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Natural Science - Year II

Unit 38: Lab - Precipitation and acid-base reactions

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Lab Assignment for Unit 38


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Unit 38 Laboratory Activity: Precipitation Reactions

Goal: To observe precipitation and solubility reactions

This set of experiments will take several days, so read the whole set of instructions through before you start and plan how you will do each step! This is based on a series of reactions described in Barron's Science Wizardry for Kids by Margaret Kenda (ISBN: 0812083644). You can stop at any point in the series, but you must do them in order to do the later ones.

Equipment and Materials:

 

Procedure

Do as many of the following reactions as possible. Notice that there is a sequence to several of the reactions.

SAFETY NOTES:When using an eyedropper or pipet for multiple solutions, be sure to rinse it between solutions.

If you are drawing your chemicals from household supplies, always pour a small amount of each into a separate container before transferring it with eyedroppers or spatulas to your experiment vessels. Never put any chemical equipment (pipets, spatulas, scoops) into a food source!

  1. Isolate a small amount of steel wool and set it aside for later comparison.
  2. Wet the rest of the steel wool and push it into the bottom of an Erlenmeyer flask (or jar or beaker). Try to get enough into the flask that it won't fall out when you invert the flask.
  3. Place a measured amount of (distilled) water in a beaker. Record the amount of water!
  4. Invert flask and place it in the beaker, so that the mouth of the flask is completely submerged in the water, but the wet steel wool is above the water level. Observe and record the water level INSIDE the flask and outside the flask in the beaker.
  5. Put the whole assembly in a place where it won't be disturbed for several days: 48-72 hours. If the water evaporates in the beaker, add more, but try not to disturb the amount of water in the flask, if any.
  6. Record your observations:
    1. Have the water levels inside or outside the flask changed?
    2. What has happened to the steel wool?

      You should have red deposits or rust (ferric oxide Fe2O3) on the wool. If none have appeared yet, but the assembly back together and leave it for another 24-48 hours.

    For the remaining steps, be sure you record at each critical point **
    • The amounts of water and each chemical that you used
    • The characteristics (clear, colored?) of the solution before and after you add another chemical to it
    • Whether addition of a chemical produces a precipitate.
     

  7. Remove the steel wool from the flask and store a small amount, as you did with your dry sample (but not together: we don't want them both to rust!).
  8. Submerge the rest of the steel wool in enough white vinegar to cover it, and bring it to a boil (you can do this on the stove or in a beaker over your alcohol lamp). Let it boil for 5 minutes, and cool. ** This is ferrous acetate, Fe2(CH2COOH)2.
  9. Pour 1/3 of the solution into a jar, label and store it.
  10. Filter the rest into another jar. Add a few (3-5) drops of hydrogen peroxide, H2O2, to the filtered solution with an eyedropper or pipet. ** The hydrogen peroxide reacts with the ferrous acetate to form ferric acetate, Fe(CH2COOH)2. Write the formula for this reaction, and determine, if it is a redox reaction, which reactant is reduced and which is oxidized.
  11. This is your store of ferric acetate for the rest of the experiment set. Store it in a labeled jar if you need to do the experiment over several days.
  12. Make some black tea (not herbal). Place a few drops (2ml) of ferric acetate on a plate or in a test tube. Add an equal amount of brewed black tea. ** You have just made a solution of ferric tannate, from the tannic acid present in black tea.
  13. Make a solution of cream of tartar (1 ml or 1/4 tsp) to water (60ml or 1/2 cup). Add some ferric acetate to your cream of tartar solution. ** The resulting solution is ferrous tartarate.
  14. Put 3-5 drops of ferrous acetate on a plate or in a test tube. Add an equal amount of ammonia. ** This is ferrous hydroxide.
  15. Add an equal amount of hydrogen peroxide. ** This is ferric hydroxide.

Report:

For your lab report, make a chart showing the compounds of iron which you created, the amounts of reactants used to create them, and their appearance, including solubility.