How can a conductivity experiment have higher voltage than battery? I did an experiment where I tested the conductivity of different solutions and some substances. 
We connected a 9 bolt battery, some aluminum foil sticks, and a multimeter that reads the voltage. 
It connects like  liquid --> aluminum --> multimeter --> battery --> aluminum --> liquid.

My understanding is that if a liquid contains more ions, it conducts electricity better.
Here's my question. We tested the battery just by itself and it read about 9 volts. When we tested with baking soda, it reads about 9.2 volts.
How can it be higher than the energy source? Does baking soda produce its own electricity?
Also, when we tested deionized water, it came out to be about 8.5. It seems close to 9, but it was one of the lowest of the tested solutions. 
Why is it like this? If it's deionzed, shouldn't it be nearly zero? since it doesn't conduct electricity at all? (although there can be impurities so it can have some, but I think 8.5 is still too much).
I don't really understand. Can someone explain why the results came out like this?
A voltmeter is a zero current(i.e. very high resistance) device. Therefore in your scheme, the readings you are getting are not reliable. The way to make that measurement is by either switching your multimeter into an ammeter in the configuration you have and measure the current, or adding a resistor to the circuit in place of where you have the multimeter now and connect the multimeter in parallel to that resistor. In either case, you will be measuring the current going through the solution, which will tell you about the conductivity.