Water is used as solvent as well as adding solubilisation of certain material during manufacturing.
Water is a neutral solvent theoretically PH of a highly pure water is 7 at temp 25, water molecule is considered a amphoteric in nature which means, water act as an acid in presence of base facilitating solubilisation of base, and water act as base in presence of a acid facilitating solubilisation of acid.
Water molecule is a highly polar molecule, which means in presence of dielectric field one end of the molecule poses a negative charge and other end a positive charge, as the electron density is distributed like wise in dielectric fields.
This property of water helps in solbulisation of many molecules by virtue of hydrogen bonding and amphoteric nature. That's why water from natural origin is rich in dissolved substances inorganic as well as organic impurities.
Water for pharmaceutical must be free from inorganic as well as organic impurities, minerals, and heavy metals. Some impurities like calcium, magnesium, ferrous are responsible for degradation of drug molecule, many cations like ferrous and calcium magnesium act as catalysts in degradation reaction of drug molecule, anions like chloride are highly active they participate in nucliophylic substitution reactions, where in they break a double bond between -C=C- in to a single bond as CL –CH-CH2- , which a reason why we observe that colour dyes tend to fed in presence of chlorine as most of the dyes used are diazo compounds which has plenty of places for nucliophylic substitution reactions, which is also a reason why stability of drug is drastically affected in presence of cations and anions from mineral origin present in water.
Why water must be free from Heavy metals
Heavy metals like lead and arsenic lead are highly cumulative neurotoxic metals, heavy metals are not eliminated out of our body easily like other drugs and molecules but heavy metals bind with proteins and tend to get accumulated in fatty tissues, nerve tissue is most likely to get damaged by heavy metals, heavy metal causes nervous tissue damage there for water must be free from heavy metals.
Free sulfate ions in water are oxidising agents which are responsible for degradation of drug molecule.
Since water is a good solvent it tend to solubilise minerals which up on evaporation again deposit as solid is called dissolved solid material, water must be free from dissolved solid material , a test which is employed to calculate content of dissolved solid material is called as total dissolved solid ideally water for pharmaceutical use is free from dissolved solid.