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Download This podcast will discuss two related misconceptions about solubility
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This podcast will discuss two related misconceptions about solubility. First is that compounds are either soluble or insoluble with nothing in between. Second is the commonly held belief that like dissolves like. This misconception seems to originate when students are first introduced to solubility at a young age in the spiraling approach. In California, solubility is introduced as early as 1st grade where students should understand that salt can be removed from saltwater via evaporation. A six year old would certainly have a difficult time understanding solubility completely. Many of my high school students didn’t know how to separate salt from salt water when they began chemistry. In reality, there is a spectrum of solubilities from nearly insoluble, to slightly soluble, to almost completely soluble. Of course the solubility depends on which solvent and the temperature of the solvent as well. A solute will have different solubility in alcohol than it will in water. For example, sugar is highly soluble in water and nearly insoluble in oil. You can also dissolve quite a bit more sugar in hot tea than you can in iced tea. So for now, let’s limit the conversation to water as the solvent. Solubility is generally measured by how many grams of solute dissolves in 100 mL of the solvent at a particular temperature, usually 25oC. As an example of the spectrum of solubilities, consider water at 80oC. You can dissolve about 1 gram of cerium (III) sulfate, 40 grams of sodium chloride, or 85 grams of sodium hydrogen arsenate in 100 grams of water at that temperature. No chemical is completely insoluble in a solvent and no chemical is completely soluble in a solvent. It is often said that like dissolves like. What is meant by this is that polar compounds dissolve polar compounds and non-polar compounds dissolve non-polar compounds. The problem is that just like with solubility, there is a spectrum of polarity from completely polar in a highly ionic bond to completely non-polar when two identical atoms bond with each other in a covalent bond and there’s everything in between. The polarity of a bond depends on how hard each atom pulls on the electrons. This property is called the electronegativity. The bigger the difference between the electronegativities of the two atoms, the more polar the molecule is. When two of the same atom bond, such as in O2 or N2, there is zero difference between the electronegativities and the molecule is completely non-polar. When a molecule is in between the two, it dissolves compounds a little more polar and a little less polar than it is. For example, water can dissolve small alcohols which are fairly non-polar as well as salt which is very polar. Water is sometimes called the “universal solvent” because it can dissolve somewhat polar molecules and somewhat non-polar molecules. Water is somewhere in between ionic and covalent. To summarize, there is a spectrum of solubilities from almost completely insoluble to almost completely soluble. It is also known that solvents can dissolve solutes that are the same polarity that they are as well as a little more polar or a little less polar.