From: Nina Vehslage (firstname.lastname@example.org)
Date: Thu Feb 13 2003 - 13:58:47 EST
Hello from Brownstown,
I too find the lightbulb demonstration a very effective way to let students 'see' what is in the beaker and put a picture in their mind about what is happening. I carry this demo one step further:
Dissolve barium hydroxide in distilled water and test it. Put sulfuric acid in a beaker and test it. You can put the sulfuric acid in a titration buret if you wish before adding it slowly to the barium hydroxide beaker. Now put the barium hydroxide on a magnetic stirrer and slowly add the sulfuric acid. I of course ask the students what do they think will happen when we mix these two just proven electrolytes together. What happens is the very bright light bulb gets dimmer and dimmer as you reach the neutralization point with the acid titration. Then the light bulb goes out. Ask them why they think this happened? What must be present in the beaker for the light bulb to stay on? Then go past the equivalence point by adding more sulfuric acid and the light bulb lights back up. Ask them why they think this happened. The sulfuric acid is .1 to .5 molar and you put 5 g of barium hydroxide crystals in about 100 ml of water.
Sometimes I add a little phenolphthalein so they can see the solution go from pink to white. It is white not clear due to the formation of baium sulfate. Take it off the magnetic stirrer and the barium sulfate quickly precipitates to the bottom of the beaker so they can 'see' how the ions made an insoluble compound.
I will definitely use the string marked off in powers of 10. Quite impressive.