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Demonstrations and Experiments

Oxygen

 

    Joseph Priestley is usually credited with the discovery of oxygen on August 1'st, 1774, but several other chemists and experimenters were working with the gas at approximately the same time. These early experiments with oxygen, an element that we take so much for granted, were quite revolutionary at the end of the 18'th century. Through carefully studying how substances reacted with Priestley's newly discovered gas when they were burned or "calcined," Antoine Lavoisier was able to determine that "oxygen" (as he came to call it) was the gas that supported all combustion. This completely overturned the old phlogiston theory and helped lay the foundation for modern chemistry. Soon after, chemists discovered that water could be produced by burning hydrogen in oxygen. This discovery, along with several other important experiments with oxygen, hydrogen, and water, eventually helped disprove the ancient Aristotelian assumption that water was an element. Through further observations of the combining weights and volumes of these gasses, evidence was gathered that later helped support Dalton's theory of atoms, one of the crowning achievements of modern science.
    Priestley produced oxygen by sealing mercuric oxide in an evacuated flask and then heating it with a magnifying glass. You can generate this historically significant gas much easier.
 

You will need: two 125ml glass Erlynmeyr flasks, a bowl filled with sand, an eye dropper, some wooden toothpicks, some matches or a cigarette lighter, a candle, a bottle of Clorox, and a bottle of hydrogen peroxide (3%.)

Procedure: Light the candle with matches or cigarette lighter and place it in the bowl of sand. Pour about 50ml of Clorox in one of the flasks, and pour 75ml of hydrogen peroxide into the other. Fill the eyedropper with hydrogen peroxide and add it to the Clorox in the flask. The mixture will hiss and bubble vigorously! Repeat this two or three more times. After you add the final portion of hydrogen peroxide to the Clorox, quickly hold your hand over the mouth of the flask. Light a wooden toothpick with your free hand (use the burning candle,) let it burn for a moment, and then blow it out, making sure that it is still glowing. Remove your hand from the mouth of the beaker and quickly insert the still glowing toothpick into the beaker. It will burst into flames! After blowing out the toothpick, you may reinsert it several more times into the flask, and it will continue to burst into flames as long as there is enough pure oxygen in the flask to speed its combustion.

Explanation: Clorox (sodium hypochlorite) and hydrogen peroxide both contain an extra atom of oxygen. (See chemical equation below.) This atom of oxygen is not bound very strongly to its parent molecule, and can be removed very easily. When sodium hypochlorite and oxygen are combined, they each loose their extra atom of oxygen. These liberated oxygen atoms quickly combine to form diatomic oxygen molecules, which leave the reaction as a gas and greatly accelerate the combustion of the toothpick. The products left behind in the flask are sodium chloride (common table salt) and water, as indicated by the equation below.


 

The formula reads: One molecule of sodium hypochlorite plus one molecule of hydrogen peroxide produces one molecule of sodium chloride plus one molecule of water plus one molecule of diatomic oxygen (i.e. two bonded oxygen atoms.)

Photo shows my own laboratory equipment. W. M. M.