It is a highly hygroscopic material so it could have water inside even when solid.
Explanation:
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It turns out that sodium hydroxide is a highly hygroscopic material, it means that it is able to catch as much water as it can from the environment and subsequently attach it to its structure even when solid, that is why a simple weight by a balance is not enough to determine the exact weight of a pure sodium hydroxide sample. Consequently, a strict standardization must be performed before its usage, since an error in the sodium hydroxide's concentration could be exhibited due to the misestimation.
Sodium hydroxide is deliquescent (absorbs moisture from the atmosphere) solid. It cannot be weighed accurately. Therefore, it is not possible to prepare a standard solution of sodium hydroxide of accurately known concentration by weighing NaOH.
An alpha particle is a helium nucleus, 2 protons and 2 neutrons, loss of an alpha particle give a new element with an atomic number 2 less than the original isotope and an atomic mass that is lower by about 4 amu.
