"an acid is a substance that forms hydrogen ions when dissolved in water." this is an example of —

Answer is: this is an example of an Arrhenius acid.

An Arrhenius acid is a substance that dissociates in water to form hydrogen ions or protons (H⁺). 

For example hydrochloric acid: HCl(aq) → H⁺(aq) + Cl⁻(aq).

An Arrhenius base is a substance that dissociates in water to form hydroxide ions (OH⁻). 
In this example lithium hydroxide is an Arrhenius base:

LiOH(aq) → Li⁺(aq) + OH⁻(aq).

percent abundance for cu-63 can be shown as:

63 x + 65 (1-x) = 62.9296

65 - 62.9296 = 2 x

x = 1.035

thus, percent abundance for cu-63 is 1.035

percent abundance for cu-65 can be shown as:

63 x + 65 (1-x) = 64.9278

65 - 64.9278 = 2 x

x = 0.0361

thus, percent abundance for cu-63 is 0.0361

the mass spectrum of cu-63 showing the intense peak as, this isotope is found in nature more abundantly around 92.38 % in comparison to cu-65 which is found 62.58 % naturally