Joseph Priestley And His Discovery of Oxygen

Hello, and welcome to our chemistry project. In this video, we will explore Joseph Priestly's discovery of oxygen. We will tell you about the chemistry when he lived, and how he changed it all. We will explore him, his life, his ideas. And his discovery. We will reveal his lead up, and his previous inventions, including the carbonated drink. We will do our own demonstration of what he discovered, and how he realised its significance, substituting some of the materials, before discussing how, and why, it was such an important breakthrough, and how he changed the world he lived in. And finally, how this set a new basis for chemistry. Thank you for watching, and enjoy! In the Middle Ages they had just started to work out the key building blocks of the world but were having trouble pinning down how processes such as combustion and respiration worked, and why they only worked in some places with some materials. The most common theory was that there were four elements; fire, air, earth and water. All liquids were made of water, all solids were made of earth and all gases were made of air. Fire was the element that governed heat and combustion. However, there were clearly many flaws with this theory; fire is not an element, air is a mixture, water is a compound and earth is another mixture. Johann Joachim Becher was a German scientist who, in 1667, first proposed a theory to solve this problem. He postulated the existence of a fire-like element called "phlogiston", which was contained within combustible bodies and released during combustion. "Phlogisticated" substances are those that contain phlogiston and are "dephlogisticated" when burned; "in general, substances that burned in air were said to be rich in phlogiston; the fact that combustion soon stopped in an enclosed space was taken as clear evidence that air had the capacity to absorb only a finite amount of phlogiston. When air had become completely phlogisticated it would no longer serve to support combustion of any material, nor would it support life, because the role of respiration was to remove phlogiston from the body. Becher had originally named this substances 'terra pinguis', but in 1703 Georg Ernst Stahl, German professor of medicine and chemistry, proposed a variant of the theory in which he renamed Becher's terra pinguis to phlogiston, and it was in this form that this incorrect theory probably had its greatest influence. In 1733, a young man called Joseph Priestly was born. He was the eldest child of six, born to a poor family near Leeds. When he was very young, he was sent to live with his childless aunt Sarah. This was important, as their beliefs were not like that of the church. Because he was not well associated with the church, many non-Christian people met there and had theological and political discussions. It was here that he was introduced to science. He went to a well-established grammar school at the age of 12, and learned mathematics and science, as well as many languages; he grew up to become a prosperous chemist. In 1765, he wrote two academic books, and received an Honor from Edinborough University for his contributions at Warrington. The next year, while on a trip he met Benjamin Franklin, one of the most important scientists in his day. He was the man who got Priestley interested in chemistry. After that, he started reading books on electricity, and conducting related experiments. Because of these experiments, he was elected to become a member of the Royal Society. His first major breakthrough was that he discovered that graphite conducted electricity. This is a great achievement because he was not very trained in scientific research. Priestley lived next to a brewery, and in 1770 became interested in the air that floated over fermenting grains. He did not know that, because of this study of gases, he would become one of the greatest chemists of all time. Priestly published his experiments a lot. By investigating this gas, he was discovering carbon dioxide, which he named 'fixed air'. This is the air that we breathe out, and he showed that it could also put out flames. He realised that when he mixed this with water, it had a pleasant and tangy taste; he had invented carbonated water, or soda pop. He sold this water to the public. For this discovery, he was awarded a medal by the Royal Society in 1773. In the same year he made another important discovery. He placed a shoot of green plant in a container of water. He then covered the container and lit a candle in it until it completely burned out. Earlier, many had found that when a lighted candle is extinguished in air, the same air cannot support a flame any more: the air was phlogisticated. But Priestly was able to burn the candle again and keep a mouse alive in the air in the presence if a green plant. He became the first ever to observe the respiration of plants - the fact that they take in carbon dioxide and produce oxygen. Priestly devised a new apparatus, the pneumatic trough, that allowed him to collect gases over mercury. No gases are soluble in mercury, so it had major advantages over water. However, it was 13.6 times heavier than water, and so it was difficult to pass a small amount of gas through it. He solved this problem in an innovative way; instead of passing the gas through the mercury, he generated it over mercury. He floated various materials over mercury that would decompose and generate a gas upon being heated, and sealed a glass vessel over the top. He heated them with a magnifying glass by focusing the sun's rays on them. The whole setup was a masterpiece of genius; had he not done this over mercury, he could not have discovered ammonia and hydrochloric acid, because these gases are highly soluble in water. In 1774 Priestly put a piece of mercuric oxide into the test chamber, and isolated a new gas. He noted that a glowing splint relights when inserted into this gas. He then put a mouse in a jar of it, expecting it to die soon. Unexpectedly, it prospered and appeared exceptionally healthy. He then tried breathing the gas himself, and noted that it felt light and easy to breathe. He even said he thought that it could become a luxury form of air, available for the rich to breathe only. Little did he know that he had discovered oxygen. Now we are going to give you a live demonstration of Joseph Priestly's famous experiment. Instead of using mercuric oxide we will use potassium chlorate with a catalyst of manganese dioxide, and rather than floating it on mercury we will heat it separately and let the produced oxygen rise through water into a gas jar. First of all we will turn the Bunsen Burner to a roaring blue flame. This will gradually heat the potassium chlorate until it decomposes into potassium chloride and oxygen. The oxygen will then travel through the tubes and into the water bowl, where it will rise, filling the gas jar. Because the water is being pushed down, the water level will rise in the bowl. Oxygen is what we breathe in, and is required by all animals. Respiration is transforming oxygen and glucose into carbon dioxide, water and energy. Oxygen is required in combustion; when you remove oxygen from a fire by smothering it, it goes out. The more there is, the stronger and more uncontrollable the fire, and so oxygen can be regarded as extremely flammable. When you blow out a splint, the end remains glowing, but if you insert this glowing (very hot) splint into pure oxygen, it instantaneously relights. Here we show you a short contest between us to see who can relight a splint the most times in a jar of oxygen. By indirectly disproving phlogiston, Joseph Priestly freed chemistry from its clutches. Before his discovery, all scientific discovery in this field was related to the phlogiston theory, and in this way research could not progress in most matters of chemistry. With this discovery, the frozen world of chemistry was set in motion again. However, it was not Priestly himself who actually disproved phlogiston - he made the discovery that could have disproved it, but, like many other scientists in the day, fell into the trap of phlogiston, reverently kept with the incorrect theory until he died. Priestley met with French scientist Antoine Lavoisier, born in 1743, in 1774, and told him all about his experiment with mercuric oxide. Lavoisier, intrigued, went back and conducted exactly the same experiment in reverse. Lavoisier carefully weighed some mercury, before strongly heating it until it had all turned into mercuric oxide. He then weighed it again, and found that the mass had increased by a substantial amount; it had taken something in. He heated it again until it decomposed into mercury, and found that the mass had gone down to the exact original amount. This substance, oxygen, was being taken in to mercuric oxide, and released from it to turn into mercury. he thus disproved the substance known as phlogiston. And therefore it was he, and not Priestley, that went down in history as 'The Father of Modern Chemistry. Priestly refused to acknowledge Lavoisier's discovery, and died naturally on the 6th of February 1804. We have talked about Priestley, his discovery and his legacy. We now know that, although Priestly set the basis for the demise of phlogiston, it was Antoine Lavoisier that famously disproved the hugely popular explanation. Without Priestly, however, none of that would have been possible, and we should thank him for this, and how far we have got with chemistry to the day, as well as for the fizzy drink and the pencil rubber. And so we bring this project to an end, reflecting on Priestly and Lavoisier's marvellous conquer of phlogiston. Thank you for watching, and goodbye!