Deionized Water (DI Water) is water that has had almost all of its dissolved ions—such as calcium, magnesium, sodium, chloride, sulfate, and bicarbonate—removed through a process called ion exchange. This purification process involves passing the water through cation and anion exchange resins, which replace positively charged ions with hydrogen (H⁺) ions and negatively charged ions with hydroxide (OH⁻) ions. These ions then combine to form pure water (H₂O), resulting in water that has very low conductivity and minimal total dissolved solids (TDS).

Deionized water is chemically pure and free from minerals and salts, making it ideal for applications requiring high water purity, such as laboratory experiments, pharmaceutical production, electronics manufacturing, and industrial rinsing. Although DI water is extremely clean, it is not suitable for drinking because it lacks essential minerals needed by the human body and can be corrosive to both metals and tissues over time.

Demineralized Water (DM Water) is water that has been purified to remove nearly all of its dissolved mineral salts and ions, such as calcium, magnesium, sodium, chloride, and sulfate. This is typically achieved through processes like reverse osmosis (RO), ion exchange, or mixed-bed deionization. In reverse osmosis, water is forced through a semi-permeable membrane that filters out most dissolved solids, while ion exchange uses special resins to replace unwanted ions in the water with hydrogen and hydroxide ions, which then combine to form pure water.

The result is high-purity water with very low electrical conductivity, making it ideal for use in industries and applications where mineral content can interfere with processes or damage equipment. Common uses include pharmaceutical manufacturing, laboratory testing, power generation (as boiler feedwater), electronics production, and cosmetics. Although it is extremely pure, demineralized water is not suitable for drinking, as it lacks essential minerals that the human body needs.

Distilled Water is purified water that has been produced through the process of distillation, which involves boiling the water to create steam and then condensing that steam back into liquid form. This process effectively removes most impurities, including dissolved salts, minerals, and many organic and inorganic contaminants, since these substances do not evaporate with the water. The result is water that is very pure, with a low level of total dissolved solids  and very few ions or particles.

The typical composition of distilled water includes H₂O molecules with very low or nearly zero concentrations of minerals like calcium, magnesium, sodium, and chloride. It may still absorb small amounts of carbon dioxide (CO₂) from the air, which can make the pH slightly acidic (around 5.5–6.5). Distilled water is commonly used in laboratories, medical equipment (like autoclaves and CPAP machines), car batteries, and steam irons.