I was reading the answer of the above question from a book. it said that while using DC current, electrolysis occurs. Hence it affects the conductance of the solution. But if any gaseous product is created then polarization happens. Then why is polarization not furthermore discussed. So I request you answer what is it and how it affects while trying to get the conductance ?
When a current is passed through an electrolyte, a chemical reaction takes place. This alters the composition of the solution over time and you won't actually be measuring the conductivity(or conductance) of the initial solution.
Since in AC current, equal current flows in both directions over a given amount of time(larger enough than time period), reaction takes place in both the directions. On an average, no chemical change in composition would be observed.(Of course, that doesn't mean no reaction has taken place.) Therefore the conductivity(or conductance) you measure will be the conductivity you wanted to measure.