The Density of Solid and Liquid Water

The large spaces within ice give the solid state a lower density than the liquid stale. When the surface of a lake freezes in winter, the ice floats on the liquid water below. If the solid were denser than the liquid, as is true for nearly every other substance, the surface of a lake would freeze and sink repeatedly until the entire lake was solid. As a result, aquatic plant and animal life would not survive from year to year. 

The density of water changes in a complex way. When ice melts at 0°C, the tetrahedral arrangement around each O atom breaks down, and the loosened molecules pack much more closely, filling spaces in the collapsing solid structure. As a result, water is most dense (1.000 g/mL) at around 4°C (3.98°C). With more heating, the density decreases through normal thermal expansion. 

Stroll through the mineral collection of any school or museum, and you ll be struck by the extraordinary variety and beauty of these solids. In this section, we first discuss the general structural features of crystalline solids and then examine a laboratory method for studying them. We survey the properties of the major types of solids and find the whole range of intermolecular forces at work. We then present a model for bonding in solids that explains many of their properties. 

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Af/lYus of water is 6024 JmoH at 273 K. The densities of solid and liquid water are 0.915 and 1.000 g mL-1, respectively. What is the freezing point of water under 100 bar pleasure ... 

Although the densities of solid and liquid water are quite similar, as is typical for most substances, water is quite unusual in that the density of its solid state is slightly less than that of its liquid state. For most substances, the density of the solid state is slightly greater than that of the liquid state. 

Density and Relative Density. Density is mass per unit volume and in SI is normally expressed as kilograms per cubic meter (density of water = 1000 kg/m3 or 1 g/cm3). The term specific gravity was formerly the accepted dimensionless value describing the ratio of the density of solids and liquids to the density of water at 4°C or for gases to the density of air at standard conditions. The term specific gravity is being replaced by relative mass density, a more descriptive term. 

Figure 1.13 The density of ice and liquid water as a function of temperature (at atmospheric pressure). Solid ice has a much lower density (0.9t5 g cm ) than water left panel). Liquid water (expanded scale, right panel) has a maximum density at around 4 °C...

The densities of solids and liquids are commonly expressed in either grams per cubic centimeter (g/cm ) or grams per milliliter (g/mL). The densities of some common substances are listed in TABLE 1.6. It is no coincidence that the density of water is 1.00 g/mL the gram was originally defined as the mass of 1 mL of water at a specific temperature. Because most substances change volume when they are heated or cooled, densities are temperature dependent, and so temperature should be specified when reporting densities. If no temperature is reported, we assume 25 °C, close to normal room temperature. 

One remarkable consequence of hydrogen bonding is seen in the densities of ice and liquid water. In most substances the molecules in the solid are more densely packed than in the liquid, making the solid phase denser than the liquid phase. By contrast, the density of ice at 0 °C (0.917 g/mL) is less than that of liquid water at 0 °C (1.00 g/mL), so ice floats on liquid water. 

You caimot predict the relative densities of ice and liquid water based on benzene. Ice cubes and icebergs float because water is less dense as a solid than it is as a liquid. Figure 13-17 shows the reason for the exception. The water molecules in ice are less closely packed together than in liquid water that is, there is more space between the molecules in ice. As a result, there are more particles per unit volume in liquid water than in solid water. 

The volume changes on mixing non-aqueous liquids, the densities of mixed liquids, of solutions of non-polar solutes in non-polar solvents, and the changes of total volume on the solution of solid salts in water, noticed at an early period and much investigated, can only be mentioned here some aspects of these will be dealt with later. Hyde found the densities of solutions of jp-nitrotoluene in carbon disulphide smaller than the density of either component, but the anomaly disappears if the p-nitrotoluene is supposed to be in the liquid state. Biron found that the volume change on mixing two liquids was Av=kx( —x where x , (1— ) are the mol fractions, and he investigated the effect of pressure on the value of Av. The apparent specific volume of alcohol in aqueous mixtures was determined by Brown, lo... 

Figure 1.19. Liquid-vapor coexistence line of SPC water and OPLS methanol (solid and dashed lines) following from the RISM/KH theory versus the simulation data (open circles and squares, respectively) and critical point estimates (closed symbols). Logarithmic and linear scales are used to resolve the density in gas and liquid state.

Under these conditions, bed density is 500 kg/m and equilibrium moisture content is zero. Specific heat of the dry solids and liquid water are 1.0 and 4.2 kJ/kg°C, respectively. 

Systems for pumping slurries around a process plant are similar to systems for pumping liquids, in regard to the calculation of performance on a basis of head. The slurry can be regarded as a fluid having a density computed from the ratio of solids to liquid (usually water) and the respective densities of the two phases. If the slurry composition is stated on a mass basis, the slurry density is calculated by calculating the volumes of each component of mass, and dividing the sum of the masses by the sum of the volumes. 

For comparing densities, the density of water is the reference for solids and liquids air is the reference for gases,... 

Currently, suspensions prepared from micronised active substances are the only marketed dehvery system for nebulisation of poorly water soluble substances such as steroids and cyclosporine [53]. Several problems are inherent in nebulising micro-suspensions and they vary from non-optimised lung deposition for the active substance to heterodispersity of the active substance concentration in the aerosol droplets and poor compatibility with different types of nebulisers, particularly ultrasonic devices. Suspensions may also have poor stability and the two components (solid and liquid) tend to separate with time within the formulation by sedimentation or flocculation, depending on the particle density relative to that of the liquid. Several jet nebulisers can deliver suspensions quite effectively, even independently of the primary particle size [54], but ultrasonic devices may convert primarily the continuous phase into aerosol whereas vibrating mesh inhalers can be blocked by particles being larger than the pore diameter of the membrane. 

Figure 3.11 Densities of solids and Uquids. (a) Water is unusual in that its solid phase, ice, will float on its Uquid phase, (b) Solid ethanol sinks to the bottom of the liquid. The solid form of almost aU substances is more dense than the liquid phase. 

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