what are redox reactions well those reactions in which oxidation and reduction occur simultaneously are called redox reactions in our previous video of oxidation and reduction we have learned all about oxidation and reduction and its link is given in the description now i teach my personal trait about oxidation and reduction 100 door here 100 stands for increased oxidation state is oxidation for example consider sodium atom when sodium atom loses one electron it becomes positive ion the oxidation state of sodium atom increases from zero to positive one so we say that sodium is oxidized on the other hand door stands for decreased oxidation state is reduction for example consider chlorine atom when this chlorine atom gains one electron it becomes negative ion the oxidation state of chlorine decreases from zero to negative one so subchlorine is reduced thus using my personal trick of hundred or we can easily spot oxidation and reduction now i will use these four rules to instantly calculate oxidation state and easily recognize redox reaction we have already discussed these four rules in our previous video and its link is given in the description for example consider calcium plus bromine gas react together to form calcium bromide according to this rule we know that in free state oxidation state is zero so oxidation state of calcium is zero and oxidation state of bromine is zero here calcium and bromine combine together to form calcium bromide so i call it combined state we know that in combined state the oxidation state of halogens like bromine is negative one there are two atoms of bromine so negative one plus negative one is equal to negative two thus the oxidation state of bromine is negative two if the oxidation state of bromine is negative two then the oxidation state of calcium is positive too because overall the charge on calcium bromide is zero now listen carefully the oxidation state of calcium increases from zero to positive two so it is oxidized while the oxidation state of bromine decreases from 0 to negative 2. so it is reduced we can see that oxidation and reduction take place in this reaction thus it is a redox reaction therefore remember that a reaction is a redox reaction if oxidation and reduction take place secondly consider carbon plus oxygen gas react together to form carbon dioxide here carbon is infrested and oxygen gas is also in free state so we say that the oxidation state of carbon is zero and the oxidation state of oxygen is also zero while here carbon plus oxygen combine to form carbon dioxide gas so it is a combined state we know that in combined state oxidation state of oxygen is negative two there are two atoms of oxygen so its oxidation state is negative four now this is a neutral molecule therefore the oxidation state of carbon is positive four now listen carefully we can see that the oxidation state of carbon increases from 0 to positive 4 so it is oxidized while the oxidation state of oxygen decreases from 0 to negative 4 so it is reduced therefore oxidation and reduction take place in this reaction so it is a redox reaction just noted down this example finally consider hydrogen gas plus fluorine gas combined together to form hydrogen fluoride now pause the video and try to solve this reaction well hydrogen is in free state fluorine is also in free state so the oxidation state of hydrogen gas is zero and the oxidation state of fluorine gas is also zero while here hydrogen and fluorine is in combined state we know that in combined state the oxidation state of halogen like fluorine is negative one this is a neutral molecule so the oxidation state of hydrogen is positive one we can see that the oxidation state of hydrogen increases from zero to positive one so it is oxidized while the oxidation state of oxygen decreases from zero to negative 1 so it is reduced therefore oxidation and reduction take place in this reaction thus it is a redox reaction thus using this simple trick we can easily recognize any redox reaction