- Density gradients are formed using water with different concentrations of salt and sugar. The different concentrations have different densities, keeping them from mixing.
Recrystallization is a process that takes an impure substance and purifies it, removing the unwanted substances from the wanted solid. This method involves heating the solid to dissolve it in a solvent , cooling it to reform the solid, then vacuuming and drying the newly formed pure crystals. First, you have to choose a proper solvent, something that the desired solid is insoluble in at room temperature, but soluble in when heated. If the solvent is too strong, the solid won’t fully reform, resulting in a poor yield of final product. If it is too weak, the start material will not fully dissolve and the final product will still contain impurities. You then dissolve the solid in the solvent using heat, when the solid is dissolved, the impurities are also dissolved and escape into the solvent. Then the solution is removed from heat and cooled to room temperature, allowing the crystals to reform, leaving the impurities in the solvent. The recrystallization process can be helped by placing the solution in an ice bath to cool it down to 0o C. Once the crystals have formed you filter out the solvent and impurities and dry the purified solid. Once you have obtained pure, dry crystals, you can take the mass of them to discover your mass yield, and test the melting point of the solid to determine the purity of the solid. For this experiment we had crystals that contained Lead bromide (PbBr2), Lead iodide (PbI2), and Lead chloride (PbCl2). We chose water as the solvent for all of our solids. We heated all of the solutions on hot plates while they were being stirred. Once all of the solid had dissolved, we cooled the solution at room temperature, continuing to stir them. Crystals as the solutions cooled, crystals began to form, see below for pictures. Since these recrystallizations were done for the purpose of show, we did not filter and dry the solids.
In this experiment, we were showing how Acetic Acid and Hydrochloric Acid can be used to dissolve the shell of an egg. The calcium carbonate in the egg shell reacts with the acid to form a salt, water, and carbon dioxide. We started by diluting the stock 15M Acetic Acid and 12M Hydrochloric Acid to the molarities that we wanted. We used 250mL of four different concentrations of each acid (Table 1).
|Acid||Molarity||Amount of stock acid used|
Overall we used 108.75mL of stock Acetic Acid. Acetic Acid is $58.00 per Liter, meaning that we used about $6.30 of Acetic Acid. We used 37.49mL of Hydrochloric Acid. HCl is $41.00 per 500mL, so we used about $3.07 of HCl.
Above is the display set up. At this point all of the eggs still had their shells and were the same as they were in the package. In day 1, pictures below, all of the eggs formed bubbles around the shells. Up close you could hear the gas, CO2 trying to escape the jar. All of the eggs still had the shell and looked the same at this time.
After week one there was some significant changes in the eggs.
.1 M HCl: The shell on the egg did not do anything and when it was broken open it looked exactly like one that came out of the carton.
.2 M HCl: This egg cracked so we were unable to observe the results.
.5 M HCl: The shell dissolved completely, leaving only the membrane. The egg also denatured and cooked inside because of the acid.
1 M HCl: The egg completely denatured and was hard inside. The shell had dissolved and only the egg’s membrane was left
.435 M CH3COOH: The egg shell halfway dissolved. The egg was still white but it bounced.
.87 M CH3COOH: The shell off the egg dissolved completely and the egg and it was able to bounce.
1.74: The shell off this egg turned into a rubber material, but it did dissolve. This is why it is white in appearance. This egg also bounces slightly.
3.48 M CH3COOH: This egg denatured just like the .5 M and 1 M HCl.