Water itself is pure; but in nature, most water exists as a mixture containing other particles or chemicals. Those other things found in water may be insignificant; but sometimes they can also be very significant.

Water is a chemically active inorganic polar compound made from two atoms of Hydrogen and one of Oxygen bonded together, also known as H₂O. Water can react with metals and metal oxides to form ‘bases’, with non-metal oxides to form ‘acids’, and with specific organic compounds to form alcohols. 

All around the earth and atmosphere, water is known to serve as a temperature regulator: in the form of ice it creates a cooling effect, in the form of steam it creates a heating effect. By being the main component of our large bodies of water, such as Oceans and Lakes, it also helps balance our global climate. Pure water is an excellent solvent yet poor conductor of electricity, however, there are various forms of water which can make them good electrical conductors, such as presence of minerals and dissolved ions, among others. 

Among the physical properties of water, we can say that it is quite versatile. First of all, water is a major component of all living things as well as occupying approximately 70% of the earth’s surface. It also forms part of the air as vapour, as ice/glaciers in rivers and lakes, and as moisture in soil and underground aquifers. Although it is a colourless fluid, it can be seen in different colours depending on its surroundings. Due to its polarity (two positively charged hydrogen atoms are attracted to the negatively charged Oxygen atom), it can be accounted for its significant thermal properties (i.e. very high heat capacity), as well as the distinguishing particularity of being able to change forms considering its cohesive, adhesive and surface tension tendencies. Water also has the unique capacity to contract (become less dense as temperature decreases) or expand (become more dense when heated), allowing thus to take 3 major forms:

  1. Solid
  2. Liquid
  3. Gas

When water freezes to a temperature at or below 0°C (32°F), the water molecules become further apart and gather in the form of ‘ice’, which is less dense and, therefore, lighter than water (ice will float). Here, the molecules are tightly compacted forming a specific

Between 0°C and 100°C, water remains in its liquid, fluid form. In this state, water molecules can move around each other, taking any form or shape, depending on where it is confined (i.e. glass of water, basin).

At 100°C (212°F) water begins to boil, changing from liquid to gas, also known as ‘water vapour’. When in a gaseous state, the water molecules do not bond with each other and move very fast through the air. 

There are also 6 other stages involved in between these 3 main states of water. These are names given when there is a change of phase from one state to another, during which there will undoubtedly be a release or absorption of heat. 



Hydrological cycle, also known as Water cycle (Hydro=water), includes ‘water processes’ occurring in a variety of physical environments all around the world. It involves the stages of different types of water movements from one process to another within the cycle. The hydrological cycle involves continuous water movements, which can occur anywhere between the atmosphere, to the bodies of water (oceans, lakes, glaciers, rivers, etc), to the underground rock layers of the earth, and back again - which is why it is known as a ‘cycle’. There are 9 main physical water processes, which can inter-relate in any order, these include:
  • Evaporation pattern with open spaces between the bonds; these ‘air pockets’ are what makes ice lighter than water.
  • Precipitation
  • Condensation
  • Percolation
  • Infiltration
  • Interception
  • Transpiration
  • Drainage (runoff)
  • Storage

Evaporation happens when water (in the form of liquid) that is found in the surface of the earth - whether it’s in the form of rainfall, bodies of water (i.e. Oceans, lakes), or even water droplets on vegetation or on any object - returns back to the atmosphere in a gaseous form. For evaporation to eventuate, a reasonable amount of solar heat needs to be present, as well as other influencing factors (i.e. atmospheric pressure, air temperature, among others). As evaporation occurs, moist air is transported around the atmosphere, it cools down and condenses to form clouds. Most evaporation processes come from the Ocean (around 80%), and the rest occurs through the rest of the earth’s waters (including a portion of rainfall), land and vegetation.

It is called precipitation when any water particle leaves the atmosphere (i.e. rainfall from clouds) and hits anywhere onto the surface of the earth, whether it’s a body of water or the ground itself. These particles of water can reach the surface either as water droplets or ice crystals (i.e. snow), depending on the occurring temperature happening at cloud level, or even during its way ‘down’, which can then dissipate in a number of different ways.

Basically condensation occurs when water vapour, in gaseous state, changes to liquid state. The presence of condensation is represented in the form of clouds, fog or dew. This mainly occurs when there is a large concentration of ‘water vapour’ up to its saturation point or when cooling of the air is present during evaporation.

Just as the word implies, percolation is when water infiltrates via gravitational forces through the soil, rocks, and other layers of the earth, into its deeper layers. This water, now referred to as ‘groundwater’, continues moving along to the deeper layers by gravity, which will eventually serve as natural water reservoirs.

This is when water particles accumulate in the surface of the earth’s soil and begins penetrating and filling  the pores of the soil into the deeper matrices.  The amount and speed of infiltration will directly depend on the porosity and permeability of the soil, as well as the composition, structure and moisture content. 

Interception is referred to when the movement of water is blocked or suspended by land ‘obstructions’ such as loose vegetation, stems of plants, branches, leaves, among others, interfering with the water's’ natural flow to reach the soil. Some factors may influence on interception such as the presence of wind, rain, or when the weight of the water stored is greater than the surface tension,

This is a process that occurs to vegetation. Depending on the amount of sunlight and the type of plant, transpiration will be more or less. When plants produce transpiration, the nutrients inside the plant move to the surface leaves and, as they transpire, the plant acquires a cooling effect from the amount of sun exposed. When there is not enough water to be transpired, plants tend to close their ‘stomata’, which are the cells specialised in letting water out. In this case, transpiration continues but at a much slower pace, allowing some of the water to be retained in the plant.

Runoff occurs when water, mainly from rain, underground infiltration and/or surface drainage flow downward and meet a stream, river or lake; which is then carried into the Ocean.

‘Stored water’ can exist, as previously mentioned, in the atmosphere (i.e. snow), on the earth’s surface and underground. Stored water can be found in lakes, oceans, glaciers, underground aquifers, and soil, among others.