急！！化学疑问！！幫忙吧

when nitric acid react with sodium carbonate, sodium nitrate,
water and carbon dioxide are formed. since carbon dioxide is
a gas, it will come out of the solution and will not contaminate
the solution. but if you use sodium chloride instead. the sodium
nitrate formed will contaminate with chloride ion. it is not
good. so we do not use it.