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

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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.

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    $\begingroup$ "This definition states that the substance present in the larger quantity is the solvent" - no it doesn't. $\endgroup$ – user2357112 supports Monica May 20 at 8:51
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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 .

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    $\begingroup$ copying from Poutnik's post on "whether a solvent can be a gas",to create a solution a solvent has to engage in inter molecular forces with the solute. But volatile materials can disperse without the presence of solvents . So it can be deduced that if the material is in gaseous surrounding, the gas acts like spectators. So, in this case, the gas is not a solvent. As a result , normal gas in general circumstances cannot be a solvent $\endgroup$ – Habib May 19 at 8:23
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    $\begingroup$ There is no such thing as no interaction situation. London dispersion forces/ Van der Waals forces are always present among gas molecules. $\endgroup$ – M. Farooq May 19 at 9:52
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    $\begingroup$ @M. Farooq It was meant as no interactions principally involved in separation of molecules of solutes. Volatile compounds would evaporate without gas. Soluble solute would not dissolve without solvent. With solvents comes solvatation with hydration as its special case. There is no solvatation in air, pulling molecules from a condensed phase. $\endgroup$ – Poutnik May 19 at 11:50
  • $\begingroup$ @Poutnik, true. This is all semantics of solvent/solute. Don't forget the principles of GC where nitrogen is the carrier gas, it does pull the molecules from the condensed (actually pretty viscous) stationary phase. $\endgroup$ – M. Farooq May 19 at 16:15
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    $\begingroup$ @M. Farooq Well, it is not true pulling. It is dynamic disbalancing of the diffusion equilibrium. Analytes are "pulled" by stopping dissolution in stationary phase, while the opposite - evaporation - continues. They would be "pulled" by vacuum as well, but we would lose the carrier functionality. $\endgroup$ – Poutnik May 19 at 16:21
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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.

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