What is the relationship between the three items in each of the following groups, and identify each as an element, compound or mixture:

hydrogen, oxygen, and water;
nitrogen, oxygen, and air;
sodium, chlorine, and table salt;
carbon, water, and table salt;

 well hydrogen (element) and oxygen(element) combine together chemically to form water (compound)

nitrogen (element) and oxygen (element) combine chemically to form reddish brown gas nitrogen oxide which is a [pollutant and pollutes air (mixture) 
or 
nitrogen (element ) and oxygen (element) are components of air (mixture) 

sodium (element) and chlorine (element) combine together chemically to form sodium chloride which is table salt (compound) 

if we heat table sugar an organic compound of carbon,hydrogen and oxygen then an oxidation reaction will occur and will result in the formation of carbon (element) as the sugar will get charred and water (compound) as hydrogen will react with oxygen.   Hope this helps!