Demonstration 2

Liberating Hydrogen

    Early natural philosophers, alchemists, and chemists were aware of the existence of hydrogen gas long before they understood its properties. Paracelsus (1493-1541) noted that an "air" or "spirit" was formed when sulfuric acid reacted with iron. Van Helmont (1579-1644) who coined the term "gas" (from the Greek word chaos) worked with an impure form of hydrogen, which he called gas pingue due to its flammable nature. Robert Boyle (1627-1691) who formulated Boyle's Law of the inverse proportion of gas volume to pressure, also worked with hydrogen. But it was Henry Cavendish (1731-1810) who first systematically studied this gas, and is credited with its discovery in 1766. Cavendish discovered that hydrogen is lighter than air, and his experiments with its flammability and explosiveness led him to equate it with "phlogiston," the theoretical substance which was invoked by chemists at that time to explain combustion. Most importantly, Cavendish found that water was produced when hydrogen burns in oxygen. Later studies of this phenomenon, which paid special attention to the combining proportions of the two gasses, provided important evidence for Dalton's atomic theory.
    Hydrogen is an easy gas to generate. It is commonly prepared in laboratories by the action of dilute sulfuric or hydrochloric acid on zinc. Our demonstration produces hydrogen by reacting sodium hydroxide with aluminum foil.

You will need: One 125ml flask, a rubber stopper with one hole, a one-and-a-half foot length of plastic aquarium tubing, a paper cup, a plastic teaspoon, a long candle, a cigarette lighter, a can of household lye, a sheet of aluminum foil, a bottle of dishwashing liquid, and a pair of scissors.

CAUTION! Hydrogen gas is extremely explosive, and lye (sodium hydroxide) is very corrosive and causes severe burns. Wear rubber gloves and goggles! Follow directions carefully! If any lye gets on you accidentally, thoroughly rinse it off with generous quantities of lukewarm running water. If any gets in your eyes, thoroughly rinse with water and seek medical help immediately!

Procedure: First, light a long candle and place it across the room from the area where you are going to conduct this experiment. Then fill paper cup with warm water and add a generous amount of dishwashing liquid. Pour 50ml of fresh water into the flask and add about 5gr (two teaspoons) of lye (sodium hydroxide.) Swirl the liquid in the flask until the sodium hydroxide dissolves. Next, cut out four 3"X 3" squares of aluminum foil, crush them into balls, and drop them into the flask with the sodium hydroxide solution. Stopper the flask with the rubber stopper, and run the length of plastic aquarium tubing from the hole in the stopper into the paper cup of soapy water. Wait a few minutes for the reaction to start. It will speed up as it progresses, and you will notice the flask becoming hot. Allow a generous amount of bubbles to collect in the paper cup, and then scoop some out with the plastic spoon. Carefully hold the spoon and slowly move across the room to the lit candle. While holding the spoon at arms length, allow the flame to touch the bubbles. They should explode with a loud pop and a flash. If no explosion happens on the first attempt, return to the flask with the aluminum foil and the paper cup. By now, the reaction should be proceeding rapidly, and all the air in the flask should be completely displaced by pure hydrogen. Scoop up some more bubbles, and try again. By this point, you should get a loud bang when the hydrogen gas contained by the bubbles explodes.

You can demonstrate that hydrogen gas is lighter than air by pulling the aquarium tubing out of the soapy water and letting a bubble form at its end. Gently blow the bubble off the tip of the tubing and you will see it float up to the ceiling. (You may have to repeat this procedure several times before it works. Sometimes the bubbles contain too much water and they sink instead of float.)

Disposal Instructions: When the reaction in the flask finally stops, allow the flask to cool, and then dump the solution into the toilet and flush. (The solution should be black by the time the reactions ceases.) Rinse the flask out with luke warm water in the sink, and wash all remaining reacted products down the drain. Keep running water in your sink until any remaining lye is completely rinsed away.

If you carefully follow these instructions, you should be able to generate hydrogen gas easily. Collecting it in bubbles actually decreases the risks involved when you detonate it. You won't have to worry about glass shrapnel flying around when the hydrogen explodes. But please be careful. Don't let an open flame or a spark get anywhere near the generator flask or the aquarium tubing, as a serious explosion could result. I know because it happened to me!