What is water?

The water cycle is undergoing changes due to climate change, something that entails great uncertainty about the potential effects on life on Earth.

1 . What is the definition of water?

Water is a substance made up of two hydrogen atoms and one oxygen atom (H2O), and can be found in solid state as ice, in gaseous state as water vapour, and in liquid state as water. The physical and chemical properties of water are very important for the survival of ecosystems.

2 . Characteristics of water

The characteristics of water include chemical, physical and biological properties, and depending on its contents there can be different types of water (freshwater, salt water, soft water, hard water, etc.). The main characteristics of water are outlined below:

The density of water is about 1 gram per millilitre. Water has the highest specific heat of any liquid (4,180 J/(K kg)), although it varies with temperature. The latent heat of vaporisation (enthalpy of vaporisation) required to break a hydrogen bond and form water vapour is very high (539 kcal/kg). The surface tension of water is very high.

Furthermore, the colour, turbidity and conductivity of water are used as water quality parameters.

3 . Water on Earth is present in different states

Water is the most abundant substance in the planet and the only one that can in found in the atmosphere in liquid, solid and gaseous state. 97% of it is water found in the ocean, and the rest is freshwater.

4 . The water cycle

The water cycle or hydrological cycle is a continuous process that includes the different states (solid, liquid and gas), and allows natural processes to take place on Earth.

The phases of the cycle are: evaporation, condensation, precipitation, infiltration, runoff, subsurface runoff, fusion, and solidification.

5 . How is freshwater obtained?

Freshwater is obtained through precipitation, which starts with the evaporation of water from the ocean to form water vapour. Gradually, rising air currents move water vapour to the upper layers of the atmosphere, where cooler temperatures allow water to condense into clouds, until water falls as precipitation.

A large part of the precipitation falls as rain and it is ends up in aquifers and permafrost, lakes, reservoirs, rivers, and in the soil, and becomes available for consumption. The remainder of that precipitation falls as snow and accumulates in the polar ice caps and in glaciers, so it cannot be consumed. 

6 . The importance of water

Water is important for ecosystems, organisms and human activity:

The water cycle is of utmost importance for the functioning of natural ecosystems, and the regulation of climate.  Water makes up 80% of most organisms: it allows tissues and organs to work and maintain vital body processes. The most common water uses are agricultural, industrial and domestic uses. Continuous population growth increases the demand for such a limited resource.