Physical Properties of Water and its Temperature Effects

Physical properties

Density of water The density of liquid water increases as temperature rises reaching maximum density of 1.000 g/cm3at 3.980C. Further warming causes the density of water to decrease One cubic meter of pure water at 10 999.70 kg; the same volume weighs 995.65 kg at 30 0C. 0C weighs

The relationship between salinity and density reveals that water at 200C with 30 g/L salinity has a density of 1.0210 g/cm3as compared to 0.99821 g/cm3for freshwater at the same temperature. This is a weight difference of 22.79 kg/m3. Differences in density related to salinity commonly cause density stratification in estuaries where rivers run into the sea.

Temperature of water Temperature is a measure of the internal thermal energy content of water. Heat content (the amount of thermal energy) is a function of temperature and volume. A liter of boiling water (1000C) in a beaker has a high temperature but small heat content when compared to water at 200C in a reservoir of million m3volume.

Water temperature in small lakes and in ponds closely follows air temperature. Because water can store much heat, larger bodies of water require time to warm up in the spring and to cool down in the temperature of large reservoirs and lakes tend to lag air temperatures. Stream temperature is effected by many factors such as slope and turbulence, precipitation, and forest cover. Nevertheless, there usually is a relatively good correlation between air temperature and stream temperature especially for smaller streams. autumn. Therefore, the

Effect of temperature and density on lakes: Heat is absorbed more strongly and more quickly within the upper layer of water than in lower layers. The transfer of heat from upper to lower layers of water depends mainly upon mixing by wind. Density temperature. Ponds and lakes may stratify thermally, because heat is absorbed more rapidly near the surface making the upper waters warmer and less dense than the deeper waters. of water is dependent upon water

Stratification occurs when differences in density between upper and lower strata become so great that the two strata cannot be mixed by the wind

Small water bodies with average depths of 1 m or less often thermally stratify during daylight hours, but they destratify at night when the upper layers cool by conduction. The stability of thermal stratification in a water body is related to the amount of energy required to mix its entire volume to a uniform temperature. The greater the energy required, the more stable is stratification.

The stratification may occur in areas where waters of different discharge into the ocean, freshwater will tend to float above the salt water because it is less dense. density of water varies with salinity, so salt contents converge. Where rivers

Color of water Many of the colors associated with water are not true colors, but the result of colloidal suspension. True Colors result from dissolved materials, most often Organics, most colors in natural waters result from dissolved Tannin extracted from decaying plant material, the result is a slightly brownish color. Many industrial waters are colored and if not properly treated can bring color to the receiving stream.

Odor Odors associated with water usually result from the presence of decaying organic matter, or in the case of mineral springs, the reduction of sulfate by bacteria to H2S gas. Decaying organic matter may accumulate in bottom to provide suitable condition for the anerobic bacteria that produce noxious gases. Source of the organic include plants wasted into the stream, died animals and waste water discharge. In addition to H2S, which has the classic odor of rooting egg, a number of other distinct smells are commonly encountered.

Particles Particles are defined as finely divided solids larger than molecules but generally not distinguishable individually by the unaided eye, although. The principal natural sources of particles in water are soil-weathering processes. Clays and silts are produced by weathering. Algae, bacteria, and other higher microorganisms are the predominant types of particles produced biologically.

In practice, the distinction between colloidal and suspended particles is blurred. However, suspended particles are generally larger than 1.0 m, while the size of colloidal particles will vary from about 0.001 to1 m. Many water treatment processes are designed to remove particles based on sedimentation and size exclusion. The type and size of various waterborne particles and processes used to remove them are presented in the figure below.

The total mass of particles may be estimated by filtering a volume of water through a membrane of known weight and pore size. Filtration of the same water sample through a series of membranes with incrementally decreasing pore sizes is known as serial filtration. Serial filtration may be used to determine an approximate particle size distribution

Electronic particle size counters estimate the particle size concentration by passing the sample through a laser beam and measuring the change in intensity due to light scattering. The change in light intensity is correlated to the diameter of an equivalent sphere.

The determination of particle size counts. A measured volume of sample is placed in a particle-counting cell and the individual particles may be counted, often with the use of a stain to enhance the particle contrast. Optical imaging software may also be used to obtain a more quantitative assessment of particle characteristics. Images of water particles are obtained with a digital camera attached to a microscope and sent to a computer for imaging analysis. The imaging software typically allows for the determination of minimum, mean, and maximum size, shape, surface area, aspect ratio, circumference, and centroid location. use of microscopic observation allows for the

Turbidity Turbidity in water is caused by the presence of suspended particles that reduce the clarity of the water. Turbidity is defined as an expression of the optical property that causes light to be scattered and absorbed rather than transmitted . Turbidity measurements require a light source and a sensor to measure the scattered light. This sensor is located at 90 to the light source. The measured turbidity increases as the intensity of the scattered light increases.

It is important to note that the scattering of light caused by suspended particles will vary with the size, shape, and composition of the particles. Also, as the number of particles increases beyond a given level, multiple scattering occurs, and the absorption of incident light is increased, causing the measured turbidity to decrease. Turbidity is expressed in nephelometric turbidity units (NTU). The regulatory standard for turbidity in finished water is 0.3 NTU, and many water treatment facilities have a treatment goal of <0.1 NTU, which is near the detection limit for turbidity meters.

In lakes or reservoirs, turbidity is frequently stable over time and ranges from about 1 to 20 NTU, excluding storm events. Turbidity in rivers is more variable due to storm events, runoff, and changes in flow rate in the river. Turbidity in rivers can range from under 10 to over 4000 NTU.