What is common in all the water soluble bases (or alkalis)?

1 Name the gas evolved when zinc granules are treated/heated with:

(a) hydrochloric acid solution

(b) sodiumhydroxide solution

2 What is the common name of water soluble bases? 4 Why does tooth decay start when the pH of mouth is lower than 5.5? 5 What is the pH of a neutral solution?