Printer Friendly

Activity: Hoffman Apparatus

NGSS Science and Engineering Practices:

NGSS Crosscutting Concepts:

NGSS Disciplinary Core Ideas:

Materials

  • Hoffman apparatus (Fig. 1.11)
  • 9-V battery
  • Two alligator clips
  • Distilled water
  • Beaker
  • Baking soda
  • Stirring rod
  • Safety goggles
  • Two glass test tubes
  • Rubber tubing ~10 cm long
  • Wooden splints
  • Candle
  • Matches
  • Table 1.6

Procedure

Safety Note: This activity involves performing a test using a burning splint. It should only be performed as a demonstration for students or under very careful instructor supervision. When performing the splint test, wear goggles. Use other appropriate safety precautions and be careful.
 

  1. Make a saturated baking soda solution.
    1. Measure approximately 100 mL of distilled water.
    2. Stir baking soda into the distilled water, until no more baking sodawill dissolve.
       
  2. Set up the Hoffman apparatus, making sure all connections are hand-tightened.
     
  3. Open the stopcocks and pour baking soda solution into the top of the thistle tube. Pour slowly enough that the solution has time to flow into the U-tube and adjoining cylinders. When the apparatus is full, close the stopcocks.
     
  4. Clip the alligator clips to the electrodes and then to the battery.
     
  5. Allow the apparatus to run until at least 10 mL of gas are produced on each side of the U-tube. The time it takes will vary based on the apparatus and the battery.
     
  6. Unclip the alligator clips from one electrode to break the circuit.
     
  7. Record the volume of gas in each side of the U-tube. Predict which gases are formed in each side of the U-tube.
     
  8. Connect the rubber tubing to the stopcock on the side of the U-tube with the lower volume of gas.
     
  9. Light the candle.
     
  10. Perform a splint test on the collected gas from the side of the U-tube with the lower volume of gas. Two people should work together to perform these steps.
    1. Place the end of the rubber tubing into the mouth of an upright test tube.
    2. Open the stopcock to release gas into the test tube.
    3. Quickly close the stopcock, while at the same time carefully removing the rubber tubing and putting your thumb over the test tube to prevent the gas from escaping.
    4. Light a splint on fire using the candle.
    5. Blow out the splint so it is smoldering (glowing).
    6. Place the smoldering splint into the test tube.
    7. Record your observations.
       
  11. Perform a splint test on the collected gas from the side of the U-tube with the greater volume of gas. Two people should work together to perform these steps.
    1. Turn a test tube upside down over the stopcock.
    2. Open the stopcock to release gas into the test tube.
    3. Quickly close the stopcock, while at the same time putting your thumb over the test tube to prevent the gas from escaping.
    4. Light a splint on fire using the candle.
    5. Place the burning split into the test tube.
    6. Record your observations.
       
  12. Repeat steps 10 and 11 if there is enough gas for a second test. The procedure can also be repeated by allowing the Hoffman apparatus to run again.
     
  13. Interpret the results of your splint tests using the information in Table 1.6.

     

Activity Questions: 
  1. What gases are formed by electrolysis of water? Give your evidence.
     
  2. Answer the following questions based on the gas formed at each electrode.
    1. How much gas formed at each electrode?
    2. How do the volumes of the gases compare?
    3. How might you explain any differences?
    4. How does this provide evidence for or against the chemical formula for water?
       
  3. Which gas formed by the electrolysis of water is more dense? What is your evidence?
     
  4. How do the properties of water differ from its individual elements?
     
  5. Is the electrolysis of water a chemical or physical change? What is your evidence?
     
  6. Was there evidence that water formed during the splint tests? If so, under what conditions was it formed?

Table of Contents:

Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.