What is distilled water? What are its features? Distilled water is defined as water, whose conductivity value is calculated as 0.055 µS/cm (25ºC) or electrical resistance as 18.2 Mega ohm-cm. Ionic purification of water may be expressed as electrical conductivity in unit of micro-siemens/cm [µS/cm] or otherwise, as electrical resistance in unit of mega ohm-cm. Because conductivity and resistance vary depending on temperature, temperatures at which these values are measured should be given in parentheses. Salts dissolved in water lead formation of positively and negatively charged ions to increase conductivity of water. Therefore, chloride and sodium ions have similar effects. Also, some gases like carbon dioxide affect ion formation as much as pH. Previously, limit values were specified for matters disturbing pureness of water separately. Now, however, a limit value for total conductivity is specified as the essential criterion describing pureness of water.