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6.6.2: Is the Formation of a Solution a Reaction?

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  • We have not yet considered what happens during a chemical reaction: a process where the atoms present in the starting material are rearranged to produce different chemical species. You may be thinking, “Isn’t the formation of a solution a chemical reaction?” If we dissolve ethanol in water, does the mixture contain chemically different species than the two components separately? The answer is no: there are still molecules of ethanol and molecules of water. What about when an ionic substance dissolves in water? For example, sodium chloride must separate into sodium and chloride ions in order to dissolve. Is that a reaction? Certainly interactions are broken (the interactions between Na+ Cl– ions) and new interactions are made (between Na+ ions and water and Cl– ions and water), but the dissolution of a salt has not traditionally been classified123 as a reaction, even though it seems to fit the criteria. Rather than quibble about what constitutes a reaction, let us move along the spectrum of possible changes and look at what happens when you dissolve a molecular species in water and it forms ions.

    When you dissolve hydrogen chloride, HCl (a white, choking gas), in water you get an entirely new chemical substance: hydrochloric acid (or muriatic acid as it is known in hardware stores), one of the common strong acids. This reaction can be written:

    HCl (g) + H2O ➞ HCl (aq)

    This is a bit of shorthand because we actually begin with lots of water, but not much of it is used in the reaction. We indicate this fact by using the aq symbol for aqueous, which implies that the HCl molecules are dissolved in water (but as we will see they are now no longer molecules). It is important to recognize that hydrochloric acid, HCl (aq), has properties that are quite distinct from those of gaseous hydrogen chloride HCl (g). The processes that form hydrochloric acid are somewhat similar to those that form a solution of sodium chloride, except that in this case it is the covalent bond between H and Cl that is broken and a new covalent bond between H and O is formed at the same time.

    HCl(g) + H2O ➞ H3O+ + Cl–

    We call this reaction an acid–base reaction. In the next chapter, we will consider this and other reactions in (much) greater detail.

    Questions to Answer

    • Can you convert the solubility of O2 in water into molarity (moles solute (O2) / liter solution)?
    • If solubility of gases depends on dipole–induced dipole interactions, what do you think the trend in solubility is for the noble gases (He, Ne, Ar, Kr, Xe)?
    • What else might increase the solubility of a gas (besides lowering the temperature)? (Hint: How are carbonated drinks bottled?)
    • Why do you think silver, copper, and gold often occur naturally as elements (rather than compounds)? Draw an atomic-level picture of what you imagine bronze looks like and compare it to a similar picture of steel.
    • Use these pictures to explain the properties of bronze and steel, as compared to copper and iron.

    Questions to Ponder

    • Why do you think the Iron Age followed the Bronze Age? (Hint: Does iron normally occur in its elemental form? Why not?)
    • How did the properties of bronze and steel influence human history?


    123 It has been noted that one reason why chemistry is so difficult is that even experienced chemists cannot agree on the terminology and this is one such example.

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