Properties of water

  • Dihydrogen Monoxide
  • μ - Oxidodiwasserstoff
  • Diwasserstoffoxid
  • Oxidan

The properties of water have basic meanings for life on Earth. These physical, chemical, electrical and optical properties are based on the structure of the water molecule and the resulting linkages and interactions of water molecules with each other via hydrogen bonds, electric dipole forces and other forces such as van der Waals forces.

In nature, water is not as pure substance ago, it practically always contains dissolved substances (mainly ions of salts ), although possibly. Barely measurable concentrations in Such solutes, the characteristics of the water change. Very pure water is prepared in the laboratory by distillation and called distilled water.

  • 2.1 Reactivity
  • 2.2 leveling effect
  • 2.4 ionic product
  • 2.5 Water Hardness

Physical Properties

Completely pure water has a molar mass of 18.01528 g / mol.

The properties of water are particularly determined by the three-dimensional chain of water molecules through hydrogen bonds, without which a substance would have very different properties at such a low molar mass as water. This is especially true for the high melting and boiling points as well as for the density is highest under normal pressure at 3.98 ° C with 0.999975 kg/dm.sup.3 (see density and density anomaly ).

From 1901 to 1964 the liter unit was defined as the volume of 1 kg of water at the temperature of its maximum density under normal pressure.

The physical properties of water are also strongly dependent on the temperature and the pressure. To take surface tension and viscosity decreases with increasing temperature. Similarly, the compressibility is dependent on temperature.

Aggregate states

Under normal conditions, water is as recognizable in the phase diagram, a liquid. It is the only known substance that exists on the earth's surface (see hydrosphere ) in appreciable amounts in all three classical states of matter. The phase diagram shows the extent to which the physical state of the water temperature and pressure dependent. The critical point of water is 373.946 ° C, and 2.2064 · 107 Pa (322 kg / m³), the triple point at 0.01 ° C and 611.657 ± 0.010 Pa.

For the properties and characteristics of the gaseous and solid phases of water, see the article water vapor and ice. In supercritical water, ie above the critical point, a distinction is whether a substance is a liquid or gaseous, not possible.

Melting and boiling point

Compared to chemically analogous compounds such as hydrogen sulfide ( -61 ° C), hydrogen selenide (-41 ° C), and telluride (-2 ° C) water will have a relatively high boiling point. The increase in the boiling point is due to the increasing molar mass and also by the higher amount of energy that must be applied to convert each substance in the gaseous state. Methane for example, has a very similar molar mass of the water and boiling under atmospheric pressure at -162 ° C. Taking the molar mass of water as a single characteristic, it would at -80 ° C and boiling would be gaseous at room temperature. However, the boiling point is 100 ° C, thus higher 180K.

The same picture can be found at the melting point; it is -86 ° C in hydrogen sulfide, -66 ° C with hydrogen selenide and -49 ° C in hydrogen telluride. With water he would have on the molar mass at about -100 ° C, but in fact it is at 0 ° C. Comparing the area occurs as a liquid in the water, so there is a margin of 20 K for the case that one considers only the molar mass. In reality, however, this range is much greater with 100K.

All these features result from the structure of the water molecule and its tendency to form hydrogen bonds via networked cluster as in the picture on the right. These additional compounds that do not occur with the other materials must be overcome or taken into account in addition in each phase transition.

Thus, boiling water under atmospheric conditions at 100 ° C and melts ice at 0 ° C. Water freezes at 0 ° C accordingly; however, it may still be present even in normal conditions below 0 ° C as a liquid. It is then subcooled water. At pressures between 1000 and 2000 bar water freezes below -138 ° C in the amorphous state. Conversely, remain firmly ice for a short time above 0 ° C as long as the temperature is not reached at the surface. The boiling point of water is highly dependent on the saturation vapor pressure. The boiling point decreases as it approaches the triple point with the boiling pressure, and at that point both reach its minimum. Water also can be heat but also about its boiling point, which is called boiling.

