Why oxide ion is called a hard ion?

Why oxide ion is called hard ion ? Explain.

Why is sodium ion stable ?

Why do calcium ions make water hard but sodium ions do not ?

Why do calcium ions make water hard but sodium do not

In the electrolysis of NaCl : Cl ion is oxidized at anode, Cl ion is reduced at anode, Cl ion is oxidized at cathode, Cl ion is neither produced nor oxidized

Which of the following ions will cause hardness in water sample?

Which of the following ions will cause hardness in water ?

Which of the following ions will cause hardness in water sample?