Re: Why does fizzing occur when you mix vinegar with hot water?

When I added vinegar to very hot water, I did not get any fizzing.

Vinegar is a weak solution of acetic acid in water. Acetic acid has a 
boiling point higher than water, so vinegar actually has a higher boiling 
point than pure water. So it is not vinegar boiling off that you saw.

You mentioned that you were preparing a cleaning solution. If you were 
using a bucket you had previously used with other cleaning products, it is 
likely that some carbonates were left behind from the previous use. Washing 
soda (sodium carbonate) is a common ingredient in cleaning products. Any 
carbonate or bicarbonate will produce carbon dioxide and fizz when vinegar 
(or any other acid) is added.

So I’m guessing that the fizz you saw was vinegar reacting with small 
amounts of impurities left behind by previous cleaning products.