Also in the water change solutes boiling and melting point. Thus, water has a molar melting point depression of 1.853 K · kg / mol and a molar boiling point elevation of 0.513 K · kg / mol.

Previously, the Celsius temperature scale was defined by the melting and boiling point of water. Due to the current definition of the Celsius scale on the Kelvin scale are melting and boiling point of water is no longer exactly 0 ° C and 100 ° C, but are at 0.002519 ° C and 99.9839 ° C ( 99.9743 ° C according to ITS-90).

A special feature of the solidification of water is named after its discoverer Mpemba effect, according to which hot water under special conditions freezes faster than cold.

Heating water

If water is heated in a pot on a stove, the water heated on the ground faster than at the surface. Thus, an unstable temperature gradient is formed which, however, soon largely disappear by convection. If the water reaches the bottom of the boiling temperature, water vapor bubbles are formed there. As you ascend, they cool off again and fall together. They produce the typical crackling noise that can be heard just before boiling. Upon further heat collapse are the tiny bubbles that rise to large. The Siedegeräusch is quieter to disappear when complete boiling of the water.

In microgravity, the vapor bubbles do not rise in the water. Instead, they remain in the vicinity of the pot bottom and conglomerate into larger bubbles and finally to a single large bubble. The lack of convection and the reduced heat conduction through the vapor bubbles impede the rapid boiling of water in a spaceship.

Sublimation and Resublimation

In the temperature range of about 0 K to 273.16 K ( -273.15 ° C to 0.01 ° C) and a pressure range from vacuum to about 0.006 bar, ie in the region below the triple point, water does not exist in liquid form, but only gaseous and solid. Ice goes in this area, so the sublimation directly into the gaseous state without a change of state takes place in a liquid. This process is referred to as sublimation, or in the opposite direction as the desublimation. In vacuum, sublimation takes place to almost 0 Kelvin ( -273.15 ° C). The upper limit, however, is given by the triple point.

Specific heat capacity

Liquid water has a very high specific heat capacity of about 4200 J / ( kg · K) (depending on temperature 4218-4178 J / ( kg · K) ). So you need to heat a kilogram of a Kelvin 4.2 kilojoules of thermal energy. This means that water can absorb quite a lot in comparison with other energy liquids, without the temperature would thereby increase significantly. This same energy is released during cooling.

Heated for 1 kg of water one (~ 1 liter) from 15 ° C to 100 ° C, then so you need 4200 J / ( kg · K) · K 85 · 1 kg = 357 kJ. 3.6 MJ are a kilowatt hour ( kWh). To bring a liter of water at line temperature under normal pressure for cooking, it does so about 0.1 kWh.

Water vapor ( at 100 ° C) has a specific heat capacity of 1870 J / (kg · K) and ice ( 0 ° C) 2060 J / ( kg · K). Solids have a much lower specific heat capacity in the rule. For example, lead has a heat capacity of 129 J / (kg · K), a copper of 380 J / ( kg · K).

Melting and heat of vaporization

For the conversion of 0 ° C ice cold 0 ° C cold water an energy of 333.5 kJ / kg has to be applied; which is exactly as much as is needed to 0 ° C to heat cold water to 81 ° C. For the conversion of 100 ° C warm water in 100 ° C hot steam 2257 kJ / kg is required. Around 0 ° C in 100 ° C to change cold water warm vapor, one needs 100 K x 4.19 kJ / (kg · K) 2257 kJ / kg = 2676 kJ / kg. The specific evaporation heat of water is much higher than the specific heat of vaporization of other liquids, methanol, in comparison, only an evaporation heat of 845 kJ / kg, and even mercury only one of 285 kJ / kg. Comparing, however, the molar heats of vaporization, so mercury with 57.2 kJ / mol higher than water at 40.6 kJ / mol.

In meteorology, the melting and heat of vaporization of great importance to come within the scope of the latent heat.

Thermal conductivity

Water, in comparison to other liquids, a high thermal conductivity, but very low in comparison with some metals. The thermal conductivity of liquid water increases with increasing temperature, ice conducts heat but much better than liquid water.

