The Nature of Pure Water

Natural water always contain dissolved and suspended substances of organic and
mineral origin. These enter the water with atmospheric precipitation and from soils with
which water comes into contact in underground streams or in surface water bodies (rivers,
lakes, etc.). Moreover, water is polluted with the metabolites of aquatic plants and animals
and the products of their decay.

Plankton: Plants and animals suspended in water are called ‘Plankton’.

Benthos: The organisms whose habitat is the bottom are called ‘Benthos’. These two aquatic
associations have a great effect on the composition of natural waters.

Ground water contains mostly dissolved substances, while surface waters are rich in
suspended matter. The large amounts of suspended solids are present in open bodies during
autumn and spring floods.

There are several classifications of natural waters. They are based on different approaches
to the problem.

By their origin, waters are classified as:

(1) Atmospheric waters (precipitates).
(2) Subterranean (spring, wells).
(3) Surface waters (rivers, lakes, seas).

By the amount and character of impurities: Waters are divided into:

(1) Fresh water
(2) Salt water
(3) Soft water
(4) Clear water
(5) Colourless water
(6) Turbid water
(7) Coloured water etc.

The composition of natural waters usually varies with time.

- Mineral and organic suspended matter gradually settles by gravity. Part of the organic
matter is consumed as food by the living organisms.
- The chemical and biological processes occurring in natural waters destroy the readily
oxidizable organic substances.
- The formation of hydroxides of iron, manganese and aluminium and the binding of
colloidal admixtures in water by these hydroxides also affect the composition of
natural water.
- The bulk of the organic matter in natural waters are humins, proteins, fats,
hydrocarbons, carbonic acids and vitamins also occur in the water, but their presence
is insignificant compared with that of organic compounds.
The mineral composition of water is quite varied. Seawater is rich in dissolved
substances and is unfit for drinking, or domestic and industrial uses.

Salt content of sea water g/litre:

 Baltic Sea — 7.5
Black Sea — 18.0
North Sea — 32.8
Pacific Ocean — 33.6
Atlantic Ocean — 36.0
Mediterranean Sea — 39.4
Red Sea — 43

The main salt contained in the water of the seas and oceans in sodium chloride (common
salt). The approximate salt composition of sea water is as follows:
NaCl – 83.67%
MgSO4 – 3.60%
KCl – 0.03%
MgCl2 – 8.50%
CaSO4 – 4.20%

The water in some lakes is even more salty, up to 5.82 g/litre in some of the seas in
Central Asia, and even 360 g/litre in Gusgundag, Transcaucasus.

Seawater is mostly water (H2O). In fact it is about 96.5 wt % water. Water affects
almost all ocean characteristics (density, salinity, gas solubility) and processes (circulation,
heat-exchange, climate, biochemistry). Thus, water has been called the universal solvent.
Water has unique and unusual properties both in pure form and as a solvent. These
properties influence many of the chemical reactions taking place in the oceans.

The water molecule is unique in structure and properties.

• H2O is the chemical formula for water.

• Unique properties of water include:
– Higher melting and boiling point than other hydrogen compounds.
– High heat capacity, amount of heat needed to raise the temperature of one
gram of water 1oC.
– Greater solvent power than any other substance.
• Water molecules are asymmetrical is shape with the two hydrogen molecules at one
end, separated by 105o when in the gaseous or liquid phase and 109.5o when ice.

Water occurs on Earth as a solid, a liquid, and a gas.

 Fills closed containers
uniformly PISTON Readily compressed
into smaller volume
Molecules are in high
speed motion More frequent collision
Density very low

Molecules close to each

Free upper surface other, glide easily past
one another
Free flowing at lower
layer

Solid (crystalline)
Molecules locked into
Strong rigid
strict geometric order
Fractures when sudden,
strong stress is applied
High density

Water molecule

A water molecule is composed of two hydrogen atoms and one oxygen atom. Water is a polar
molecule, having a positive and a negative side.
Melting point of water is 0oC
Boiling point of water is 100oC

Water and heat

Heat capacity is the measure of the heat required to raise the temperature of 1 gram of a
substance by 1oC. Different substances have different heat capacities. Because of the great
strength and large number of the hydrogen bonds between the water molecules, water can
gain and lose large amounts of heat with very little change in temperature. Thus water has a
large thermal buffer capacity and acts as a climate buffer. Energy transport by water transport
in currents is large. This thermal inertia moderates the temperatures worldwide. Of the
common substances, only liquid ammonia has a higher heat capacity than liquid water.

Heat capacity of common substances

Substance Heat capacity (cal/g/oC)
Silver 0.06
Granite 0.20
Aluminum 0.22
Alcohol (ethyl) 0.30
Gasoline 0.50
Acetone 0.51
Pure water 1.00
Ammonia (liquid) 1.13
Water temperature and density

The density of water at 20oC temperature is 0.9981 g/cm3. As the heat is removed density
slowly increases (at 10oC temperature density is 0.9997 g/cm3) and attains a maximum
density of water (1 g/cm3) is at 3.98oC. The density of water at 0oC is 0.999 g/cm3 and abrupt
decrease in density is observed with freezing of water to ice to 0.917 g/cm 3. That’s why ice
floats on water.

