About Water

What is water?

A colorless, transparent, odorless, liquid which forms the seas, lakes, rivers, and rain and is the basis of the fluids of living organisms.

Where does your water come from?

A public water system? A private well? A plastic bottle?
No matter what the source of your water, you need to know the facts about what you drink.

Properties of Water

Water exists in all the three states. In solid state it is termed ‘ice’ and exists in this phase at temperature 0 deg C or below, at atmosphere pressure. It is liquid at atmospheric pressure and in the temperature range 0 deg – 100 deg C. At 100 deg and atmospheric pressure water boils to the vapour phase, it is called ‘steam’ in the gas phase.

The following are the important physical properties with respect to water treatment.


Pure water has a density of 1.0 kg/l at 4 deg C and atmospheric pressure. The density decreases at other temperature. In practice the numerical value of the density of pure water is considered as unity.

The density of water increases with salinity. Sea water with a salinity of 3.5 g/l has a density of 1.028 kg/l at 0 deg C.

Thermal Properties

The specific heat of water at 0 deg C is 1 kcal/kg deg C. The latent heat of fusion is 79 kcal/kg and the latent heat for vaporization is 539 kcal/kg at normal pressure and at 100 deg C. Due to this large specific heat and latent heat of vaporization, water used as a heat transfer fluid.


This is the ability of a liquid to resist movements such as flow. It is the basic cause of head loss and therefore plays an important part in water treatment. Viscosity of water reduces with increase in temperature. At 20 deg C, the absolute viscosity of pure water is 1.007 mPa s (1 centi poise approx.)

Viscosity of water increases with higher content of dissolved salts. Sea water is therefore more viscous than river water.

The absolute viscosity of sea water with salinity of 3.5 g/l is 1.085 mPa s 20 deg C.

Surface Tension

This is properly peculiar to boundary surfaces of two phases. It is a tensile force which is exerted at the surface of the liquid and which tends to reduce the area of this surface to the greatest possible extent.

Surface tension diminishes with a rise in temperature. The addition of dissolved salts generally increases surface tension. The substances which reduce surface tension are called surface-active agents.

Osmotic Pressure

This parameter is essential in the sizing of reverse osmosis systems. Osmotic pressure describes a phenomenon which occurs between liquids with different concentrations that are separated by a semi-permeable membrane.

Sea water with salinity of 35 g/l exerts an osmotic pressure of 14.38 x 105 Pa (208.5 psi) at 15 deg C.

Electrical Conductivity

Pure water is slightly conductivity. The conductivity of pure water is 0.037 µS/Cm at 15 deg C.

The presence of dissolved salt in water increases its conductivity which varies with the type of salt, its concentration and the solution temperature.

Characteristics of Water

Water found in nature is never pure. Water may be characterized on the basis of contaminants, also by its physical parameters, chemical composition and biological impurities.

Physical Parameters

The physical parameters comprise mainly suspended solids, turbidity, silt density index, color, odor and taste.

Suspended Solids
This includes all the matter suspended in water that is large enough to be retained on a filter with a given porosity.

Together with the measurement of suspended solids, this gives an indication of the level of fine suspended impurities, matter of inorganic or organic origin etc. that are so fine that they achieve colloidal dimensions.

Silt Density Index
It is a measure of the fouling potential of water. It is also linked with suspended solids and is essential in water treatment using membrane processes.

Colours in the water are due to dissolved matters & not suspended solids.

True color after filtration is most often due to the presence of dissolved or colloidal organic matter. There is no relationship between color and the quantity of organic matter. Color is measured by comparing the specimen with a reference solution (Platinum – Cobalt method)

Odour and Taste
The bad taste and smell of water is generally caused by the presence of bacteria, germs and various microorganisms and their secretions and excretions in water. It is due to organic matter and gases such as hydrogen sulphide dissolved in water.

Chemical Composition

The chemical composition of water gives the parameters and the constituents of various chemical species present in water.

It has the chemical formula H2O.

Sources of Water

The sources of natural water are Groundwater, Surface water and Sea water

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