It is very important to use pure copper sulfate when growing crystals with it. This is because the purer your raw materials, the nicer the crystals will be.
The importance of purification
Crystals grown properly from pure solutions have flat faces and sharp edges. They are also quite translucent. On the other hand, crystals grown from dirtier solutions have rough faces, jagged edges and white inclusions.
Below is a photo of a crystal grown from an impure solution (left) vs a relatively pure one (right).
There are two situations when you’d want to purify the copper sulfate on hand.
- When your solution looks cloudy/dirty.
This is very easy to spot. A pure saturated solution is a striking shade of blue, with a very slight purple tint. Meanwhile, an impure solution of the same concentration is also blue – but is tinted greenish instead. The more it leans towards green, the more impurities it contains.
Note the color difference between an impure saturated solution (left) and a pure one (right).
- When you experience problems with crystal growing.
There are 3 ways impurities in copper sulfate solution negatively affect crystal growth.
- It leads to the formation of jagged and uneven faces.
This is likely due to the presence of iron salts. These salts also cause the shape of the crystals to flatten.
- It causes small needle-like crystals to form.
These are probably crystals of calcium sulfate, which are usually relatively insoluble in water. They stick all over the place, and often embed themselves inside your copper sulfate crystals.
- It encourages crystal crust to creep up the sides of the container.
This crust sucks out your solution and grows upwards. Eventually, the crust might cover the entire container and spread to the surface underneath. Oxidation of iron salts might also cause the crust to take on a yellowish color.
All 3 problems mentioned are visible in this photo.
The easiest way to purify copper sulfate is via re-crystallization.
Re-crystallization procedure
In this process, crystals are allowed to grow in an impure solution. These crystals will consist of only copper sulfate ions while excluding impurities. Then, the plan is to take these crystals out, wash them, then re-dissolve to form a purer solution. We can repeat this process to obtain crystals of even higher purity.
First, dissolve a large amount of impure copper sulfate with hot water in the ratio of 50 g copper sulfate to 100 ml water. If the copper sulfate does not dissolve, heat it lightly and stir. My powder was bought online and had a bunch of dirty stuff inside.
Look at how murky the thing is!
After that, filter the warm solution using a coffee filter/filter papers. Collect the filtrate in a dish and discard the residue, which is mainly dust, sand and insoluble copper compounds.
If you added 50 g copper sulfate to 100 ml water, the solution is supersaturated at room temperature. Therefore, crystals should begin to form immediately. If that doesn’t happen, drop a few grains of crystal powder into the solution and crystallization should start immediately.
Now, don’t get too excited over the crystals. These are low-quality ones.
Place the solution into the fridge for a day. By cooling it down, you are decreasing the solubility of copper sulfate in water, forcing even more crystals to grow.
Note: Copper sulfate is mildly toxic, so putting it with your food is a bad idea.
After one day, there will be a lot of crystals at the bottom of the container. Pour the remaining solution into another container.
Wash the crystals using cold water, and store them properly. We will get back to these crystals later.
Heat the remaining solution to evaporate off even more water.
Wait until about 70% of the solution is left, then wait for it to cool down.
Sprinkle some copper sulfate powder to induce crystallization, then put it back into the fridge.
Remove the crystals after a day and wash them with cold water. Store them together with the old batch.
Then continue heating the remaining solution.
Repeat these steps until your solution is down to 10-20% of its original volume. It is now “waste water” as it contains a lot of impurities.
Meanwhile, you should have collected a lot of purified crystals. These are the fruits of your labor. They are reasonably clean, and can be used for crystal growing by re-dissolving in water.
You might have noticed that your new solution is much clearer and bluer than your original one. This is proof that your purification is successful!
Compare the dirty waste water with the stunning blue solution. Also compare it with the dark, murky solution I began with. (pic above)
Further purification
If you want even purer stuff, you can choose to do a second re-crystallization – or even a third one. For crystal growing purposes, 1-2 will be enough.
For my run, I did two re-crystallizations. I started with 148g of copper sulfate and ended up with 121g, which was a percentage yield of 81%.
Having purified our copper sulfate, it’s time to start growing actual beauties. I’ll show you exactly how in this article: The Best Way to Grow Big Copper Sulfate Crystals
Alternatively, you can use it to grow copper metal crystals.
If you have any questions/thoughts, drop me a comment below.
Happy growing.
Hi Chase! I was wondering:
1. Will the purified crystals from the process be enough to start growing a single copper sulfate crystal? Or do I have to repeat this purification process many times until I get enough crystals to make a pure saturated solution that lets me grow a big single crystal?
2. When growing the seed crystals, you said that the first saturated solution should be left for 1 day forming crystals before pouring a part of it into a petri dish. After waiting 1 day and the solution is way cooler, won’t it be too cool for it to form more crystals?
3. When growing the single crystal, does the water have to be hot? Because first time I did this before doing a little research about the process, the seed crystal dissolved in the water 😅
1. It all depends on how big you want your crystal to grow. If you end up with 100g of purified crystals, then naturally the maximum single crystal you’ll be able to grow is 100g. If you want to grow a bigger one, you’ll need to purify more raw copper sulfate.
2. If the concentration is correct, the solution should still be saturated after cooling. Therefore, it will still form crystals, only much slower. But this is what we want, because crystals that form more slowly are usually more transparent and beautiful.
