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A Solution to Your Mix-up (1 Favorite)

LAB in Separating Mixtures, Mixtures, Solubility, Phase Changes, Physical Change. Last updated November 5, 2018.


Summary

In this lab, students will perform a step by step process of mixing and separating substances based on their states and solubility in order to solve a problem.

Grade Level

Middle school

Objectives

By the end of this lab, students should be able to

  • understand the terms: mixture, solution and miscible.
  • know the difference between chemical and physical changes.
  • understand states of matter and changes of state.
  • make a prediction based on experiment observations.
  • describe their thought processes for problem solving a real world problem.

Chemistry Topics

  • Solutions
  • Mixtures
  • Separation techniques
  • States of matter

Time

Teacher Preparation: 30 minutes

Lesson: 1 class period -45 minutes

Materials

Per lab group:

  • 30 ml tap H2O premeasured in flask, corked
  • 30 ml 91% isopropyl alcohol premeasured in flask, corked
  • 5 grams NaCl
  • (4) 250 ml beakers
  • Glass stirring rod
  • 5 glass marbles

Safety

  • Always wear safety goggles when handling chemicals in the lab.
  • A lab coat or lab apron should be worn by students.
  • Students should wash their hands thoroughly before leaving the lab.
  • When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.
  • Alcohol is highly flammable, keep away from heat source and open flames.

Teacher Notes

  • Teachers should premeasured flasks of 30ml of tap water and 30ml of 91% isopropyl alcohol for each of the lab stations. These flasks should be corked with a rubber stopper.
  • The teams have at least two ways to explain the missing solvent:
  1. Usually the teams deduce that the technician forgot to return the cork to the bottle and the high temperatures in the lab allowed the alcohol to evaporate into the air (they should note this is a safety/ flammability issue). We discuss the evaporation / boiling point of rubbing alcohol (this does not allow the material to be reclaimed).
  2. After the initial evaporation explanation, I try to push them into considering an error that would still provide the opportunity for the next day’s shift to reclaim the alcohol. That would assume that the earlier shift forgot to decant the isopropyl out of the marble cleaning beaker and it therefore became a colorless solution (miscible) along with the saltwater. They would then have to figure out a way for the materials to be separated. (Please note, they are aware of distillation and vaporization temps, but we do not use flames in the lab with alcohol. They are not familiar with the miscible solutions with water.) It then gives the instructor the opportunity to add 1 or 2 drops of green food coloring to the solution and show them the salting out process. After mixing, the pigments in the food coloring will separate out between the two once clear solvents and we are left with a density column of green saltwater on the bottom and light green isopropyl alcohol on the top. Voila!

For the Student

Lesson

Background

A chem-maniac sleuth is needed to decipher a problem. Our sleuth needs to perform a series of tasks in the proper order to find the solution to a mix-up at a local manufacturer. Failure to complete the tasks in the proper order will result in the unleashing of the noodle of great moisture and wrath upon the town of Chemville. Are you up for the challenge?

Acme Chemical Company had a mix-up on one of its saline solution production lines resulting in a large quantity of isopropyl alcohol coming up missing. You are needed to find a solution to their dilemma. You will be given all of the components of their product line and their procedure for mixing saline solutions. You and your lab partner have the task of finding a solution to their mix-up.

You and your lab partner will be given an opportunity to retrace the technician’s steps to describe how the solvent came up missing. You will deduce how the procedure can be changed so that it doesn’t happen again. You may also wish to describe how you would recover the lost rubbing alcohol if at all possible to save ACME money.

Objective

Please deduce what has happened to the missing isopropyl alcohol by retracing the technician’s steps.

Materials

  • 30ml tap H2O
  • 30ml 91% isopropyl alcohol
  • 5 grams NaCl
  • 4, 250ml beakers
  • Glass stirring rod
  • 5 glass marbles

Safety

  • Always wear safety goggles when handling chemicals in the lab.
  • A lab coat or lab apron should be worn.
  • Wash your hands thoroughly before leaving the lab.
  • Follow the teacher’s instructions for clean-up of materials and dispose of chemicals.
  • Alcohol is highly flammable, keep away from heat source and open flames.

Procedure

  1. Following the outline steps to retrace the actions of the technician in order to find a solution to their mix-up. Here is what they do. You and your lab partner have the task of finding a solution to their mix-up by retracing their steps.
  2. The technician starts with putting on his/her protective wear, which includes goggles, a lab coat (even though it is 350 C in the shop!) and their hard hat.
  3. The marbles used to mix the saline solution must first be cleaned. Put the marbles in a beaker and uncork the flask containing pre-measured isopropyl alcohol. Add all of the 30ml of isopropyl alcohol to the marbles. Swirl the isopropyl alcohol to clean the marbles and with the help of the glass stirring rod, decant off the solvent and return it to the flask and cork it.
  4. Now pour in the pre-measured 30ml of tap water onto the marbles in the beaker and stir with glass rod.
  5. Continue stirring while you add 5 gram of NaCl. Stir until you have the finished clear saline solution. The marbles should be reclaimed by decanting the saltwater out into the final beaker. This final beaker is the finished saline solution product.
  6. Separate the marbles into their original beaker and set aside for cleaning by the next day’s shift.

Analysis

When the technician on the assembly line for the next day comes in they find that:

  • the saline solution is in its proper beaker (but too full)
  • the marbles are in the beaker ready for cleaning
  • the isopropyl alcohol is gone
  1. Where is the isopropyl alcohol? Can we get it back?
  2. After retracing the technician’s steps and following the procedure, please explain your findings in a complete report to the plant manager. Don’t forget to use full sentences and reference your work.
  3. Do you need any more information to complete this assignment?

Stay tuned for further instructions from the plant manager.