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My daughter has a chemistry test that I am helping her prepare for. One question is

Nickel sulfate, $\ce{NiSO4(aq)}$ is a green solution. Nickel chloride, $\ce{NiCl2(aq)},$ is a yellow solution. If you add nickel, $\ce{Ni(s)},$ to hydrochloric acid, $\ce{HCl(aq)},$ what color solution do you expect?

Wikipedia states that it will turn green but doesn't give an explanation for why. Can anyone fill in the blanks for us? (This is for high school intro to chem so try to keep it at that level.)

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    $\begingroup$ Could you add a link to where it says that the solution will turn green? Just for the record. And here's a related question. $\endgroup$ – Felipe S. S. Schneider Oct 15 at 12:49
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    $\begingroup$ I think the question is trying to get the student to think about what salts come from what acids. Since HCl does not produce sulfates, one can rule out the green option based on the information provided by the question. $\endgroup$ – t9050b Oct 16 at 0:09
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    $\begingroup$ High school science teacher here, so definitely biased, but I'd contest the notion that the question is badly worded. Notice that it asks what outcome you expect, not what the outcome will be. The latter sets the the student up for rote learning, while the former sets them up for deductive learning, a central concept in science. If the outcome is not as you expected, then that in in itself is interesting, and something we can build on further: why is that? $\endgroup$ – AkselA Oct 16 at 9:55
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    $\begingroup$ @AkselA Agreed, the question gives you information to get an expectation - all the teacher is asking is that you know that nickel will form salt with chlorine. $\endgroup$ – Tomáš Zato Oct 16 at 10:15
  • $\begingroup$ @TomášZato actually, all the teacher is asking is that you know that sulfur and oxygen cannot be spontaneously created, and that what comes out of a reaction can only contain elements which are supplied as input ;) $\endgroup$ – Syndic Oct 18 at 6:37
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The question is really badly worded. For starters, let’s look at solutions of nickel(II):

Colour comparison of various nickel(II) solutions
Figure 1: Nickel(II) solutions. From left to right: $\ce{[Ni(NH3)6]^2+}$, $\ce{[Ni(en)3]^2+}$, $\ce{[NiCl4]^2-}$, $\ce{[Ni(H2O)6]^2+}$. Image taken from Wikipedia, where a full list of authors is available.

You can ignore the left two but the rightmost is a standard nickel(II) solution (could be $\ce{NiSO4}$) while the second from the right is a chlorido-complex—basically $\ce{NiCl2}$ but dissolved in hydrochloric acid. I wouldn’t call the chloride solution ‘yellow’, more like ‘yellowish-green’ but your mileage may vary. Next, let’s look at solids.

Nickel(II) chloride hexahydrateanhydrous Nickel(II) chloride
Figure 2: salts of nickel(II) chloride. Left or top: hexahydrate. Right or bottom: anhydrous. Images taken from Wikipedia (hexahydrate, anhydrous), where a full list of authors is available.

The picture on the right is dry, i.e. completely water-free nickel(II) chloride (anhydrous). On the left, the salt contains tightly bound water molecules which can be removed by further drying although the salt itself would be dry in everyday terms; this is the hexahydrate of nickel(II) chloride.

It might seem easy that the salt on the left gives a greenish solution while the salt on the right gives a yellowish one. Here’s the catch: they should both give a greenish solution, because there is no excess of chloride as was required in the solution in figure 1.

But what about the actual question? Well, dissolving nickel in hydrochloric acid means that there should in fact be excess chloride, meaning that yellow (or yellowish green) would be the correct answer.

tl;dr: That question is a perfect opportunity to complain if it was marked as ‘incorrect’.

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Jan already gave a great answer explaining the real chemistry behind this, which you and your daughter should absolutely read. This answer also makes the point that the question is poorly worded and could be contested on that basis—that is absolutely true, but unfortunately rather common in high school chemistry courses.

In high school chemistry, one of the things they're trying to drill into young minds is how reactions proceed and which products will form. That's what they're trying to test here: can the student draw a conclusion about which product(s) will form when given the reactants?

Here, the reactants are given as excess aqueous hydrochloric acid and free nickel. Formulaically, that is the following reaction:

$$\ce{HCl(aq) + Ni(s) \rightarrow ?}$$

The student is supposed to be able to figure out that this is a single replacement/substitution reaction, a characteristic reaction of acids with metals, in which the ionic compound nickel (II) chloride will form while hydrogen gas is liberated:

$$\ce{2HCl(aq) + Ni(s) \rightarrow NiCl2 + H2(g)}$$

Once you figure this out, the question is designed to give you all of the information you need to answer the question insofar as it tells you what the colors of two different nickel salts are:

Nickel sulfate, $\ce{NiSO4(aq)}$ is a green solution. Nickel chloride, $\ce{NiCl2(aq)},$ is a yellow solution.

Essentially, you're supposed to decide which of these two salt solutions will be formed by the described reaction. Since the described reaction will yield nickel chloride (not nickel sulfate), the question is expecting you to answer that the solution will be yellow in color.

The question does not expect you to know, be able to figure out, or go research what color the solution will actually be if you do the reaction in the laboratory. Overthinking it may cause you to arrive at a different answer than was expected, which will lead you to getting the question wrong. You can absolutely contest this, but at least now you understand what the question wanted you to say and how they expected you to arrive at that conclusion.

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Anhydrous nickel chloride is yellow. However most simple divalent salts of nickel are green. This should be good enough for high school. The question is poorly worded because it is ambiguous (hopefully the textbook will improve it). If they discuss metal complexes it can be mentioned that $\ce{[Ni(H2O)6]^2+}$ is green in color.

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The other answers provide valuable insight into what actually happens and are very helpful in understanding the chemistry behind it.

But to correctly answer the question, a much more basic understanding of chemistry (and how tests work) would suffice.

Let's look at what is given in the Question:

Nickel sulfate, NiSO4(aq) is a green solution. Nickel chloride, NiCl2(aq), is a yellow solution. If you add nickel, Ni(s), to hydrochloric acid, HCl(aq), what color solution do you expect?

So what the teacher gave us is:

  • two solutions with their names, colors and formulas:
    • Nickel sulfate, NiSO4(aq), green
    • Nickel chloride, NiCl2(aq), yellow
  • two chemicals to be mixed, with their formulas:
    • Nickel, Ni(s)
    • hydrochloric acid, HCl(aq)

Now, if we can assume that the student in question knows the notations, she can figure out that the elements going in are Ni, H and Cl. Let's look at the options we were given for output:

  • One of the two possible output solutions contains Ni and Cl - these are going in, so this solution would be possible to produce.

  • The other one contains Ni, S and O - Ni is going in, but where would the S and O come from? (Technically the O could come from the air I guess, but even allowing for that the S would still be unaccounted for)

So with basic knowledge of chemistry and the notation used, basic logic can give us the answer. There's no way to produce Nickel sulfate because there's no sulfur or oxygen going in, which means we should expect to get the other one. Which is colored yellow, making "yellow" the answer.

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