Lab 20: Solubility: A Guided Inquiry Lab
Introduction
Purpose
The purpose of this lab was to allow students to put the principles of solubility in practice and thus further understand them. This required an understanding of the terms solubility (the ability of a minor substance, known as a solute, to dissolve in a major substance, referred to as a solvent) as well as its properties.
The purpose of this lab was to allow students to put the principles of solubility in practice and thus further understand them. This required an understanding of the terms solubility (the ability of a minor substance, known as a solute, to dissolve in a major substance, referred to as a solvent) as well as its properties.
Summary
In this lab students were asked to identify an unknown soluble solid by referring to a solubility graph. Students had to design their own procedures in order to find the identity of the solid and had to present the teacher with their findings for confirmation.
Procedure
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| Setup and materials for the lab. |
2. Measure out 10 mL of water into the graduated cylinder. Pour this into a beaker and heat it up on the hot plate; try to reach 50 degrees Celsius*, monitor this with the thermometer.
3. Measure out five grams of the unknown substance into a weigh boat and set this aside; this will be used in determining if the substance is NaCl.
4. Once the water has reached 50 degrees Celsius carefully retrieve it and place the unknown substance in. Stir it with the stirring rod for about five minutes. If necessary, place the beaker within another beaker partially filled with water at a similar temperature to maintain this temperature. If not, you may need to reheat the beaker after stirring for a few minutes.
5. Determine if the solute has dissolved completely yet. If so, continue testing the 8 grams of substance at 40 degrees Celsius* to determine if the substance is sodium nitrate or potassium nitrate. If it has not dissolved completely, then the substance is most likely sodium chloride.
6. Continue testing if necessary; if doubtful, you may run another trial.
7. Record and analyze data.
*Note: These temperature and mass values were taken from the graph, chosen at points where the three substances' solubility curves were easily distinguishable. Exercise judgement in choosing these points. In this procedure we chose 50 degrees Celsius as an ideal temperature to work with.
Data
During this experiment, we performed a total of two trials to confirm the identity of the substance.
Temperature (degrees Celsius) Mass (grams) of Substance Dissolved Completely?
Trial 1 50℃ 4.0 g No; some on bottom
Trial 2 70℃ 4.0 g No; some on bottom
Qualitative Data
We observed that even after ten minutes of continuous stirring with the first trial, the substance was not fully dissolved. The water remained clear; however there was a large amount of powder collected on the bottom of the beaker that remained no matter how long we stirred. The same results were found for the second trial.
Quantitative Data
The choice of temperature and mass were chosen based on the difference between According to the solubility graph, the conditions we put the solid in corresponds with the solubility of the ionic compound sodium chloride; when these conditions are plotted on the chart, they fall above the solubility line of sodium chloride, indicating that the solution would be saturated and that therefore excess salt would collect on the bottom of the container.This also applies to the figures for the second trial, which is found slightly above the line. This also indicates that there will be visible solid left at the bottom of the container due to the saturation.
Discussion/ Concluding Summary
Unknown Solid Identity
Our solid turned out to be the compound NaCl, sodium chloride.
Our solid turned out to be the compound NaCl, sodium chloride.
Evidence A
Even after continuously stirring the solution from our first trial (4 grams of solid, 10 mL of water at 323 Kelvins) for over ten minutes the substance still did not dissolve completely; we had a considerably large amount of substance remaining at the bottom of the solution. Thus, the substance could not be any of the substances with higher solubility such as sodium nitrate or potassium nitrate.
Even after continuously stirring the solution from our first trial (4 grams of solid, 10 mL of water at 323 Kelvins) for over ten minutes the substance still did not dissolve completely; we had a considerably large amount of substance remaining at the bottom of the solution. Thus, the substance could not be any of the substances with higher solubility such as sodium nitrate or potassium nitrate.
Evidence B
In our second trial, we raised the temperature to about 343 Kelvins; even still, the solute did not dissolve completely after about four minutes of stirring. According to the graph there is a large gap between sodium chloride and the other two substances in terms of solubility at this temperature, which would indicate that the substance was definitely sodium chloride.
In our second trial, we raised the temperature to about 343 Kelvins; even still, the solute did not dissolve completely after about four minutes of stirring. According to the graph there is a large gap between sodium chloride and the other two substances in terms of solubility at this temperature, which would indicate that the substance was definitely sodium chloride.
Solubility and Temperature: The Relationship
Solubility and temperature, according to the graph, have a positive, mainly linear correlation (when the substance is solid anyway). As the temperature increases, the solubility of the substance in grams per 100 grams of water also increases; each substance, however, increases at a different rate, with NaCl having a smaller sloe relative to the steep slopes of the other two substances.
Solubility and temperature, according to the graph, have a positive, mainly linear correlation (when the substance is solid anyway). As the temperature increases, the solubility of the substance in grams per 100 grams of water also increases; each substance, however, increases at a different rate, with NaCl having a smaller sloe relative to the steep slopes of the other two substances.
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