The Work of Priestley

Joseph Priestley was born in Yorkshire in 1733 and became a Nonconformist minister as his family wished. He was very interested in chemistry and became an expert in preparing and handling gases, being one of the first to use a pneumatic trough, a special piece of apparatus, for gas collection. There was no Bunsen burner in those days, Robert Bunsen was not born until 1811 and did not introduce his burner until 1855, so it was not an easy matter to heat sub­stances without destroying them. Priestley was presented with a large lens, one foot in diameter by some admirers in 1774 and he used it to direct the sunlight on to substances and so raise their tempera­ture. One of these substances was the red calx of mercury, now called mercuric oxide. He found that when this substance was heated, it gave off a colourless gas which he collected and he called this gas "dephlogisticated air", we now call it oxygen.

Red calx of mercury — Mercury + Oxygen

Priestley found this gas very interesting as substances burned better in it than they did in air, for instance a candle burned much more brightly in the gas. In 1775 he visited Paris and met Lavoisier, the leading French chemist of that day, who found Priestley's work very stimulating and himself carried out what has now become a classical experiment.

The Liebig Condenser

This consists of a glass tube surrounded by an outer jacket through which water can be passed. As the water vapour enters the glass tube it is cooled by the outer water jacket and is changed back to water. The water is thus condensed and can be collected.

Oxygen - Breathing Stimulant

Oxygen is used as a breathing stimulant, that is it helps people who are having difficulty with breathing to breathe more naturally. It is used in hospitals for patients suffering from lung disorders or from paralysed muscles. The patients are put in oxygen tents. It is also used in mine rescue work, where there may be pockets of poi­sonous gas, and at high altitudes where there is a lack of air.

Lime-water Test

Collect two jars of oxygen. Burn a small piece of carbon in one and a small piece of calcium in the other. The carbon will produce a colourless gas, carbon dioxide, and the calcium will burn to a white powder, calcium oxide. Dissolve this in water - it is not very soluble, and after shaking for some time you may have to filter some of the solid off as you want to use the clear calcium hydro­xide solution which we call limewater. Pour this clear solution into the jar of carbon dioxide and shake up. Note what happens - you ought to know what to expect. The milky appearance is due to the formation of insoluble calcium carbonate. This reaction is used to test for carbon dioxide.

Industrial Production of Oxygen

Oxygen is so important in many ways that it is made in large quantities in industry. Manufacturers have an abundant free supply of it in the air, and it is obtained by cooling the air until it becomes a liquid (just as steam condenses to water). This liquid air contains both liquid oxygen and liquid nitrogen, and when the temperature of the mixture is allowed gradually to rise it boils and the oxygen and nitrogen change back to gases. They do this, however, at different temperatures and can therefore be collected separately. Oxygen is stored under considerable pressure in steel cylinders painted black. Its release from the cylinder is controlled by a valve, and the gas can easily be obtained whenever it is required.