This article incorporates, in modified form, material from Illustrated Guide to Home Chemistry Experiments: All Lab, No Lecture.
Differential solubility was one of the earliest methods developed for separating mixtures. Differential solubility depends on the fact that different substances have different solubility (ability to be dissolved) in different solvents. In this lab, we’ll examine the simplest example of differential solubility: separating a mixture of two compounds, one of which is freely soluble in water, and one of which is insoluble in water.
Required Equipment and Supplies
- goggles, gloves, and protective clothing
- balance and weighing papers (optional but recommended)
- microwave or conventional oven (optional)
- evaporating dish (2)
- test tubes (2)
- stopper, solid (to fit test tube)
- test tube rack
- filter paper (or white coffee filter)
- dry sand (3.0 g)
- sucrose (table sugar) (3.0 g)
- tap water
All of the specialty lab equipment and chemicals needed for this and other
lab sessions are available individually from Maker Shed or other laboratory
supplies vendors. Maker Shed also offers customized laboratory kits at special
prices, including the Basic Laboratory Equipment Kit, the Laboratory Hardware Kit, the Volumetric Glassware Kit, the Core Chemicals Kit, and the
Supplemental Chemicals Kit.
Although the chemicals used in this laboratory session are not hazardous, it is a matter of good laboratory practice to wear splash goggles, gloves, and protective clothing at all times. Be careful when heating the separated compounds in the microwave.
Substitutions and Modifications
- If you do not have a balance, you can substitute a set of kitchen measuring spoons, using a quarter teaspoon of sand (~ 3 g) and 3/8 teaspoon of sugar (~ 3 g). After you separate and dry the two compounds, measure the volume of each to determine if you ended up with as much as you started with.
- If you do not have evaporating dishes, you can substitute microwave-safe dessert plates, saucers, or similar flat dishes.
- If the sand is not completely dry, spread it in a thin layer and dry it in a microwave or conventional oven.
- You may substitute ordinary table sugar for the sucrose.
Paul Jones comments:
Make observations of the physical appearance of the sand and sugar. Does it change as you go through the process? This is a good rule of thumb to consider during any physical or chemical manipulation.
- If you have not already done so, put on your splash goggles, gloves, and protective clothing.
- Weigh about 3.0 g of dry sand and about 3.0 g of sucrose, record the masses on Lines A and B of Table 6-1, and add both of them to a test tube.
- Fill the test tube about 90% full with tap water, stopper it, invert it several times to dissolve the sucrose, and replace the tube in the rack.
- Weigh a piece of filter paper, record the mass on Line C of Table 6-1, and fan-fold the filter paper.
- Place the funnel atop an empty test tube in the rack and put the fan-folded filter paper into the funnel.
- Swirl the solution in the first test tube to suspend as much sand as possible and pour it into the filter funnel, capturing the filtrate in the second test tube.
- Transfer the filtrate from the second test tube to one of evaporating dishes, replace the tube in the rack, and put the funnel back in position.
- Add a couple mL of water (~ 1/8 tube) to the first test tube, swirl the contents to suspend any remaining sand in the test tube, and pour it through the filter funnel, capturing the sand on the filter paper and the filtrate in the second test tube.
- Repeat step 8 as many times as necessary to transfer all of the sand from the first test tube to the filter paper. Once you’ve finished, remove the filter paper and spread it out on the second evaporation dish to dry. Add the additional filtrate from the second test tube to the filtrate already present in the first evaporation dish.
- Allow the water to evaporate from both evaporating dishes. You can hasten drying by heating the dishes in a microwave oven at a low to medium setting until they are completely dry.
- Weigh the filter paper and sand and record the mass on Line D of Table 6-1. Subtract the mass of the filter paper to determine the mass of the sand, and enter that value on Line E of Table 6-1. Calculate the percent yield of sand for this operation and enter that value on Line F of Table 6-1.
- Place a weighing paper or weighing boat on the balance pan and tare the balance to read 0.00 g. Carefully transfer as much as possible of the sucrose from the second evaporating dish to the weighing paper, determine the mass of the sucrose, and enter that value on Line G of Table 6-1. Calculate the percent yield of sucrose for this operation and enter that value on Line H of Table 6-1.
Figure 6-1. The filter paper captures the sand, allowing the sucrose solution to pass into the receiving vessel
Table 6-1. Differential solubility – observed and calculated data
|A. sand, initial||___.___ g|
|B. sucrose, initial||___.___ g|
|C. filter paper||___.___ g|
|D. filter paper + sand||___.___ g|
|E. sand, final (D – C)||___.___ g|
|F. sand, percent yield [(E ÷ A) × 100]||___.___ %|
|G. sucrose, final||___.___ g|
|H. sucrose, percent yield [(G ÷ B) × 100]||___.___ %|
All of the waste material from this laboratory can be disposed of with household waste.
Q1: After this procedure, you should end up with separate piles of sand and sucrose with masses that are the same as the initial masses. How closely do your experimental results correspond to the expected results? Propose an explanation for why the final masses of the sand and/or sucrose might be higher or lower than the starting masses.
Q2: You are mixing concrete, but the only sand available is heavily contaminated with salt, which weakens concrete. You have plenty of water available, but no means of filtering. How might you remove the salt from the sand?
Q3: You are presented with a mixture of two compounds, both of which are freely soluble in water. What might you do to separate these compounds using differential solubility?
August 24, 2009