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Chemistry

Separating mixtures, pouring liquids safely, cleaning equipment.

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Laboratory Exercise

Goal: Develop Basic Lab Skills

Parents and students: Please read these instructions and make sure that anyone performing experiments works with adequate supervision and care. I expect everyone to show up next week with all skin and digits intact!

Goal: Develop basic chemistry lab skills.

Equipment: (If you are substituting equipment, please see the chemistry lab list for appropriate substitutes).

Procedure:

Read the safety rules. Be sure that you understand them.

Weighing liquids and solids on your scale.

If necessary, reassemble your equal arm scale. If your scale is strong enough, use it for all of the following exercises. If not, use a postal scale or diet scale for the exercises involving larger masses.

Calibrate your equal arm scale:

Add a nickel (standard 5gm mass!) to each side of the scale.

Joggle one side of the scale slightly, and watch the swing range of your pointer against the grid. How far does it swing to the left? [WRITE IT DOWN!] How far to the right? If the swings are equal, you are well-balanced. If not, adjust your arm slide to make the swings equal. Repeat the test at least twice.

For an adjustable postal or kitchen scale:

Place a known mass on the scale, of sufficient weight to read 20% of the scale range. For example, if you have a 100g scale, put something which is at least 20g (4 nickels would do it) on it.

Joggle the scale, and watch it come back to equilibrium. Does it read correctly? If not, adjust the scale so that it reads properly. Then repeat the test 2 more times.

Be sure to record all your measurements for the exercise below.

 Trial Dry tube/stopper Dry beaker volume of water Mass of Tube + water Mass of tube + beaker  Mass of water
 1            
 2            
 3            

 

  1. Determine the dry weight of one test tube and no-hole stopper.
  2. Determine the dry weight of one small beaker or jar.
  3. Add a measured amount (10 - 20 ml) of water to the test tube, stopper and reweigh.
  4. Add a measured amount (10 - 40 ml) of water to the beaker, and reweigh.

 

Dry all your equipment. Recalibrate your scale and repeat ALL your measurements two more times.

Estimate the amount of error in your measurements and suggest how you might reduce it.

Filtering

  1. Cut circles of filter paper twice the diameter of your beaker or jar.
  2. Fold the filter paper in half, and then in half again, so that you have a quarter circle.
  3. Hold the filter paper curved side up, point side down, and open it so that three sheets of paper are to one side, and one to the other, forming a cone.
  4. Place the filter cone point down in your beaker.
  5. Fill a second beaker 1/4 full of water. Add a tablespoon of salt and stir until the salt is dissovled.
  6. Add a teaspoon of sand or dirt to the salt water and stir.
  7. Pour the water through the filter into the first beaker.
  8. Let the filter assembly sit until all the water has filtered through.
  9. Remove the filter paper, open it carefully so that nothing spills, and let it dry.

If your water looks muddy, filter it again.

Examine your filter paper.

Evaporating

If you have heat-resistant glass, you may want to try evaporating your filtered salt solution.

  1. Set up your heat source (hot plate, alcohol burner) according to the instructions that came with it. If you are using a candle, be sure that it is securely seated in its holder. Be sure to read and follow the safety precautions for working with an open flame on the safety page.
  2. If you use your beaker with an alcohol burner, set up the support stand for it and test its stability with a beaker of cold water before putting the stand over flame.
  3. When you are sure that your heat source is safe, pour plain water into your beaker and set a lid or watch glass on top of it.
  4. Bring the water to a boil, and pour a small amount of liquid into the watch glass. The boiling water below will cause the liquid in the glass to evaporate uniformly, leaving behind your salt deposit.

Why didn't we just boil the salt water in the beaker above the flame?

 

POURING AND DECANTING

 

This technique is commonly used for pouring from large-mouthed vessels into smalle ones. It takes some practice, so be prepared for spills.

  1. Fill a beaker 2/3 full of water.
  2. Hold a glass rod (or plastic straw) at an angle so that it touches the inside of a test tube, but not the lip, as shown.
  3. Pour from the beaker down the rod into the tube.

With practice, you can do this quickly and without spills. So practice!

CLEAN YOUR EQUIPMENT

  1. Wait until any hot glass you have used has cooled off before touching it with your hands.
  2. Use a bottle brush to help remove residue.
  3. Thoroughly rinse all your equipment with water until the water sheets on the class. If the water beads up, your glassware is not clean.
  4. Let it dry before putting it away.
  5. Properly cap and store all fuels or chemicals away from flame and small children.

Lab report: Please send me your table for the mass measurements with your scale, along with your recommendations for how you could improve the accuracy of your mass measurements.

If you are seriously interested in doing much home chemistry, I highly recommend the book Explorations in Chemistry by Charles A Gray.

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