[CopperPiggy94]

Hello everyone,
I am at the end of my straw here.
I tried to precipiate basic copper carbonate from a solution of copper II chloride through the addition of a saturated solution of sodium carbonate. Both solutions were at around room temperature. The copper chloride was dissolved in just about enough distilled water.

At first, I slowly added the sodium carbonate solution to the solution of copper chloride, but after the short burst of bubbling subsided, I dumped the rest in, which resulted in the precipitation of a solid copper compound.
I knew right away that something was off: The basic copper carbonate I previously made was the expected green-blue color that leaned more towards green. I believe I made that batch using copper II acetate instead of the chloride I used this time. The precipitate that formed was clearly more towards the blue end of the spectrum and had a harder time settling to the bottom after stirring was stopped, which told me that something went wrong.

I took a sample of the solution with some re-suspended precipitate and heated it, closely monitoring the temperature. At around 70°C, the solution turned pitch black.
After stirring was stopped, a deep blue solution and a black precipitate at the bottom remained.
Basic copper carbonate shouldn't decompose at these temperatures if I am not mistaken, which is why I tried heating the solution. To the best of my knowledge, I just made copper hydroxide instead of the compound of my interest.

Can someone tell me what I did wrong and how this could even happen? I read somewhere that is was fairly tricky to produce copper hydroxide, but for me, the opposite is true. Any advise highly appreciated, I am going nuts!

[Aldebaran]

Firstly can I ask if you used pure reactants ( copper chloride and sodium carbonate) from a reliable supplier or did you make the copper chloride yourself as preliminary step?

[CopperPiggy94]

The sodium carbonate is sourced from washing soda that contains pure powdered sodium carbonate. The copper chloride was made from copper metal and 35% hydrochloric acid. Filtered the solution and let the remaining acid evaporate. I tried using the acid + copper salt solution directly, but the results were the same. This time, I have used the dry crystals and added enough de-ionized water to dissolve everything.
/edit:
I should've clarified: the copper metal I have used was pure copper, no alloy or coating was present when I made the copper chloride.

[Aldebaran]

Have you used hydrogen peroxide at all in these reactions?

[CopperPiggy94]

In this case, I have not. I opted to bubble air through the solution, since I ran out of hydrogen peroxide. I wrapped the beaker with cling film and bubbled air through the solution over the course of a couple of days until the solution was a saturated green color.

[Aldebaran]

My first thought regarding the black precipitate was the possibility of copper oxide. The use of hydrogen peroxide to produce copper chloride from metal and concentrated HCl is well-known and if there was some remaining H₂O₂ it was not inconceivable that some copper peroxide/oxide might be formed.
However if you are certain there is no H₂O₂ present then I have no explanation for your black precipitate.
My suggestion is to repeat the experiment and  isolate a fresh sample of each solid product at each stage of the process and then test it.
So first produce the copper chloride solution and evaporate to crystals then recrystallize. (It might be interesting to check the pH of the solution of the chloride crystals to check you have driven off all the HCl).
Then add sodium carbonate solution to the re-dissolved chloride solution and collect the precipitate. If you again get a black ppt. you could attempt to check if it is indeed copper oxide by reduction with methane.
All this might be rather tedious but it’s experimental chemistry. Sorry I can’t give you a ready-made answer!

[CopperPiggy94]

Well, I was already sure that the precipitate must be copper oxide, as the decomposition of the unknown compound matches the decomposition temperature of copper hydroxide, yielding copper oxide in the process. I have used recrystallized copper chloride crystals for this, which makes it even more amusing that this experiment goes wrong.
I don't have access to methane, so I can't test the precipitate that way. However, I re-dissolved the dried precipitate in HCl, yielding the expected green color. Now I am out of HCl however. I still have the main batch of the (suspected) copper hydroxide containing solution, which I would rather not let go to waste. I am thinking I could just thermally decompose it all and get my copper back that way, but I'd rather find a better solution to get my basic copper carbonate - especially considering I am now HCl-less.

[Aldebaran]

Not sure where you’re located but bear in mind household natural gas is around 90% methane so if you are connected to a gas supply you can use that. 
😊

[CopperPiggy94]

I'm in Europe - unfortunately, I don't have access to natural gas in my flat. But in any case, I am now convinced that I am simply producing copper hydroxide everytime I try to do this experiment. I have heated the remaining solution with the blue precipitate from the original attempt to around 60-70°C and the solution turned black/brown. After filtering and washing the precipitate, I re-dissolved it with acetic acid and got a solution of copper II acetate with no other undissolved material in the beaker. To the slightly warm solution, I have added a slightly warm solution of sodium carbonate again - I got the same precipitate as before.
Just to be clear: Basic copper carbonate is not meant to decompose at the temperatures I have mentioned above, right? I am genuinely baffled by how I managed to produce basic copper carbonate once in my life and now got stuck supposedly making copper hydroxide. 

/edit:
As a side note, can anyone tell me a fool-proof method, one that they themselves have used to make basic copper carbonate, so I can try it with a new solution of copper chloride? I'm especially curious about the temperatures, concentrations of the solutions and carbonate-solution-addition rate involved.

[rolnor]

Here they use copper sulfate, its hard to say what you got, but this seems like a nice procedure.

https://youtu.be/Ttf80a2zZ9w

[CopperPiggy94]

I was first introduced to copper chemistry through copper sulfate. Using copper sulfate would probably be the most reproducible way to do it, but I'm really into doing things from scratch - if I want basic copper carbonate, buying copper sulfate for it seems kind of redundant. On the other hand, since I have no idea why this is failing, I'm almost inclined to just buy some copper sulfate to at least see if I get better results.

[rolnor]

Is there any particular reason you want to do it from copper metal? Basic copper carbonate is not expensive? Do you need it for some reason or is it a interesting experiment?

[CopperPiggy94]

Valid question - mainly for experimentation and getting better at practical chemistry. After all, if I can't get a common reaction like this right, surely I can't expect to make cubane next month.
/edit:
I just realized that I completely missed your first question, sorry about that.
I use copper metal as I want to synthesize copper compounds from the metal itself - if I used copper sulfate, it wouldn't serve the same purpose.