At 20 ° C, water has a thermal conductivity of 0.60 W / (m · K). For comparison, copper 394 W / ( m · K ), and silver 429 W / (m · K). Even the worst heat conductor among all metals, bismuth comes to 7.87 W / ( m · K).

The thermal conductivity of the water in the form of ice at -20 ° C is at least 2.33 W / (m · K).

Density and density anomaly

Water has a density of about one kilogram per liter (one liter corresponds to a cubic decimeter ). This circular relationship is no accident: it goes back to the unit Grave, which forms one of the historical roots of today's international system of units (SI). A Grave was defined as the mass of one liter of water at freezing point.

At atmospheric pressure, water has its highest density at 3.98 ° C, indicating a density anomaly. This is that there is water below 3.98 ° C with further decrease in temperature, even when going to the solid state, expands again, which are known only for a few substances.

In addition to temperature, dissolved substances in water also affect its density, which can be measured with a hydrometer. , Since there are dissolved particles between the water molecules and the increase in volume is low, thus increasing the density. The increase in density corresponds approximately to the mass of solute per volume and plays an important role for large-scale water movements, for example in the context of the thermohaline circulation or the dynamics of freshwater lenses.

Smell and taste

Water is tasteless and odorless in its pure state.

Optical Properties

Refraction and reflection properties

Water is in the range of visible light, a refractive index of about 1.33. When light strikes the interface of air ( refractive index ≈ 1 ) and water, it is therefore to Lot broken down. The refractive index is low compared to many other materials, so the refraction is less pronounced due to water than for example in the transition from air to most types of glass or even diamond. There are, however, materials such as methanol, having a lower refractive index. The refraction of light leads to optical illusions, so that you can see an object under water at a different location than where it actually is. The same is true for a view from the water into the air space. Specialized in fishing animals such as herons, or fish that hunt for insects over the water may recognize this image - transfer and take their prey therefore most easily.

The reflectivity of the surface air-water is by the Fresnel formulas for normal incidence about 2%. As with all materials takes this value with shallower angles of incidence and at grazing incidence is approximately 100 %. The reflection behavior is, however, as with all materials, depending on the polarization of the light. While parallel polarized light generally has a lower reflectivity than perpendicular polarized light, that is, upon impinging on the interface of air and water, light is polarized. By the relatively low refractive index of water, however, this effect is less pronounced than with many other ( transparent ) materials having a higher refractive index. The polarization effect is generally stronger, the flatter the light hits the water surface. This is for example used in photography, here is a certain polarization is filtered out using a polarizing filter, thereby disturbing reflection effects can be reduced.

When light from the water to the water - air interface, so it comes as a direct result of the law of refraction from a critical angle of 49 ° to a total reflection. This means that flat light impinging rays do not escape to the boundary surface of the water, but are reflected.

Some optical effects in the atmosphere are subject to the refractive properties of water. For example, a rainbow is caused by water droplets or ice crystals by a halo phenomenon in which the light is refracted and thereby split by spectral colors. The darkening of the earth through clouds based on light refraction and total internal reflection in or on water droplets.

Absorption behavior and color

Water absorbs light in the visible spectral range is very weak, that is, the imaginary part of the complex refractive index ( extinction coefficient) is approximately 0 water is therefore generally considered to be transparent and colorless. The large light transmittance of the water allows the existence of algae and plants in the water, which need light to live.

However, also has the low extinction coefficient in the visible spectral changes by several orders of magnitude (see figure ). In the wavelength range of about 400-440 nm, the extinction coefficient, and therefore the absorption (cf. Lambert Beer 's Law) at the lowest. This has the result that the light of the wavelengths is completely absorbed after several meters. In the red visible and near infrared range, the extinction coefficient increases slightly. Long-wavelength (red ) light is therefore absorbed more strongly than short-wavelength (blue ) light. Receiving water by a faint bluish tinge. This is, however, imperceptible to the naked eye only in thicker layers from a few meters. In the ultraviolet region ( λ <350 nm), the extinction coefficient increases even faster than the long-wavelength ( red ) range, below 240 nm it is approximately 0.1. UV light is therefore absorbed completely after only a few centimeters.