Why lakes don't freeze solid all the way to the bottom? Or how fish survive under freezing
water conditions?

The key is that the density of freshwater reaches a maximum at 4 oC (see above Fig.). This
explains why lakes and streams freeze from top down and rarely freeze solid all the way to
the bottom. As the lakes get colder in the winter the water gets denser and sinks to the bottom
until it gets colder than 4°C, then the water is lighter and can't displace the deeper water. So it
only freezes at the surface and the bottom water remains at 4 oC all winter. That way the fish
survive.

Temperature, Salinity, and Water Density

Density of water depends on salinity levels as well as temperature. As the salinity of water
increases the density of water also increases. Irrespective of the salinity levels, as the
temperature of water increases, the density of water decreases.

Sea water

Sea water consists of water with various materials dissolved within it.

• The solvent is the material doing the dissolving and in sea water it is the water.
• The solute is the material being dissolved.
• Salinity is the total amount of salts dissolved in the water.
– It is measured in parts of salt per thousand parts of salt water and is
expressed as ppt (parts per thousand) or abbreviated %.
Average salinity of the ocean is about 35%.
Solutes in water: Ionic salts

99% of all the salt ions in the sea are sodium (Na +), chlorine (Cl-), sulfate (SO4-2), Magnesium
(Mg+2), calcium (Ca+2) and potassium (K+).

• Sodium and chlorine alone comprise about 86% of the salt in the sea.
• The major constituents of salinity display little variation over time and are a
conservative property of sea water.

Solutes in water: Nutrients and Organics

Nutrients are chemicals essential for life.

• Major nutrients in the sea are compounds of nitrogen, phosphorus and silicon.
• Because of usage, nutrients are scarce at the surface and their concentrations are
measured in parts per million (ppm).
• Concentrations of nutrients vary greatly over time and because of this they are
considered a neoconservative property of the sea.
Marine organic compounds occur in low concentrations and consist of large complex
molecules, such as fat, proteins, carbohydrates, hormones and vitamins, produced by
organisms or through decay.

Solutes in water: Gases and Trace elements

In order of decreasing abundance the major gases in the sea are nitrogen, oxygen, carbon
dioxide and the noble gases, argon (Ar), neon (Ne) and helium (He). Nitrogen and the noble
gases are considered to be inert because they are chemically non-reactive. Trace elements
occur in minute quantities and are usually measured in parts per million (ppm) or parts per
billion (ppb). Even in small quantities they are important in either promoting life or killing it.

Addition of salt modifies the properties of water.

• Pure water freezes at 0oC. Adding salt increasingly lowers the freezing point because
salt ions interfere with the formation of the hexagonal structure of ice.
• Density of water increases as salinity increases.
• Vapor pressure is the pressure exerted by the gaseous phase on the liquid phase of a
material. It is proportional to the amount of material in the gaseous phase.
• Vapor pressure decreases as salinity increases because salt ions reduce the
evaporation of water molecules.

Properties of seawater

Water is called the universal solvent because of its ability to dissolve at least a little of
virtually every substance. Water is a particularly good solvent for substances held together by
polar or ionic bonds. Indeed, the most abundant substances dissolved in seawater are ionic
solids (salts such as sodium chloride).

The density is increased: Any substance dissolved in a liquid has the effect of increasing the
density of that liquid. The greater the amount of solute, the greater the effect. Since the
density of seawater increases continuously with increase in temperature, and there is no
density maximum like for freshwater, we never see the strange density inversion like in
freshwater lakes.

The freezing point is depressed: This is why salt is spread on frozen roads. Salts also lower
the temperature at which water reaches its maximum density. That is because dissolved salts
inhibit the tendency of water molecules to form direct bonds with other water molecules.

The boiling point is elevated: The salts have the effect of making the water molecules
cluster and become more ordered, thus harder to pull apart and evaporate.

The conductivity is increased: If an electromagantic field is applied to seawater, the ions

will migrate, producing an electric current.

Comparison of seawater and pure water properties

Property Seawater (35%) Pure water
3
Density (g/cm ) 1.02412 1.0029
Specific conductivity ohm-1 cm-1 25oC 0.0532 --
o
Temperature of maximum density, C -3.25 3.98
o
Freezing point C -1.91 0.00
Surface tension, dyne cm-1, 25oC 72.74 71.97
Specific heat, J g-1 oC-1, 17.5oC 3.898 4.182
Viscosity, millipoise 9.02 8.90
-1 o
Velocity of sound, m s , 0 C 1450 1407
Vapor pressure, mm Hg, 20oC 17.4 17.34
Many factors affect water quality

 Sedimentation
 Runoff
 Erosion
 Dissolved oxygen
 pH
 Temperature
 Decayed organic materials
 Pesticides
 Toxic and hazardous substances
 Oils, grease, and other chemicals
 Detergents
 Litter and rubbish