can gold disappear after it was dissolved in agua regia?
I do not disagree with 4 Metals, but I guess I will just say if a magician held a gold coin in his hand and then wet it to where I could no longer see it I would say it disappeared in front of my eyes, I know it is somewhere but I cannot see it anymore, in chemistry it is like magic (at least in my small and simple mind).
Yes (in my mind), gold can disappear after being dissolved, at least we cannot see it anymore, it is not magic but chemistry, we cannot see it but it is still there in solution but not as the same gold we held in our hands before dissolving it. When gold is in the metallic state it is a lump of gold atoms with a full shell of electrons.
When we dissolve the gold we Oxidize (Oxidation the loss of electrons) the gold in a chemical reaction where the acids involved in this reaction are Reduced (Reduction a gain of electrons), so in this reaction the atoms of the gold is oxidized (the atoms of gold have lost their electrons) to form a salt of gold (positive ions called Cations) and the acid is reduced to a salt of the acid (negative ions called Anions) together these positive and negative ions make up a salt of the gold in solution we call gold chloride Au+Cl3-.
Gold chloride will tint the solution yellow if dilute to red burgundy when very concentrated, but we cannot see the gold as it basically disappears from our eyesight, and other metals will also give color to a solution, iron ions in solution can look like gold (yellow) or look like copper (green) or red and several other colors depending on its oxidation state... With other metals giving colors to a solution much easier than gold the gold in our solution basically disappears (at least from our eyesight) the gold in solution cannot be seen but it can be detected. The color of a solution is not a very good way to tell what is in solution, although it can sometimes be an indicator, testing is the only way to verify what the color actually comes from or what metal ions are in solution...
How do we detect for gold in solution if we cannot see it?
Well, we just use a little magic we call chemistry.
We have to make the gold reappear to our eyesight.
How do we do this?
We have to give the gold ions back their electrons so they will make atoms of gold with full orbitals shells, and we have to get enough of these small gold atoms to join together with each other in a big enough clump that we can see them with our naked eye (make the gold reappear magically well chemically but magic sounds more fun), to do this we must Reduce the gold giving it electrons, we do not need to reduce a large amount of gold to see it in solution we can use a tin salt called stannous chloride, which reduces a small portion of the gold ions in solution to a certain size of gold in nanoclusters of atoms of reduced gold called colloidal gold which give a purple color to the solution called purple of Cassius, even though these clusters of gold atoms are actually metallic gold with full shell of electrons in the atoms of gold, the clusters colloid and will not grow large enough to precipitate from solution the cluster gain an electron charge which makes them repel other clusters of gold atoms, ad so are virtuall locked in a cycle which is not easily broken (but thats a subject for another question).
So gold does disappear from our eyesight, but the gold is still in solution as gold atoms with missing electrons or gold ions a salt of gold.
Stannous chloride SnCl2 can make the gold reappear in solution as a purple colored colloidal solution we call the purple of Cassius test, where the clusters of gold atoms color the solution violet in the test.
Well if I have seen gold and someone put it into a solution, for my eyes it disappears.
But just because it disappears it does not mean it is gone.
Your gold will disappear from your possession if you do not know where it is and you throw it out with your waste, with stannous chloride we can detect very small amounts of gold in solution (by a reduction reaction), which we can see with our eyes.
Without using the stannous chloride test you are just guessing, and have no real idea if gold is in solution or not, this is why we call SnCl2 the eyes of the refiner, it allows us to make the gold reappear to our eyesight.
Like a magician I can make gold disappear where you cannot find it-- unless you also are a magician and can make it reappear with magic (chemistry), your magic will come with a chemical reaction using stannous chloride, or ferrous sulfate...
I do not believe in magic a magician cannot make anything disappear (even gold), he can hide it or change its form or make it into something else, but he cannot make it disappear.