To kick start this series, let’s take a moment and just think about water. What is water, really? What makes up this life-giving element that takes up over 70% of Earth’s surface?

Did you know that water molecules are one of the most versatile molecules in the natural world? Allow me to take you back to grade/primary school science for a brief moment. Remember the water properties that make it unique? Polarity? Hydrogen bonds that make surface tension, cohesion and adhesion possible? Universal solvent? Any of these ring any bells? No…? Okay — let me give you a quick refresher course on some of the amazing properties that make water the most awesome molecule in the whole natural world — so much so that, regardless of how contaminated water is, it can be recovered, treated and reused over and over again.

Water Properties
Polarity: A water molecule consists of an oxygen atom (O)covalently bound to two hydrogen atoms (H). Since O is more electronegative than H, an unequal sharing of electrons occurs. This creates a polar covalent bond with H having a partial positive charge and O having a partial negative charge. All of water’s unique physical properties are caused by water’s polarity.

Bonds that hold water molecules & constituent atoms together. [Source]

Hydrogen Bonds: Water molecules have a weak partial negative charge at one region of the molecule (the oxygen atom) and a partial positive charge elsewhere(the hydrogen atoms in water). When the water molecules are close together, opposites attract (positive regions are attracted to negative regions and vice versa). The force of attraction is called a hydrogen bond. Each water molecule is hydrogen bonded to four others.

It is because of these hydrogen bonds that water has the following properties:

Cohesion and adhesion. [Source]
  1. Cohesion:- water molecules sticking to each other. This is the reason water is found in drops; perfect spheres
  2. Adhesion:- water molecules stick to other surfaces.
  3. Surface Tension:- the hydrogen bonds between neighboring molecules cause a ‘film’ to develop at the surface. This gives water the ability to support small objects like insects
  4. Capillary action: — A combination of cohesion and adhesion enables water to ‘climb’ structures against gravity e.g. in plant stems or a water straw / paper towel
  5. Universal Solvent: Water is able to dissolve a wide variety of substances so it is considered to be the universal solvent. The polarity of water attracts or dissolves any other polar or charged particles by forming hydrogen bonds with them. Because of this property, substances can generally be described as:

Hydrophilic (water-friendly) — a substance that has an affinity for water. These substances can interact or be dissolved by water e.g. detergents, alcohols and salts.

Hydrophobic (water-fearing) — substances that repel water. They are non-polar and interact well with other non-polar solvents e,g. oils and wax.

Worth mentioning also are the thermal properties of water. Water changes into different states (liquid, gas and solid) depending on the conditions it is exposed to through the processes of evaporation, condensation, freezing, melting and sublimation. However, for these changes to take place, they require high amounts of energy i.e

  1. High specific heat — energy required to change water temperature
  2. High heat of vaporization — energy required to boil water
  3. High heat of fusion — loss of energy required to freeze water

Application of Water Properties

Now that we are up to date with our water-science knowledge — you might ask: Why are these properties important? What consequences do they have in real-life applications, especially in the context of water and sanitation?

For starters, the thermal properties of water coupled with the strong hydrogen bonds between polar water molecules cause water to generally resist change thus offering a stable environment for aquatic organisms. During winter, when lakes begin to freeze, the surface freezes into ice as it expands, allowing it to float. Because of this, the ice surface insulates the water beneath from the cold temperature outside, allowing it to stay liquid and maintain a temperature adequate for ecosystems living in the lake to survive.

Perhaps one of the most important applications of the water physical and thermal properties is during the water cycle. Water’s ability to change states from liquid to vapor through evaporation allows not only the separation of salts from the water molecules in the ocean but also the movement of water molecules to the atmosphere where it condenses (thermal property) to form clouds. Clouds are formed when many water molecules (cooled water vapor) collide with and stick to tiny particles in the air called condensation nuclei (e.g salts and dust) because of adhesion (with the condensation nuclei) and cohesion (between the water molecules). When precipitation occurs ( in the form of rain, snowfall or hail), water falls in drops because of cohesion. The water drops stick to leaves and surfaces (adhesion) and flow as surface runoff in the form of streams and rivers or subsurface flow because of infiltration(cohesion & adhesion). Substances along the water-flow paths will either be dissolved or be repelled because of the dipolarity nature of water and carried along until the water finds its way back to the ocean where the water cycle continues.

The Water Cycle. [Source]

In the context of water and sanitation — water needs to be cleaned and treated before it can be used for drinking, cooking or any other human activity- domestic or economical. Manipulating the properties of water (both physical and thermal) makes treating and cleaning water possible. Since water is a scarce resource, water usage needs to be conservative and sustainable.

Water scarcity — insufficient freshwater resources to meet human and environmental demands of a given area — is a looming problem of global scale that is projected to only get worse. There is a need to recycle waste water as well as abstract water from alternative sources such as the ocean as one of the ways to stem this looming problem. Nature recycles water at a grand-scale through the water cycle making this is our primary source of freshwater. Borrowing a leaf from nature as well as understanding and harnessing the unique properties of water will allow us to further develop and improve technology that will enable desalination of brackish groundwater and seawater and effective wastewater recycling and reuse.

To sum all this up — water has amazing properties regardless of the source and level of contamination. We can harness and manipulate its physical and thermal properties to not only recover clean drinking water but also tap into alternative water sources such as brackish groundwater, seawater or recycled wastewater. This will provide reliable and sustainable water resources to meet both human and environmental needs. So next time you are about to have a glass of water or you are by the lake or the ocean somewhere, take time to consider the amazing properties of water and how, just like nature, we can harness these properties to provide sustainable and reliable access to clean water and sanitation for all.

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References

  1. Greater Egg Harbor Regional High School District (2019, October 15). Water Properties. Retrieved from https://www.gehrhsd.net/site/handlers/filedownload.ashx?moduleinstanceid=14177&dataid=4277&FileName=Properties%20of%20Water%20Foldable.pdf
  2. UCDavis, University of California LibreText (2019, October 15). Unusual Properties of Water. Retrieved from https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Physical_Properties_of_Matter/States_of_Matter/Properties_of_Liquids/Unusual_Properties_of_Water
  3. Green, W. (2019, October 15). Water. Retrieved from https://www.mrgscience.com/topic-22-water.html
  4. Petruzzello, M. (2019, October 15). Water Cycle. Retrieved from https://www.britannica.com/science/water-cycle
  5. Petruzzello, M. (2019, October 15). Water Scarcity. Retrieved from https://www.britannica.com/topic/water-scarcity