Magnetism is the thread connecting the functioning of a household compass and the auroras observed at poles. One could safely assume that the existence of life on earth is next to impossible if not for the magnetic shield that protects us from the dangerous radiation coming from space. Magnetism is not a new thing; most of us must have tried magnetizing an iron piece in our childhood and would have pondered on why wood or plastics cannot be magnetized. Depending on their magnetic properties materials are classified into paramagnetic, diamagnetic, ferromagnetic etc. Paramagnetic and ferromagnetic materials are attracted towards the magnetic field. An interesting thing is not just solids like iron possess the property to be attracted to magnets but also gases like oxygen show similar signs. But how do we confirm the magnetic property in gases experimentally? Let’s discuss an experiment that confirms paramagnetism in oxygen.
Oxygen gas molecules contain an even number of electrons and generally paramagnetism is associated with the presence of an unpaired electron in the system. To simplify how exactly an unpaired electron affects magnetic property; we know that time varying electric field results in magnetic field and vice versa, a moving electron results in magnetic field as learnt in the topic of Electromagnetism during our Physics tuition classes. Electrons pair up in an atom and the resultant magnetic field is zero, but when an unpaired electron is present this process does not occur and it results in a system that is attracted by the magnetic field. Theoretically, more precisely as per Molecular orbital theory, one unpaired electron is present in oxygen molecule though there are even number of electrons in it. To experimentally verify the same one can think of passing oxygen gas through magnetic field and see if it is deflected. But, because the kinetic energy of the gas molecules is very high, these molecules are generally not affected by the magnetic force acting on them. In a gas, unlike liquids, molecules are separated by large empty spaces and the interaction between the molecules can be safely assumed to be null, due to this the molecules are free to move anywhere within the vessel containing them. Thus the kinetic energy of the molecules in a liquid is much less than that of the molecules (of the same type) present in gases. So, in order to circumvent our problem oxygen needs to be cooled down until it turns into liquid and see if the liquid is affected by magnetic field.
Oxygen gas needs to be cooled down to very low temperatures in order to obtain its liquid form. This is done with the help of liquid nitrogen. A coiled tube is immersed in liquid nitrogen and oxygen gas is pumped into the tube from one end and oxygen liquid can be collected at the other. When liquid oxygen is poured down through magnetic poles, some of it gets collected at the magnet. This is because of the paramagnetic property of oxygen i.e., oxygen is attracted by the magnetic poles. A ferromagnetic substance, like iron, retains magnetic property even after the field is removed. On the contrary, oxygen does not retain this magnetic property once the field is removed and hence it is a paramagnetic substance.