What does the dot mean in the middle of chemical compounds? (1 Viewer)

[calamebe]

So my teacher set some homework but I've come across something we haven't yet learnt. We are given two compounds (I think) malachite, Cu(OH)₂.CuCO₃, and azurite, Cu(OH)₂.2CuCO₃, and we have to do stuff but that doesn't matter. My question is what exactly the dot means in the substances. I'm not sure whether it represents that the substance is a compound or a mixture. I'm guessing it's some kind of mixture as copper hydroxide and copper carbonate are neutral and so they wouldn't form a more complex compound. But if it did represent that, I would guess that in malachite, both copper hydroxide and copper carbonate would be found in equal amounts, and in azurite, there would be 2 copper carbonates for every copper hydroxide. However, a mixture would be able to have varying compositions, and so there can't be an exact composition for every ore. As you can tell I'm a bit confused and any help would be appreciated, thanks!

 

[BlueGas]

So for example Cu(OH)2.CuCO3, for every 1 Cu(OH)2, there's 1 CuCO3

and

Cu(OH)2.2uCO3, for every 1 Cu(OH)2, there's 2 CuCO3

 

[calamebe]

But it's a mixture right? If it is a mixture how can we have exact ratios? Mixtures have a variable composition and so we can't have exact ratios. If it's a compound that problem would be fixed, but I don't think it's a compound for reasons I stated in the first post.

 

[Red_of_Head]

I was taught that it was the ratio of a salt to its water in a hydrate. You might learn more about it when efflorescence and hygroscopy comes up. The only significance of the dot in HSC chemistry that I recall is that it denotes the ratio, like BlueGas said.

IIRC, a hydrate has water as part of its structure. If there was excess Cu(OH)2, there would be as much Cu(OH)2.2CuCO3 that could be formed + the excess, so 2 different substances.

So if there were 10 mols Cu(OH)2 and 10 mols CuCO3, there would be 5 mols Cu(OH)2 left over.

 

[calamebe]

Ok, that makes enough sense for now. Thanks for that!

 

[porcupinetree]

It doesn't represent a mixture. It represents an actual compound with a definite ratio of constituent parts.

 

[calamebe]

So copper hydroxide and copper carbonate bonded together, but how would that work if they are neutral compounds?

 

[someth1ng]

The dot means that the crystals that formed were made so the crystal structure contains both of those molecules at a particular ratio. That is, the crystal structure is still repeating but it contains both compounds. This is why different hydrates of what otherwise appear to be the same compound can have different properties.