What must happen to the liquid as it passes from the stomach to the small intestine for digestion?

The liquid should be neutralized.

In the stomach, the pH value is around 2.0, which is because of the hydrochloric acid in the stomach. It can provide a optimum pH for the stomach enzymes.

However in the small intestine, the enzymes there works in a optimum pH of around 8.0, therefore the liquid must be neutralized before going into the small intestine, or else the enzymes in the small intestine will be denatured due to extreme pH.

Neutralization can be done by the sodium hydrogen carbonate, which is a alkali produced in small intestine.