatively charged; the hydrogen, losing some of its electrons is slightly positively charged. This is because oxygen is more electronegative than hydrogen. If you haven't come across electronegativity before, then you can just remember that in general, molecules with different atoms are polar whereas molecules with same atoms are non polar (e.g. O=O is non-polar as its 2 atoms are the same whereas H-F is polar as its 2 atoms are different).

Polar solvents will tend to dissolve polar compounds more than non-polar compounds and vice versa. Water is a polar solvent, and so will tend to dissolve other polar compounds better. If your mystery compound is polar e.g. H-F, then it will be very soluble; if it is non-polar e.g. O=O, the it won't be very soluble.

This explains why we have a specialized protein for transporting oxygen in our blood. O=O is non-polar and therefore doesn't dissolve well in our blood, so to carry it in high concentrations, we need to bind it to haemoglobin protein. Carbon dioxide, on the other hand, O=C=O, is polar because of the large difference in electronegativity between O and C and can therefore just be carried by dissolving it in the blood.

Hope this helps.

Reply 9

ScienceConfusion

Original post by qiaoyu.he

THANK YOU SO MUCH. I'm eternally grateful! You've helped me so much with Chem revision, I thank you again. Fantastic answer, thanks for your help. :rolleyes: