Water

Deionised water has had its dissolved mineral ions removed, leaving water with very low conductivity and almost no scale-forming content.

That matters in several common applications. Lead-acid batteries need topping up with deionised or distilled water, because the calcium, magnesium and chloride in tap water attack the plates and shorten battery life. Coolant concentrate should be diluted with deionised water for the same reason, since minerals in tap water form scale on hot surfaces and reduce heat transfer, and chlorides accelerate corrosion in aluminium engines. It is also used for steam irons, laboratory work, final rinse in cleaning processes and humidifier systems.

Supplied in bulk on a per-litre basis. Store it in a clean, sealed container, as deionised water picks up contamination readily from dirty vessels and from the air.