How is the concentration of hydroxide ions $(OH^-)$ affected when excess base is dissolved in a solution of sodium hydroxide?

How is the concentration of hydronium ions $(H_3O^+)$ affected when a solution of an acid is diluted?

You have been provided with three test tubes. One of them contains distilled water,and the other two contain an acidic solution and a basic solution,respectively. If you are given only red litmus paper,how will you identify the contents of each test tube?

Fresh milk has a $pH$ of $6$. How do you think the $pH$ will change as it turns into curd? Explain your answer.

Write an equation to show the reaction between Plaster of Paris and water.

Write word equations and then balanced chemical equations for the reactions taking place when:
$(a)$ Dilute sulphuric acid reacts with zinc granules.
$(b)$ Dilute hydrochloric acid reacts with magnesium ribbon.