Can a solvent be a solid?

Question

Can a solvent be a solid? I do not think so. But is it possible for a solvent to be a solid?

Answer 1

When most people think of the term solvent, a liquid medium comes to mind; however, in the technical sense of the definition, this does not have to be the case. Here is the definition of a solution present in the IUPAC gold book:

A liquid or solid phase containing more than one substance, when for convenience one (or more) substance, which is called the solvent, is treated differently from the other substances, which are called solutes. When, as is often but not necessarily the case, the sum of the mole fractions of solutes is small compared with unity, the solution is called a dilute solution. A superscript attached to the ∞ symbol for a property of a solution denotes the property in the limit of infinite dilution.

The definition of a solvent is incorporated into the definition of a solution. This definition states that the substance present in the larger quantity is the solvent while that present in the smaller quantity is the solute. Based on this definition, it is entirely possible to have a solvent that is in the solid phase. For example, brass is a mixture of 64% copper and 34% zinc. According to the IUPAC definition, a solution can be in the solid phase. Because copper is present in a larger amount, it is considered the solvent -- a perfect, everyday example of a solvent in the solid phase.

Answer 2

The term solvent is not restricted to liquids. You can very well use it for gases, it may not be wrong to say that in air, nitrogen is the solvent in which oxygen is dissolved. We regularly use supercritical $\ce{CO2}$ in chromatography. There is no harm in calling a solid solvent- think of alloys.

You can read the concept of solid solutions here .

Answer 3

Both your question and the answers seem to rely on a common linguistic/mental shortcut which regards phase as some kind of inherent property of a compound, but I would argue that it becomes a confusing oversimplification when regarding this specific question.

I would say that the proper statement here would be "During the process of dissolving, the solvent has to be in a liquid state", or maybe "in a fluid state" depending on how you like to define the semantics of a solution. The reason for this is that the solvent has to get "between" the molecules of the solute to do its job, and that can only work when the solvent flows.

As for the answers which discuss solid solutions, they are correct, under the implicit assumption that the phase change happened after the dissolving process was finished. Sure, there are some kind of edge cases of diffusion between metals, but really, nobody makes brass by putting a block of copper and a block of zinc in a press at room temperature and waiting a century, to then harvest a fraction of a millimeter thick alloy. So, if we discount such unrealistic edge cases, the statement as I formulated it above is still true, and absolutely compatible with the statement "There are solid solutions" which could be rewritten more explicitely as "A liquid solution can change its phase to solid". (The opposite is of course also true - you can melt an alloy - but not especially relevant to the apparent contradiction).

The comments under M. Farooq's answer do a good job of describing the point about gases. In that case, you have a simple linguistic/categorization problem. The prototypical process of dissolving something involves a liquid, and the process of mixing two gases is in some ways similar and in some ways different. So it is up to the individual language user to "decide" (typically not a conscious decision, but a gut feeling that can be rationalized post-hoc) whether they consider this process to be similar enough to deserve to be called "dissolving" or not. Depending on that decision, you can have a gaseous solvent (at the time point of creating the solution) or not. The above point about phase changes still apply - if you would mix, say, water vapor and ethanol vapor and then cool it down to a liquid, I am pretty certain that everybody would term it "a solution of ethanol in water" (or water in ethanol, depending on ratios) even though there will be a dispute whether the process of dissolving happened when the mixture was still in a gas state.

A separate linguistic case which deserves a mention: there is a widespread usage of "dissolving" among laypeople that is actually incorrect by scientific standards. It is used to describe a situaion in which you have a chemical reaction by a corrosive gas or liquid with a solid. If you pour liquid acid over a basic salt, chances are that you will get a chemical reaction - but many people don't recognize that detail and, seeing the crystal disappear, will state that the acid dissolved the substance (they are also unlikely to call it "the salt", btw, reserving that name for sodium chloride only). Even if one of the two reactants happened to be in a gaseous phase, I would say that it is incorrect to consider this an example of a solution with a gaseous solvent, even though you might meet people (without advanced scientific education) who would classify it as such.

Answer 4

It is more a question of appearances. An acquaintance was playing with some mercury and got a drop or two on his gold wedding ring and watched it disappear into the gold the ring broke in two. What dissolved in what? Place a cup of rice in two cups of boiling water and simmer for about 20 minutes. Where did the water go? Do the same with equal volumes of pasta, then with sugar and water. Hydrogen that is generated on some metals such as Pt, Pd, and some high nickel stainless steels will pass thru thin sheets of the metal most likely as H atoms. This works best if the H is generated galvanically by corrosion processes [hydrogen embrittlement] but can happen with very high pressures of H2 gas [ask the petroleum people]. In real life real things happen that do have relatively simple answers based on chemistry but require some imagination beyond the textbook.