Re: What is correct mix ratio for baking soda/vinegar?

There is quite a lot to this question, Sarah, and I cannot give you an exact answer. But I can go a 
long way towards it. You will have to do a little bit of experiment and trial and error.

Vinegar is not a pure substance. It is a solution of a chemically active material -- acetic acid -- in 
water. Typically vinegar is about 3 parts acetic acid in 20 parts total (3 parts acetic acid mixed with 
17 parts water). It can vary quite a bit though for different types and different brands of vinegar.

The exact mix ratio for a reaction between baking soda and acetic acid is 5 parts of acetic acid to 7 
parts of baking soda (by weight). If you go on to study chemistry in later years of school, you will 
learn about moles and molar masses, and how to work these formulas out.

To make a rocket work, you want the reaction to go really quickly. If you have the exact mix, the 
reaction will start to slow down as it runs out of acetic acid. So it is best to have just a little bit 
extra vinegar above what the formula says (an excess, a chemist would say).

There are two tricks that will help. First, the reaction will go faster the warmer the mixture is. If 
you can warm up the vinegar to about the temperature you would run your shower, the reaction 
will go really fast. Any hotter, and the vinegar will get too smelly, and there may be a risk of 
getting burnt. The second point depends on how much "messy" matters. You get more thrust from 
a rocket the more mass of material you throw out the exhaust. So if you produce a foam and squirt 
that out the back, you will get a lot more thrust than just squirting gas out the back (but also a 
huge mess). If mess does not matter too much, you might help it along with just a dash of a 
foaming detergent. You may need to experiment to get a good foam: some detergents will not 
produce good foams with salty mixtures, and this mixture will be very salty!

There is no way of knowing hole size other than trial and error. I think the hole should be fairly 
large -- probably between half and one centimetre across.

So here is a recipe that might work. You are using a 2 Litre bottle. You can probably have it just 
under half full of vinegar -- say 900 mL. That would mean about 135 gram of acetic acid. If we 
allow for a small excess, we could say that 120 gram of acetic acid will react with our baking soda, 
which means 168 gram (= 6 oz.) of baking soda (5 to 7 ratio). Use solid baking soda to drop into 
the vinegar. If everything works well, this could produce about 48 litres of foam! Have fun cleaning 
it up!

I have just done a web search using "vinegar baking soda rocket". It turned up a lot of interesting 
sites, but I did not quickly find one that I would strongly recommend. Many were just about 
allowing pressure to build up inside a bottle and blowing the cork off. I have assumed that this is 
not what you were meaning, but rather that you were going to put the bottle on wheels, and try to 
use the reaction to propel the bottle along.