Separation of a Mixture
To separate a mixture based on the properties of its substances. Mixtures that are not chemically combined can be separated based on their unique physical properties. The use of phase changes support the separation of distinct substances in a heterogeneous mixture. Simple techniques such as sublimation, decanting, evaporation and extraction can be used for the different types of phase changes.
The procedure of separating a mixture of three solids (NH4Cl, SiO2 and NaCl) will be accomplished through the processes of sublimation and decantation. Heat is applied to sublimate the NH4Cl solid to remove it from the mixture. The sublimation process causes a phase change of the solid state to a vapor state. The remaining two solids go through a decantation process where water is used to separate the two solids. Since NaCl is soluble in water, an aqueous solution is formed and therefore making it possible to remove it from the insoluble solid SiO2. Following the separation of the final two solids, heat is applied to both the wet solid (SiO2) and solution (NaCl) to evaporate out the water. This will make it possible to measure each solid separately. The mass of the three separated solids, totaled together, should be no less than 99% of the total mass to begin with.
This procedure will separate three components of the mixture: NH4Cl, NaCl, and SiO2. Refer to Chart 1 for the flow chart process. The NH4Cl will be sublimed from the mixture while the NaCl will be decanted from the SiO2. Begin the procedure by measuring the mass of the empty Crucible 1 on the pan balance and record this data. Shake the bottle mixture to have uniformity of the contents and then add 2 – 3 grams of the mixture in to Crucible 1. Measure the total weight in grams using the pan balance to the nearest .001 g, record this data.
Sublime Crucible 1 by placing it on the wire gauze, ring and ring stand (see Figure 1) and heat until the white “smoke” of the NH4Cl stops. Occasionally slightly shake the crucible during the sublimation process. When the white “smoke” has stopped the NH4Cl is now sublimed from the mixture. Let the crucible cool and then weigh and record the remaining solids on the pan balance.
Measure and record the mass of the empty Crucible 2 on the pan balance and record this data. Measure using the pan balance and record the data for Watch glass 1. Repeat this step for Watch glass 2 and record the data. These watch glasses will be used with Crucible 1 and Crucible 2, respectively. Start the decantation process by adding 25-mL of water to the solid in the Crucible 1. Using the glass rod stirrer, stir for five minutes. Then carefully decant the liquid from Crucible 1 in to Crucible 2. Do not allow for any of the solid to transfer in to Crucible 2. Add another 10-mL of water to Crucible 1 and stir for five minutes. Again transfer the liquid from Crucible 1 to Crucible 2. Repeat this process one more time with another 10-mL of water.
Crucible 1 now contains SiO2 and Crucible 2 contains NaCl. The next step is to heat both crucibles. Set up a second ring stand, see Figure 1. Using low to moderate heat, evaporate the water from both crucibles. Avoid over heating that will cause boiling or splattering. Place the watch glasses on the crucibles as the water level approaches a minimum. Reduce the heat to a low setting. Water will continue to condense on the watch glasses. Continue to evaporate the water until there is no more water on the watch glasses. When no more water condenses on the watch glasses, let the crucibles cool to room temperature.
After the crucibles have cooled, measure Crucible 1 on the pan balance and record the data. Measure Crucible 2 on the pan balance and record the data.
Figure 1. Heating requires the setup of two ring stands (picture provided from the lab manual).
DATA AND RESULTS:
Chart 1. Separation of a mixture