The calculated theoretical absorption excluding the effect of the hydration shell was larger than that of the measured absorption, indicating that the hydration shell surrounding MgSO 4 had a lower absorption coefficient than the bulk water that it replaced which is in line with expectation as the dipoles in the hydration shell tend to exhibit slower relaxation behaviour. The results also inferred that the hydration shell encompassed more than the 7 water molecules that form part of the MgSO 4 heptahydrate crystal because of the observed difference between the measured and calculated absorption. This is in line with other observations that also found extended hydration shells when probing samples with THz-TDS. An increase of the overall absorption coefficient at 0.5 THz and 1.0 THz as seen in experiments when crystals grew hence corresponded to water being expelled from the hydration shells into the bulk phase when the crystals formed. The bulk aqueous phase is pushed toward the field of view sampled by THz-TDS during the growth of the crystals before the crystals themselves enter the field of view of the terahertz beam. Therefore, the growth of MgSO 4 heptahydrate, that started at one end of the cell, increased the local concentration of bulk water in the centre of the cell, where it was probed with THz-TDS. This explained the initial slight increase in absorption that was observed at both 0.5 THz and 1.0 THz, given that the absorption coefficient of bulk water is much higher than that of the solution mixed with MgSO 4 or the heptahydrate.