Steve Marsden’s

Most of the elements present on this planet have few or no naturally occurring unstable isotopes, except for the very long-lived ones like uranium and thorium isotopes and an interesting pair of isotopes which illustrate how elements may be transformed into other elements by "building up" rather than decay processes.

Carbon-14 and Hydrogen-3 (tritium) are both formed in the upper atmosphere as a result of collisions with neutrons from solar radiation which reaches the earth:

When ions in aqueous solution experience a greater attraction for one another than the separate attractions they may have for water molecules, a precipitate forms. The solution becomes saturated and equilibrium is established between the undissolved (or precipitated) solute and the dissolved solute that stays in solution as ions. The rates of precipitation and dissolution are equal as long as the temperature remains constant and no water is added or removed.