Physical Properties of Ozone
- Ozone absorbs radiation strongly in the ultraviolet region of the atmospheric spectrum between 220-290 nm. This protects the Earth and its inhabitants from the harmful ultraviolet radiation of the Sun. Without this protective layer, more ultraviolet radiation would reach the surface of the Earth and cause damage to plant, animal and human life.
- Pure ozone is a blue gas, with a strong irritating smell. When inhaled, it causes headache and nausea. In smaller proportions it smells pleasant.
- It is about 1.5 times heavier than air and has a vapor density of 24, corresponding to the formula O3.
- It is more soluble than oxygen in water, about 49% by volume at 0°C.
- It gets liquefied to a deep blue colour liquid, when cooled in liquid air.
- It boils at 161.2 K and solidifies to violet-black crystals, which melt at 80.6 K.
- It dissolves readily in turpentine oil and acetic acid.
Chemical Properties of Ozone
Ozone is an unstable compound. Pure ozone decomposes explosively,while ozonised oxygen decomposes slowly at room temperature. Decomposition is instantaneous at about 573 K.
The decomposition is accelerated by the presence of manganese dioxide, platinum black and copper oxide etc.
Ozone acts as a powerful oxidizing agent due to the reaction,
The nascent oxygen formed due to its decomposition is responsible for the oxidation of a number of substances. Typical oxidation reactions are given below:
It oxidises lead sulphide to lead sulphate
It liberates iodine from a solution of potassium iodide
Halogen acids are oxidized to corresponding halogens(e.g. hydrochloric acid is oxidized to chlorine.)
It oxidizes sulphur dioxide to sulphur trioxide
It oxidizes moist iodine to iodic acid
It oxidizes potassium ferrocyanide solution to potassium ferricyanide
It oxidizes acidified stannous chloride to stannic chloride
Silver metal when warmed with ozone gets blackened due to reduction of the oxide formed in the initial stages of the reaction.
When ozone is passed through mercury, it loses its meniscus and sticks to the glass due to the formation of mercurous oxide. This is called tailing of mercury. The meniscus can be restored by shaking it with water.
Ozone acts as a good bleaching agent for vegetable coloring matter (due to its oxidizing nature)
Ozone reduces peroxides to oxides and in turn gets reduced to oxygen. For example, with H2O2 and BaO2, it gives H2O and BaO respectively.
Ozonides are addition products, which are formed when unsaturated organic compounds containing double bond react with ozone.
These ozonides are decomposed by water or dilute acids giving aldehydes and hydrogen peroxide in most of the cases.
The position of the double bond can be located in the original unsaturated molecule by this reaction. This reaction is termed 'ozonolysis'.
Uses of ozone
Ozone is used
- For air purification at the crowded places like cinema halls and tunnel railways. Due to its strong oxidizing power it also destroys the foul smell in slaughter houses.
- In sterilizing drinking water by oxidizing all germs and bacteria.
- For preservation of meat in cold storages.
- For bleaching delicate fabrics such as silk, ivory, oils, starch and wax.
- It helps to locate a double bond in any unsaturated organic compound by ozonolysis.
Tests of ozone
Either of these three tests help in identifiying ozone.
- With starch iodide (KI + starch solution), ozone oxidizes KI to iodine, which gives ozone a blue color.
- Tailing of mercury test.
- It turns benzidine paper brown.