Another crucial factor that affects the optical properties of water, in the water and dissolved substances in the water floating particles. In water, dissolved substances can lead to a significant change in these properties, which is described by the spectral absorption coefficient. Small particles with a diameter in the range of wavelength, however, lead to scattering of the light, the water then acts slightly cloudy (or milky umgangssprachig ). Color and turbidity of the water, depending on the substances contained in it play an important role as indicators of water quality, as well as a research method in water analysis.

Electromagnetic resonances

Water has different resonance frequencies. The lowest resonant frequency of the free water molecule is located at 22.23508 GHz. The integer multiples of this frequency, in turn, are resonant frequencies. The widespread assumption that the frequency of the microwave oven of 2.455 GHz was a resonance frequency of the water, is wrong.

Resistivity and electrical conductivity,

Chemically pure water at a pH of 7, only to a small extent dissociated OH in the electrical charge carriers and H3O . It therefore has a high specific resistance of 18.2 megohm- cm (= 1.82 × 1013 Ω · mm ² / m ) at 25 ° C. This corresponds to a specific conductance of 54.9 nS · cm -1. The temperature dependence is in this case about 1.5 to 2% per Kelvin. Dissolved salts and acids increase the carrier concentration. Already tap water depending on mineral content reaches to about 10,000 times the average conductivity of 500 ĩS · cm -1, sea water reaches values ​​of 50 mS · cm -1.

When extinguishing fires, it may happen that a conductive connection through the clear water between current-carrying parts (cables, electrical appliances ) to the so-called short-circuit or that the deleted person himself across the water is part of a circuit and receives an electric shock.


Viscosity ( strength ) of the water at 20 ° C is 1.0 mPa · s so that it has a higher viscosity than petroleum (0.65 mPa s at 20 ° C) but also a lower than, for example, mercury (1.5 mPa.s at 20 ° C). The viscosity of water decreases by the decreasing number of hydrogen bonds with increasing temperature and reaches the boiling point of 0.283 mPa s

The viscosity is changed by solutes. In addition to the concentration, the type of the dissolved substance is critical to the viscosity of the solution.


The diffusion of the water molecules within the water or aqueous solution is referred to as self-diffusion, and described by the " self-diffusion coefficient " D; he is at 25 ° C: D = 2.299 · 10-9 m2 · s -1. The size D describes the translational mobility of water molecules within the liquid water. This motion is coupled with high Newtonian fluids in the viscous behavior, thus decreasing the temperature increases the viscosity of the water is connected to a self- increasing diffusion coefficient.

The temperature dependence of the self-diffusion coefficient is measured very accurately and often serves as a reference set of values ​​in the study of diffusion in liquids other.

In addition to the translational diffusion are in the water - as in other liquids as well - the rotational diffusion, namely the random change in orientation of the axes of symmetry of the water molecules in random movements ( random walk ) within the liquid. The this Umorientierungsbewegung characterizing correlation time, that is about the time during which a water molecule in the liquid, once rotated by random small steps by himself, at 25 ° C in the range of a few picoseconds, how and by nuclear magnetic relaxation dielectric relaxation was measured. It is extremely fast, random reorientations of the water molecules and hence to the same rapid changes in the microstructure of the water.

If water is present with a different isotopic composition, such as heavy water D2O, then occurs a so-called dynamic isotope effect, which affects both the translational and the rotational diffusion. As the low molecular weight of the water, the relative change in weight by isotopic substitution is comparatively large, contact the water, compared to other known liquids, the most isotope effects. Thus at 25 ° C 23% D 2 O has a lower diffusion coefficient than H2O.

Solutes, such as salts, can also self-diffusion coefficient and the rotational diffusion of the water both lower ( " structure-forming " salts with small ionic radii, such as lithium chloride) as well as enhance ( " structure -breaking " salts having large ionic radii, such as cesium iodide ). Structure -breaking salts in which the anion of the water causes the refractive structure, are often chaotropic salts. Dissolve non-polar or electrically uncharged species in water, occurs, a hydrophobic effect, slows down in addition to the rotation, the diffusion motion of the water molecules in the vicinity of the " hydrophobic " species and thus decreases the average diffusion coefficient of water in the solution.

Surface tension and wettability

Water has a relatively large surface tension, because the water molecules attract each other relatively strong. The surface tension is approximately 73 mN / m at 20 ° C and decreases at increasing temperature. Because of the large surface tension, for example, water striders can move on the water. Washes in surface tension is a hindrance, and therefore (surfactants ) are included in detergent surfactants that reduce the surface tension. However, their occurrence is low in natural waters.

Wherein a smooth surface contact angle of 120 ° can be achieved. In roughened surfaces with hydrophobic character, however, this angle can be up to 160 °, which is called super hydrophobia. This make a lot of plants to use on the lotus effect.

Bulk modulus and sound velocity

Water has a bulk modulus of about 2.06 GPa at a temperature of 4 ° C under atmospheric pressure - therefore compressed at 100 MPa ( a thousand times normal pressure or water pressure in less than 10 km depth) is about five percent. According to the density of 1 kg/dm.sup.3 or incurring propagation velocity of sound in water gives of 1435 m / s

Isotope fractionation

Water molecules from different isotopes of hydrogen (eg Protium 1H 2H or deuterium ) and oxygen (for B.16O or 17O ) exist, each of which occur in different concentrations. This so-called isotope effects occur. For certain processes such as precipitation and their phase transitions, it is used to this isotope fractionation, ie the water here changes its isotopic composition. Depending on the ambient conditions and the original composition result from this specific isotope signals that can act as a kind of fingerprint for different processes and areas of origin. Application locates the appropriate methodology especially in hydrogeology and paleoclimatology.

Water as a solvent

Water is characterized by its dipole an excellent polar solvent for many substances. In general, the water solubility increases with increasing polarity of the substance. Water has a relatively high dielectric constant of 80.35 ( 20 ° C).

The solubility in water is often heavily dependent on the temperature. Here, solids and gases behave differently. Gases dissolve proportional to the partial pressure of the gas in water without a fixed limit of the releasable amount ( Henry's law ). The case called " solubility" equilibrium concentration per unit pressure decreases with increasing temperature. In contrast, solids dissolve with increasing temperature usually better in water, but of which there are many exceptions, such as lithium sulfate.

Some substances, such as acetone or ethanol are miscible with water, so soluble in one another in any ratio. In other cases there is mutual solutions with a miscibility gap, for example with phenol or chloroform.

Normally this is that a molecular substance is dissolved in water the better, the more polar groups are present in this material. However, Supercritical water shows similar solubility as nonpolar organic solvents.

Upon dissolution of ionic substances in water endothermic lattice reduction and the exothermic hydration expire, allowing heat mixtures (sulfuric acid in water) and cold mixtures ( salt in water). This determines the difference between the exothermic hydration and the endothermic decomposition grid, whether heating or cooling occurs. Salts with the ratio of the lattice energy of hydration of the ions involved, and decides on the solubility, defined herein as the product of the molar ion concentration in equilibrium with the crystalline substance ( solubility product ). As a rule of thumb for solubility of ionic compounds can be considered: the higher the charge number of the ions involved, the less soluble the substance in water.

In contrast to simple compounds such as sodium chloride, the ionic bonds of complexes are not cleaved. We distinguish between two groups. On the one hand the strong complexes, such as the cyanide complexes of heavy metals, and on the other hand, the weak complexes ( aqua complexes ) of metal ions with sulphate, hydroxy, or carbonate ions. The nature and occurrence of the various metal species are important issues of chemical water analysis and water treatment.

Wherein molecules of different polarities such as the many amphiphilic lipids, the solubility in water or affinity for water depends on its orientation. This effect makes almost all living things with their biomembranes advantage. One speaks in this context of a hydrophilicity or hydrophobicity.

Chemical Properties

Water has a molar mass of 18.01528 g · mol -1. Water is a catalyst in many reactions, that is, without the presence of water would a reaction take place much more slowly and with higher activation barrier. Many reactions are enabled or accelerated even by the normal humidity. That falls through the almost always present traces of moisture in our environment practically non-existent, because it is the normal case. Only when removed by special drying process even the smallest amounts of moisture and chemical tests are carried out in closed systems, which is demonstrated. So do not burn in this environment, for example, carbon monoxide into oxygen and alkali metals do not react with sulfuric acid and chlorine.


Water is amphoteric, ie it is a substance which - depending on the milieu - can act both as an acid and as a base.

Water reacts with anhydrides to acids or bases. Examples:

  • Phosphorus pentoxide ( anhydride ) reacts with water to form phosphoric acid ( acid):
  • Sodium oxide ( Basenanhydrid ) reacts with water to form sodium hydroxide ( base):

Water reacts with base metals to form hydrogen to metal oxides, but these metal oxides are Basenanhydride and dissolve mostly right back into water to bases, as has just been described. An example:

  • Magnesium reacts with steam to form magnesium oxide and hydrogen:

Leveling effect

In aqueous solutions, strong acids and strong bases dissociate completely H3O - and OH - ions. Thus, the different acid strengths of, for example, hydrochloric acid and perchloric acid can not be distinguished in water based on the pH. This is called a leveling effect ( v. French: niveler = same make ) of water. In order to distinguish even very strong acids relative to the acid strength is determined equilibrium constants in non-aqueous solutions and transfers it approximately to the solvent water.

Chemically pure water of 22 ° C has a theoretical pH value of 7 ( the equilibrium constant for the dissociation of water is then exactly 10-14). This value is defined as chemically neutral. However, chemically pure water has no buffer, responding to the slightest contamination with a significant pH change. So it turns into previously chemically pure water with air, due to dissolution of CO2 immediately a pH value between 4.5 and 5.

If water is contaminated by salts (eg bicarbonates ), it reacts much less sensitive to contamination by acids and bases.

Ionic product

The ionic product of water is the product of the concentration of H3O or OH - ions in water. In 1894 studied Friedrich Wilhelm Georg Kohlrausch and Ernst Heydweiller of water by distillation under complete exclusion of air, the conductivity of distilled water (see dissociation). From these measurements and from knowledge of the equivalent conductivities of hydronium ions and hydroxide ions, the ion product of water could be calculated.

In a conductivity measurement of distilled water occurs, a small current flow. This is an indication of ions in the water that can be created only by the autoionization of water, according to the following reaction:

On the protolytic equilibrium, the law of mass action can be applied:

Since the concentration of the water is almost constant even when a shift in the equilibrium ( 55.5 mol / l), can include the value of the constant with.

At 22 ° C Kw = 10-14 applies (mol / l) ². This very strong, the equilibrium is on the side of the water. The concentration of H3O or OH - ions are respectively 10-7 mol / L. The pH is thus 7

Is the concentration of either ion increases, the ion product of 10-14 is maintained, i.e., decreases the concentration of other ions. The sum of pH and pOH must therefore always be 14.

The pKW of water changes depending on the temperature.

( experimentally determined by measuring the conductivity values)

With knowledge of the ion product of water, the pH values ​​of dissolved salts, acids, bases let in water (eg, sodium acetate, sodium carbonate, calcium oxide, hydrochloric acid, sulfuric acid, caustic soda ) calculated.

Water hardness

Hardness describes the equivalent concentration of dissolved ions in the water of the alkaline earth metals. The " hardness minerals " include mainly calcium and magnesium ions and traces of strontium and barium ions. These cations have a large, positive physiological significance, however, interfere with some uses of the water.

New insights

Two American researchers, Emily B. Moore and Valeria Molinero at the University of Utah have theoretically demonstrated that pure water - H2O so without the presence of any nuclei - only freezes at -48.3 ° C. This is done by crystallization in a tetrahedral shape; in the center of the crystal is a water molecule, which is surrounded by four other molecules. At the temperature above there are only these crystals and no free water molecules more.