What would you predict to be the effect of temperature on solubility? That is if you raise the temperature, does solubility of a solute increase or decrease? Based on your experience, it would be reasonable to assume that increasing temperature increases solubility. But remember both ΔH and ΔS have a role, and a temperature increase will increase the effects of changes in entropy. While dissolving solute into solvent is likely to increase entropy (if ΔS is positive), this is not always the case. Consider what happens when you heat up water on the stove. Long before the water reaches its boiling point, bubbles of gas are released from the liquid. At low temperatures these bubbles contain air (primarily N₂, O₂) that had been dissolved in the water.

6.1 Solutions
6.2 Solubility
6.3 H-bonds
6.4 Free Energy
6.5 Polarity
6.6 Temperature

Why? because the solubility of most gases (in water) decreases with temperature. The reason can be traced back to the entropy of solution. Most gases have very small intermolecular attractions (that is why the gas molecules don’t stick together and form solids and liquids), and therefore tend not to have very high solubility in water. As an example, the solubility of O₂ in water is 8.3 mg/L (25 ºC and 1 atmosphere.)

Most gases have a small favorable (that is negative) enthalpy of solution and small unfavorable negative entropy of solution. The effect on enthalpy can be traced to the dipole induced dipole attractions formed when the gas dissolves in the solution. The decrease in entropy results from the fact that the gas molecules are no longer free to roam around – their positional entropy is more constrained within the liquid phase that it is in the gaseous phase. When temperature is increased, the gas molecules have more kinetic energy and more of them can escape from the solution, increasing their entropy as they go back to the gas phase. Thus the solubility of O₂ (and other gases) decreases as temperature increases. This can produce environmental problems, since this means that less oxygen is available for organisms in the water to breathe. A common example of this type of thermal pollution occurs when warm water from power plants and manufacturing facilities is expelled into the environment.

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 for the noble gases is? (He, Ne, Ar, Kr, Xe)
• What else do you think might increase the solubility of a gas? (besides lowering the temperature) (hint - how are carbonated drinks bottled?

Solutions of Solids in Solids: Alloys

Another type of solution occurs when two (or more) elements, typically metals, are melted and mixed together so that their atoms can intersperse, forming an alloy. Upon re-solidification the atoms are now fixed in space relative to each other and the resulting alloy will have different properties than the two separate metals. One of the first known alloys was bronze; the major component of which is copper, with tin as the minor component (although other elements such as arsenic or phosphorus may also be included). The “Bronze Age” was a significant leap forward in human history: bronze is harder and more durable than copper, and therefore artifacts (weapons, pots, statues etc) made from bronze were prized. Before this, the only metals available were those that occurred naturally in their elemental form – typically silver, copper, and gold.

Bronze (which is typically a solution of about 90% copper and 10% tin) is harder and more durable than copper because the tin atoms substitute for copper atoms in the solid lattice. The resulting structure has stronger metallic bonds – making the alloy harder and less deformable, and with a higher melting point than the copper itself. However the bronze alloy retains the typical metallic properties – since the structure is still that of a metal, with some of the atoms replaced by a different metal. Steel is another example of a solid-solid solution. Steel can be described as an iron solvent with carbon solute. In this case the carbon atoms do not replace the iron atoms, but fit in the spaces between them (this is often called an interstitial alloy). This has the effect of making steel more dense than iron (more atoms per unit volume), harder, and less metallic. Since the carbon atoms are not in the original lattice, they affect the metallic properties more, making it harder for the atoms to move relative to each other. Steel is more rigid, less malleable, and conducts electricity and heat less effectively than iron.

Question to answer:

• 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. Use it to explain the properties of bronze.
• Draw an atomic level picture of what you imagine steel looks like. Use it to explain the properties of steel.
  

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?

Is the formation of a solution a reaction?

Up to now we have not considered what happens during a chemical reaction - that is, a process where the atoms present in the starting material are rearranged to produce different chemical species. The question arises - is the formation of a solution a chemical reaction? As we have so often seen the answer is not so simple. For example if we dissolve ethanol in water does this mixture contain chemically different species than the two components separately? The answer is no, there are still molecules of ethanol and molecules of water. But what about when an ionic substance dissolves in water? For example, sodium chloride must separates into sodium and chloride ions in order to dissolve. Is that a reaction? Certainly bonds are broken (Na+ Cl– ionic bonds), and new interactions are made (solvated Na+ and Cl– ions), but traditionally this would not have classified the dissociation of a salt as a reaction, even though it would seems to fit our criteria. There are arguments on both sides, but rather than quibble about what constitutes a reaction, let us move one place down 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 short hand because we begin with lots of water, and 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. At the same time 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 by which hydrochloric acid is formed 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 is formed between H and O.   

HCl(g) + H₂O ↔ H₃O+ + Cl–

We call this reaction an acid-base reaction. In the next chapter we consider this and other reactions in some detail.

6.1 Solutions
6.2 Solubility
6.3 H-bonds
6.4 Free Energy
6.5 Polarity
6.6 Temperature