Water
Properties
| State | Liquid at room temperature |
| Color | Colorless |
| Solubility | Miscible with most polar solvents (methanol, ethanol, acetone, DMSO); immiscible with most nonpolar solvents |
| Melting Point | 0 °C (273.15 K) at 1 atm |
| Boiling Point | 100 °C (373.15 K) at 1 atm |
About Water
Water, H2O, is the molecule that anchors most of the constants and reference points you encounter in a chemistry lab. Two O-H bonds at a 104.5 degree angle, a sizeable dipole moment, and an extensive hydrogen-bond network give water its unusually high boiling point (100 °C at 1 atm), high specific heat capacity (4.184 J/g/K), and a density anomaly that makes ice float. Pure water at 25 °C autoionizes weakly to give [H+] = [OH-] = 1.0 × 10^-7 M, which is what fixes neutral pH at 7.00 — that number is not arbitrary, it falls out of Kw = 1.0 × 10^-14 at 25 °C and shifts with temperature (Kw at 100 °C is closer to 5.5 × 10^-13, so neutral pH is roughly 6.13 in boiling water). Water is the solvent in which most aqueous acid-base, redox, and complexation chemistry is defined; the calorie was originally tied to it; the kilogram was tied to it through 1901; the Celsius scale brackets its triple-point and atmospheric boiling point. As a chemical reactant water shows up in hydrolysis, hydration, condensation, and acid-base neutralization, and as a solvent it dissolves more ionic and polar substances than any other common liquid.
Where you'll encounter it
If you've ever calibrated a pH meter with the standard 4, 7, and 10 buffers, run a titration to a phenolphthalein endpoint, or watched ice cubes drop into a glass and wondered why they float, you've used water as both reagent and reference. In a Type I purification system feeding ICP-MS or HPLC, water is polished to 18.2 megohm-cm resistivity at 25 °C, less than 5 ppb total organic carbon, and run through a 0.22 micron final filter — those specifications fall directly out of how much instrument noise residual ions and TOC will inject. Calorimetry students spend their first lab measuring the specific heat of an unknown metal by dropping it into a known mass of water and applying q = mc(deltaT), with water's 4.184 J/g/K specific heat as the calibration anchor. Drinking-water utilities run jar tests on raw source water with alum or ferric chloride to optimize coagulant dose before the water hits the rapid mix at the plant inlet.
Common Uses
- Solvent of record for aqueous acid-base, redox, and coordination chemistry
- Coolant and heat-transfer fluid in nuclear, automotive, and industrial process equipment
- Reactant in hydrolysis, hydration, and acid-base neutralization reactions across organic and inorganic synthesis
- Reference substance for the kilogram, calorie, Celsius scale, and specific gravity definitions
- Calibration medium for densitometers, refractometers, and calorimeters in analytical labs
- Mobile-phase component for reverse-phase HPLC and ion chromatography separations
- Working fluid in steam turbines, hydraulic systems, and pressure vessels for power generation
Safety Information
Pure water is non-toxic, non-flammable, and the substance against which most workplace exposure limits are normalized. Drinking water at typical municipal supply rates carries no acute toxicity, but rapid intake of several liters within a few hours can cause hyponatremia (water intoxication) by diluting serum sodium below 135 mEq/L, with seizures and cerebral edema as terminal symptoms. There is no OSHA PEL or GHS classification for water itself. Note the practical hazards of state changes: steam at 100 °C delivers the latent heat of vaporization (2257 J/g) to skin on contact and causes deeper burns than equivalent-temperature liquid water, and contact between liquid water and molten metals or strong dehydrating agents like H2SO4 can be violently exothermic. Always add concentrated acid to water, not the reverse.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.