The solidification of sodium acetate trihydrate in a commercial heat pack is investigated in a series of three articles. In the first work, the classic nucleation theory is used to predict the range of temperatures and pressures for which a subcooled solution of sodium acetate solidifies spontaneously. The nucleation temperature was determined experimentally and it was shown that it is insensitive to concentration, at atmospheric pressure. Using the theoretical results for high pressure and the experimental measurements at atmospheric pressure, an equation for the nucleation temperature was derived as a function of pressure. This relation takes into account heterogeneous nucleation on ever-present impurities suspended in the solution.