Lab 8: From Topic I, Recrystallization and Salting Out

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From this week on, we are going to start working through the Home Scientist Lab Manual. Did you download the Home Scientist CK01A Standard/Honors Home School Chemistry Laboratory Manual already? You’ll need to use it for the rest of the course.

Important Note:  We will not be working through the manual in order, but instead we will be performing labs as they align to the concepts presented in the textbook. Therefore, you will have to pay close attention to which labs are assigned, and how they are being modified.

Reading:  Topic I. Separating Mixtures. We will be doing Session I-1. Recrystallization  pp. 21-26 and Session I-4. Salting Out, pp. 39-41.

Experimental Title: Lab 8:  Investigating Solubility, Recrystallization and Salting Out

Date of laboratory:  July 22, 2014

Purpose: The purpose of this laboratory is to examine concepts relating to solubility. We will investigate how temperature affects solubility, how to isolate pure sodium bicarbonate via a recrystallization technique, and how to utilize differential solubility to separate water and isopropyl alcohol by a salting out procedure.

Introduction:

You may skip reading the introduction in the lab manual, pp.19-20 because it pertains more to separating mixtures than solubility, which is what we are interested in this week. Instead, use the background information from each lab session for your own lab notebook introduction. The background information can be found on pp. 21-24 and pp. 39-40. You don’t need to write down everything word-for-word, but you might want to jot down important concepts. You can also supplement information from the text/lesson if you find it useful.

The solute is the smallest part of a solution, or the substance being dissolved.

The solvent is the larger part, or the part doing the dissolving.

A solution is a solute dissolved in a solvent.

Miscible means two substances that can mix, and particularly that they can form solutions in any proportion. That means that both substances can be present as the smaller portion or as the larger portion.

Recrystallization involves making a supersaturated solution and then adding more solute. This causes the solute to fall out of solution in the form of pure crystals.

Special safety concerns for Lab 8:

Please start by carefully reading the Lab Safety section, pages 10-13.

  • If anything spills, please clean it up immediately with a paper towel and let your instructor know.
  • Alcohol is flammable, so please keep it away from any sources of heat or sparks.
  • If glass breaks, do not pick it up with your bare hands. Notify your instructor immediately.
  • Be sure to wash your hands when you are finished with this lab.
  • We will discuss whether to actually do the tasting he suggests for session I-1.

Materials: See lists page 21 and 39.

Procedures:

Session I-1 Recrystallization

Go ahead and write down the Procedure on pages 24-25 the lab manual with these adjustments:

For each measurement, use the quantity he lists (for example one teaspoon),  and then weigh it on the scale to convert to grams. Adjust your methods accordingly. (He assumes that the people using the kit might not have scales).

Edit:  Further details about changes made 7/21/2014. Please follow the procedure in the manual with the following changes:

Step 2. Weigh a small paper cup on the scale. Record the weight in grams. Add one teaspoon of table salt and 3 heaping Tablespoons of baking soda to the cup. Mix thoroughly and weigh again. Record the weight. Subtract the weight of the cup and record the weight of the mixture in grams.

Step 3. Use 100 mL of cold tap water.

Step 4. Once you have added the mixture until just a few grains remain undissolved, weigh the paper cup with mixture again so you know exactly how many grams you used. You don’t need to keep track of the teaspoons.

Step 5. As the solution is heating, add the mixture again until just a few grains remain undissolved. Take the temperature and weigh the cup to calculate how much mixture you used. This will give you an additional data point between room temperature and boiling.

Step 6. Once again, at the end weigh the cup plus mixture when you have added the necessary amount, and record the final weight. Subtract the weight of the cup you recorded earlier for the weight of the mixture used. Record the final mass in grams.

Note:  Don’t leave the stirring rod in the beaker between stirs. It will get very hot.

During Step 7 we will use the time we are waiting for the solution to cool to set up the salting out experiment (Session I-4) and to graph the results you obtained.

Step 14. We will replace the hydrochloric acid with acetic acid, which also bubbles when baking soda is present.

Be sure to leave room in your notebook to record the temperature, amounts of materials used, observations, etc.

Session I-4:  Salting Out

Again, write out his instructions on page 40. We will be using food coloring, because otherwise the two layers might be hard to tell apart.

Conclusions:

Once you have completed both sessions, sit down and write a sentence or two to explain the results.

Discussion:

Record any thoughts you have about the experiments, including:

  • Possible improvements to the procedures or how to tweak techniques
  • How the results differed from your expectations
  • Whether the goals were met
  • Suggestions for other experiments
  • The answers to the review questions he provides.

If there is time, we’ll go over the review questions together at the end of lab.

Please leave a comment or send an e-mail if you have any questions before our meeting.

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