Question

Why can't we change subscripts when we balance chemical equations? I don't understand why we can change coefficients but not subscripts. Thanks.

Answer 1

There is a VERY good reason. BECAUSE NaCl is sodium chloride. Not NaCl2, Na2Cl, NaCl3 or any other arrangement. NaCl is the ONLY one that is a 1:1 ratio. Balancing equations would be SO MUCH easier if we could change subscripts BUT then you have changed the reactant or the product and only NaCl is NaCl.
NaCl + Pb(NO3)2 ==> PbCl2 + NaNO3.
So what you want to do is to rewrite this as
NaCl + PbNO3 ==> PbCl + NaNO3.
Easy, huh? BUT PbNO3 is NOT lead nitrate and PbCl is NOT lead chloride. In fact, PbNO3 and PbCl do not exist.

Answer 2

Regarding changing coefficients, we can change coefficients because we are simply saying we have twice as many or three times as many or any times as many; for example, 2NaCl instead of 1NaCl or we have 3AgCl instead of 1AgCl. Look at it this way. We go into a grocery store to buy jelly + bread + banana. Now we can buy 2 jars of jelly, 3 loaves of bread and 5 bananas and we haven't changed what we went to the store to purchase. BUT, if instead of this I purchase 1 lb of hamburger, a loaf of bread and a banana, I have changed the shopping list and my family will be mad at me, especially if they wanted a jelly/bread/banana sandwich. They aren't likely to like a hamburger, bread, and banana sandwich. In the first case (changing coefficients), my family can make more jelly/bread/banana sandwiches but if I change the shopping list they can't make any sandwiches.