Properties of Water

Physical And Chemical Properties of Water

Water is the most abundant compound on Earth and the only common compound to exist as a solid, liquid, and gas on Earth's surface. It is also the 3rd most abundant molecule in the universe.


Chemical formulaH2O
Molar mass18.01528(33) g/mol
AppearanceAlmost colorless liquid with a hint of blue, White crystalline solid and colorless gas
DensityLiquid: 0.9998396 g/mL at 0 °C 0.9970474 g/mL at 25 °C 0.961893 g/mL at 95 °C Solid: 0.9167 g/ml at 0 °C
Melting point0.00 °C (32.00 °F; 273.15 K) 
Boiling point99.98 °C (211.96 °F; 373.13 K)
Solubility in waterN/A
SolubilityPoorly soluble in aliphatic and aromatic hydrocarbons, haloalkanes, ethers. Improved solubility in carboxylates, alcohols, ketones, amines. Miscible with ethanol, propanol, methanol, isopropanol, acetone, glycerol, 1,4-dioxane, tetrahydrofuran, sulfolane,  dimethoxyethane,  dimethylformamide,  dimethyl sulfoxide,  acetaldehyde, acetonitrile. Partially miscible with   Methyl Ethyl Ketone, Dichloromethane, Diethyl ether, Ethyl Acetate, Bromine.
Vapor pressure3.1690 kilopascals or 0.031276 atm
Acidity (pKa)13.995
Basicity (pKb)13.995
Conjugate acidHydronium
Conjugate baseHydroxide
Thermal conductivity0.6065 W/(m·0)K
Refractive index(nD)1.3330 (20 °C)
Viscosity0.890 cP

Water is amphoteric compound, meaning that it can display properties of an acid or a base, depending on the pH of the solution that it is in; it readily creates both H+ and OH ions. Linked to its amphoteric properties, it experiences self-ionization. The product of the activities or the concentration of H+ and OH is a constant, so their respective concentrations are inversely proportional to each other.

Physical properties of water

Density of water

The density of water is around 1 gm /cubic centimeter (62 lb/cu ft): this association was initially used to define the gram. The density changes with temperature, but not linearly: as the temperature rises, the density increases to a top at 3.98 °C (39.16 °F) and then drops. This rare negative thermal expansion below 4 °C (39 °F) is also seen in molten silica.  hexagonal ice is also less thick than liquid water—on freezing, the density of water drops by about 9%. 

These effects are because of the decrease of thermal motion with cooling, which lets water molecules to make more hydrogen bonds that avoid the molecules from coming near to each other. While below 4 °C the breaking of hydrogen bonds because of heating allows water molecules to pack closer in spite of the rise in the thermal motion (which tends to increase a liquid), above 4 °C water increases as the temperature increases. Water near the boiling point is about 4% less thick than water at 4 °C.

Vapor pressure

The pressure at which water vapor is thermodynamically stable along with its condensed state is the vapor pressure. At greater pressures, water will condense. The water vapor pressure is the fractional pressure of water vapor in any gas combination in balance with solid or liquid water. As for another compound, the water vapor pressure is a function of temperature and can be explained with the Clausius–Clapeyron relationship.

Vapour pressure of water (0–100 °C)
T, °CT, °FP, kPaP, torrP, atm


At 0 °C, at the bound of zero pressure, the compressibility is around 5.1×10−10 Pa−1. At the 0 pressure limit, the compressibility touches a minimum of 4.4×10−10 Pa−1 about 45 °C before growing again with rising temperature. As the pressure is increased, the compressibility reduces, being 3.8×10−10 Pa−1 at zero °C and 100 megapascals (1,000 bar). The bulk modulus of H2O is about 2.2 GPa. The low compressibility of non-gases, and of water in particular, which will lead to their often being expected as incompressible. The low compressibility of water means that in the deep oceans at 5 km depth, where pressures at 40 MPa, there is only a 1.8% drop in volume.

Heat capacity and heats of vaporization and fusion

Water has a high specific heat capacity around 4.1914 J/(g•K) at 24 °C – the second highest among all the heteroatomic species, as well as a high heat of vaporization at the normal boiling point, both of which are an outcome of the wide hydrogen bonding between its molecules. These two uncommon properties permit water to moderate Earth's climate by buffering large variations in temperature. Most of the extra energy kept in the climate system since 1970 has gathered in the oceans.

The specific enthalpy of fusion of water is 333.55 kJ/kg at zero °C: the exact amount of energy is needed to melt ice as to warm ice from −160 °C till its melting point or to heat the exact volume of water by about 85 °C. Of common compound, only that of ammonia is greater. This property discusses resistance to melting on the ice of glaciers and drifting of ice.

Triple point

The temperature and pressure at which normal solid, liquid, and gaseous water exist in balance is a triple point of water. This statement had been used to describe the base unit of temperature, the Kelvin but, the Kelvin will be described using the Boltzmann constant, rather than the triple point of water H2O. Due to the presence of many polymorphs of ice, water has other triple points, which have either three polymorphs of ice or two polymorphs of ice and liquid in stability. Gustav Heinrich Johann and Apollon Tammannin Göttingen formed data on numerous other triple points in the early 20th century. 

Melting point

The melting point of ice is zero °C (37 °F; 273 K) at typical pressure; however, pure liquid water can be supercooled well below that temperature without freezing if the liquid is not disturbed. It can persist in a fluid state down to its similar nucleation point of about 231 K (−42 °C; −44 °F). The melting point of hexagonal ice falls somewhat under moderately high pressures, by 0.0083 °C (0.0181 0°F)/atm or about 0.7 °C (0.90 °F)/70 atm as the equilibrium energy of hydrogen bonding is surpassed by intermolecular repulsion, but as ice converts into its allotropes  above 209.9 MPa (2,072 atm), the melting point grows decidedly with pressure, that is reaching 356 K (82 °C) at 2.286 GPa (21,770 atm) (triple point of Ice VI).

Chemical properties: 

At standard conditions, water is a highly polar liquid that marginally dissociates excessively into a hydroxide ion and hydronium ion.

2 H2O ⇌ H3O++ OH

The ionic yield of pure water, KW has a value of around 10−14 at 25 °C. Pure water has a concentration of the hydroxide ion (OH) identical to that of the hydrogen ion (H+), which gives a pH around 7 at 25 °C.