Ask A Scientist Chemistry Archive

Question:
My mom always cleans her windows with a solution of mostly
water, and equal parts ammonia and vinegar.  Since both
are of equal concnecentration and of equal strengths, I
claim she is just making more water.  Yet she swears
by this method.. So what actually does the cleaning?
 jon rienstra

Answer:
I think you must be thinking "OK, ammonia is a basic solution,
and vinegar is acidic. So if I mix equal volumes of equal
concentrations of acid and base together, I'll just have
water because the acid neutralizes the base." Eh?
But this is incorrect for several reasons. (1) to neutralize
a strong acid with a strong base, you must combine equal numbers
of moles of acid and moles of base. Thus, just because you use
equal volumes of the two substances doesn't mean that you'll end
up at pH=7; they could be in different concentrations in the
two solutions.  (2) Acetic acid is a weak acid and ammonia is
a weak base. Therefore, mixing together equal amounts of these
two substances will result in a solution with a pH which is
not 7, i.e., you won't have a neutral solution.  (3) When
you neutralize an acid with a base, one does not only end up
with water. Consider the reaction of HCL with NaOH; you get
H2O and NaCl(aq), which is in the form of Na+(aq) and Cl-(aq).
In your mom's economical and effective homemade window cleaner,
there's other stuff floating around than protons (H3O+) or
hydroxide (OH-).  There's going to be a mixture of acetic acid and
acetate (both from the vinegar) as well as other things, and
NH3 and NH4+ (from the ammonia).  What you've basically got is
a buffer solution with a lot of ions floating around (which will help
dissolve charged dust particles) and organic compounds (which will
help dissolve organic muck).  OK? Hope this helps - topper