The method to judge the acidity and alkalinity of the solution is as follows:
1. Add colorless phenolphthalein solution: after adding the solution, the color that does not change is acidic, and the color that turns red is alkaline.
2. Add purple litmus reagent: add litmus reagent to turn red into acid, and turn blue into alkaline.
3. Use pH test paper: according to the standard colorimetric card, pH less than 7 is acidic, and pH greater than 7 is alkaline.
4. The aqueous solution of strong acid and strong alkali salt is neutral, the aqueous solution of strong acid and weak alkali salt is acidic, and the aqueous solution of weak acid and strong alkali salt is alkaline.