3. I highly recommend using room temperature water. In theory, you can grow crystals inside a solution of any temperature as long as it is saturated (with respect to that temperature), but if the temperature keeps changing, then you’ll have a hard time keeping track of the saturation of the solution. For example, a copper sulfate solution that is saturated at room temperature is undersaturated when hot. Therefore, a crystal that would have grown in room temperature solution would dissolve in hot solution.
Hey Chase! Thank you for this blog… I just finished 50 g copper sulfate to 100 milligrams of distilled water. I am wondering about putting a lid on it so it doesn’t get all over the fridge (I only have one and don’t want to infuse everyone’s food with it) – if so, a glass lid? plastic?
This will be my first time – is it evaporating in the fridge? Is that why it remains uncovered?
Big thanks from the newbie!
Yeah, do put the lid on. Both glass or plastic work. It’s almost always a bad idea to keep it close to food – but if you have to, consider wrapping the whole container, with the lid, inside another container/plastic bag to avoid accidental spillage. Also, make sure no one decides to try some “delicious” blue juice they found.
This technique is based on cooling the solution and reducing its solubility to force the crystals to appear – hence no evaporation is necessary. Good luck!
Hey Chase! Thank you for answering, I am just seeing this now. Have you seen any evidence of bio-mimicry when growing crystals? Crystals that you have purified, then they keep “growing” even when out of the solution?
Is this the process when purifying all of the elements? If I wanted to repeat this with selenium, or molyB, or Zinc, is it the same instructions or are they all different?
Thank you so much for your assistance with this!
No crystals can grow when they are out of solution – as the solution literally provides the ions for crystal growth.
This method of purifying chemicals only works for soluble compounds. Therefore, it’ll work for water soluble salts like alum & copper sulfate, or other compounds that dissolve in organic solvents but not metals.
is it okay to heat the liquid copper sulfate in a metal pot to reduce it instead of a glass one?
Try not to keep copper solutions in metal pots, as they tend to corrode the metal. If you really have to, use a stainless steel pot that isn’t used to hold food.
Hi Chase. Thanks for the information. I have some questions:
1. Can these final crystals then be used to make a copper solution?
2. If so, then how is this done?
3. Is it better to use distilled water when making the crystals?
4. Your photos all show the process being carried out in glass vessels. Do these need to be of a special kind so that they don’t break when heat is applied?
1. I’m assuming you mean a copper sulfate solution. Yes, of course.
2. Just dissolve them like normal copper sulfate powder.
3. Distilled water is perfect. Based on my experience, tap water is already good enough though.
4. Yes. My beakers are made from borosilicate glass. Regular glass would shatter under strong heating, and you definitely don’t want that.
Is crystal powder another product or do you mean more copper powder? Thanks in advance!
“Sprinkle some crystal powder to induce crystallization”
More copper sulfate power.
I should have made that clearer. Fixed it. Thanks!
Hey chase would you mind clarifying this step?
“First, dissolve a large amount of impure copper sulfate in a beaker. My powder was bought online and had a bunch of dirty stuff inside.
Next, dissolve all of it in hot water using the ratio 50 g copper sulfate to 100 ml water. If the copper sulfate does not dissolve, heat it lightly and stir.”
I don’t understand the first paragraph. Do I need to make 2 solutions or is this all going into one solution?
Ah, I didn’t realize I repeated the step twice. You only need to make one solution. Here’s the corrected version:
First, dissolve a large amount of impure copper sulfate with hot water in the ratio of 50 g copper sulfate to 100 ml water. If the copper sulfate does not dissolve, heat it lightly and stir. My powder was bought online and had a bunch of dirty stuff inside.
Thanks for pointing it out!
Would you happen to know if this would also work with zinc sulfate? Thanks.
Yes, recrystallization works with any water soluble salt. But in my opinion, zinc sulfate crystals are not as nice as copper sulfate crystals.
I was thinking you could electroplate the remaining copper out and then redissolve it in H2SO4 – but presumably you need slight excess acid – and then how do you deal with the excess acid… and… yeah… chasing yield on cheap reagents is a trap I feel…
But as a source of copper for metal crystals it sounds perfect…
That’s chemistry in a nutshell. Hahahaha
Hi, thanks for this explanation, how do you dry out the crystals?
Just place them on a piece of filter paper or tissue paper and wait 1 minute to let it absorb the solution. But don’t leave it on the paper for too long, because it will get stuck there. After 1 minute, wipe any remaining solution dry and store.
Thank you very much! 😀 Love your blog.
I heated a glass jar directly on electric stove once. Never again. Bottom separated rather cleanly. Doing it slowly enough or heating in water bath is good though. Just don’t apply too much heat unevenly.
You’re right; I should probably be more careful about heating those next time.
Have you ever had a flaky whitish percipitate form when heating the solution in the evaporation step? This happens with my solution way before a significant amount of water has evaporated and it dissolves when a few drops of concentrated sulfuric acid are added. It can’t be copper sulfate because at that point we’re far away from its solubility product and it doesn’t behave like copper sulfate at all. So does this sound familiar to you?
Yep, I’ve faced the exact same problem. It occurs quite rarely, and only when my copper sulfate isn’t pure. I believe the precipitate is a mixture of insoluble copper carbonates and hydroxides. They can really mess up the process, but since you discovered that adding acid works (by dissolving the precipitate and acidifying the solution hence preventing further precipitation), it should not be a big issue.