Some Chemistry Quizzes

I've just been doing a Chemistry quiz and for some I couldn't quite work out the answer. Any help would be great :smile:

How would you tell the difference between salt and sugar without tasting them? I guess sugar will combust while salt won't, but why is that?

Why is CO2 a simple molecular structure while SiO2 is a giant covalent structure? I think I may have missed something very obvious...:\

What makes things transparent, opaque or translucent? It's something to do with absorbing/transmitting light but I don't really know... Incidentally, I know how colour arises in transition metal complex ions, and also in flame tests; but surely that isn't the only reason why everything is coloured? For example, why are halogen vapours coloured?

Thanks!

Reply 1

Cataclysm

Hey Excalibur,

1) Burn the Sugar and Salt separately. If you burn sugar, preferrably if they are crystals for long enough then it will go a brown (caramel) colour and then a black tarry substance. Salt will just make the flame brighter and yellow/golden for a second. (I've tried it, so it does work!).

Or, you could put the 'supposed' sugar or salt in separate beakers and use Benedict's reagent.

Or, you could put the 'supposed' sugar or salt in separate beakers and set up an electrical experiment, the salt should complete the circuit and conduct well.

Or, you could do a melting point test and check the values in a data book and compare your results.

Reply 2

eggy789

SiO2 forms giant covalent structures by forming covalent bonds between molecules, in much the same way as carbon in diamond. That is why it is a giant covalent structure. (I apologise if you already knew that and wanted to know why it does this).

You can see the structure and bonding in detail on this website http://www.chemguide.co.uk/atoms/structures/giantcov.html

Reply 3

Cataclysm

2) I'm not completely sure but here is my reasoning.

Carbon is relatively smaller than Silicon and therefore it's electron cloud is smaller. It is able to have overlaps in it's pi orbitals and sigma orbitals with oxygen atoms to form double bonds. This gives rise to it's 'molecular structure' which is smaller in comparison to giant covalent.

In addition, Silicon is a relatively larger than Carbon and only the sigma orbitals are able to overlap. This means that silicon forms single bonds with Oxygen. Since, it is able to form four bonds with single oxygen atoms it results in a large network of Si and O atoms combining to form a large giant covalent or marcomolecular structure.

Reply 4

Cataclysm

3) AFAIK, you're right, it has to do with the absorption of light by the molecules in the substance. Perhaps in mixtures such as air, molecules don't absorb a lot of light etc.

Reply 5

eggy789

Oh and another thing, doesn't sugar exhibit optical isomerism. That would be a way to tell the difference using a polarimeter. Although I'm not sure if sugar is a racemic mix, which would make that pretty pointless.

Whether something is transparent or opaque has a lot to do with its structure. Many crystalline materials tend to be transparent whereas amorphous materials are often opaque. If you take transparent plastic (which is fairly ordered) heat it to a liquid and then cool it quickly, it becomes opaque because the molecules do not have enough time to reorder themselves. I think that in crystals there are many paths that light can take that would lead to it not interact with molecules giving it a fairly transparent appearance. The same cannot be true of disordered structures.

Some Chemistry Quizzes

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