Solubility Tests for Unknowns

Week 2 Lab Handout

Determine the solubility of your unknowns in water, 5% sodium hydroxide (NaOH), saturated sodium bicarbonate (NaHCO3), and 5% hydrochloric acid (HCl).

The best way to test for solubility is to add a spatula tip of solid unknown (about 10 milligrams), or 1-2 drops of liquid unknown, to about one milliliter of solution in a small test tube or vial and then mix. In certain cases it may take a minute or two for all the unknown to dissolve, so be patient. With the solid it will usually be easy to see whether or not it has dissolved, but with the liquid you will have to look closely. If the solution is cloudy, the liquid unknown has probably not dissolved, just become dispersed in the aqueous layer. If you think that the unknown is partially soluble, you may try heating the solution gently to see if this causes it all to dissolve.

If your liquid unknown does not dissolve in water, you should note whether it appears on the top of the water layer or the below the water since this will tell you something about the density of your unknown. Water has a density of about 1.0 g/mL. Most organic liquids have a density of less than 1.0 and hence float on water (look at what happens when you mix oil and water when making a salad dressing). Organic liquids with density greater than 1.0 often contain halogens (Cl, Br, or I). They will sink to the bottom when added to water.

These aqueous solubility tests are particularly effective for determining the presence (or absence) of acidic or basic functional groups in your unknown (Scheme 1).

Solubility in H2O

If your unknown is soluble in water, it suggests that you have at least 1 functional group capable of hydrogen bonding with the water per 4-5 carbon atoms. For example, simple alcohols containing 1-3 carbons (methanol, ethanol, propanol) are completely soluble in water. Butanol and pentanol (containing 4 and 5 carbon atoms respectively) are slightly soluble in water. While hexanol (6-carbons), and larger homologues, are essentially insoluble in water.

If your unknown is soluble in water, then you can determine whether it contains an acidic or basic functional group by testing with litmus paper. Dissolve about 100 mg of your sample in 1 mL of water and then test with red and blue litmus papers. If the red paper turns blue when you dip it into the solution of your unknown in water, then your unknown contains a basic functional group. In simple organic compounds, such as those we are using, this normally indicates the presence of an amine. If the blue litmus paper turns red when you dip it into the solution, then your unknown contains an acidic functional group. In simple organic compounds this normally indicates the presence of a carboxylic acid. Of course it is possible that your unknown has no acidic or basic functional groups, in which case you will see no change in the color of the litmus paper.

If your unknown is soluble in water then there is no reason to test solubility in any of the other aqueous solutions (5% sodium hydroxide, 5% hydrochloric acid, or saturated sodium bicarbonate) since these solutions are predominantly water and consequently your unknown will dissolve in them as well. However, even if your unknown is insoluble in water it may still dissolve in one of these aqueous solutions if it reacts to form an ionic (charged) species. For example, decanoic acid is insoluble in water but when treated with a base, such as sodium hydroxide or sodium bicarbonate, it reacts to form a carboxylate salt which is soluble in water.

Solubility in Aqueous NaOH

If your unknown is insoluble in water, but does dissolve in 5% sodium hydroxide (NaOH) solution, then your unknown probably contains an acidic functional group (pKa < 15) that is deprotonated by the sodium hydroxide producing an ionic compound. Two common functional groups with this property are carboxylic acids (pKa ~5) and phenols (pKa ~10) (Scheme 2).

Solubility in Aqueous NaHCO3

Sodium bicarbonate (NaHCO3) is a weaker base than sodium hydroxide. Thus, if your unknown does not dissolve in 5% sodium hydroxide there is no reason to test its solubility in a solution of sodium bicarbonate. However, if your unknown does dissolve in the hydroxide solution, it is useful to test the solubility in sodium bicarbonate since sodium bicarbonate will deprotonate functional groups with a pKa < 8, but not those with a pKa > 8 (Scheme 3). Thus, if your unknown dissolves in the aqueous NaOH, but does not dissolve in aqueous NaHCO3, it probably has a functional group with a pKa of between 8 and 14, possibly a phenol. If, on the other hand, your unknown is soluble in the bicarbonate solution, it is likely to have an acidic group with a pKa < 8, probably a carboxylic acid.

As an additional observation, when bicarbonate solutions are treated with acids they react to form CO2 (a gas) which bubbles out of solution. Look closely to see if you observe any bubbling. You can even try this test if your unknown is soluble in water as an additional way of confirming the presence of a carboxylic acid.

Solubility in Aqueous HCl

If your unknown is not soluble in water, but does dissolve in 5% HCl, then your unknown probably contains a basic functional group that is protonated by the hydrochloric acid producing an ionic compound. The most common organic functional group with this property is an amine (Scheme 4).