Use blue litmus paper. This is an indicator that can safely determine whether it is a base or an acid by changing color in response to the substance. This color indicates whether it is an acid or a base. Refer to the pH scale to see if the substance is basic or acidic.
Isotopes are the same element with a different mass number. They are similar because once again the protons are equal to the atomic number which keeps the identity of the atom the same. But they are different because the masses of each is different.