WATER

The Importance of Water

Water is essential for all living things, and there is no one who can survive without it. About 3/4 of the Earth is covered by water, but most of it is saltwater, which makes up 72.8% of the total. Unfortunately, saltwater is not drinkable. The remaining 21.2% of water on Earth is freshwater, which we can use for drinking and other needs.

However, not all freshwater is easily accessible:

• About 60% of the freshwater is locked up in glaciers and ice caps, so it’s not usable.

• Around 30% of the freshwater is found underground in aquifers and wells.

• The remaining 10% is found in rivers, lakes, and other surface water that we can use.

Water in the Human Body

Water makes up about 60-70% of the human body (Khan Academy, 2018), meaning it is a crucial part of our physical makeup. In fact, it's not just humans who need water—most animals and even bacteria are also made up of water.

PROPERTIES OF WATER :

1. What is Polarity?

Water is a polar molecule, which means that its electric charge is not evenly spread out. This happens because of how its atoms—oxygen (O) and hydrogen (H)—share electrons.

Why is Water Polar?

• Oxygen is stronger than hydrogen when it comes to pulling electrons, so it keeps them closer to itself.

• This makes the oxygen side of the water molecule slightly negative (𝛿⁻) and the hydrogen side slightly positive (𝛿⁺).

• Since it has both a positive and negative side, water acts like a small magnet.

How Does Polarity Affect Water?

Because of its polarity, water molecules stick together through hydrogen bonds. This happens when the positive hydrogen of one water molecule attracts the negative oxygen of another. These bonds make water unique and give it special properties, like surface tension.

Why Can Water Dissolve So Many Things?

Water's polarity allows it to dissolve many substances, including salt (sodium chloride - NaCl):

1. Salt is made of sodium (Na⁺) and chloride (Cl⁻) ions, which are held together in a solid crystal.

2. When salt is placed in water, the negative oxygen side of water attracts the positive sodium ions (Na⁺), and the positive hydrogen side attracts the negative chloride ions (Cl⁻).

3. This pulls the salt apart and dissolves it into the water.

2. What is Hydrogen Bonding?

Since water molecules are polar, they are naturally attracted to each other. The positive end of one water molecule (hydrogen) is drawn to the negative end of another water molecule (oxygen). This attraction forms a hydrogen bond.

How Hydrogen Bonds Work

• Hydrogen bonds are weak interactions that happen between:

  • A hydrogen atom with a partial positive charge (𝛿⁺)

  • A more electronegative atom (one that pulls electrons strongly), like oxygen (O), nitrogen (N), or fluorine (F)

• For hydrogen bonding to happen, the hydrogen atom must be directly attached to one of these electronegative atoms.

Why Are Hydrogen Bonds Important?

• In Water: Hydrogen bonds hold water molecules together, which gives water its unique properties, like surface tension and capillary action (how water moves up plant roots).

• In Biology: Hydrogen bonds are also important in DNA and proteins, helping to hold their structures together.
  
  
  

3. Cohesion and Surface Tension in Water

Cohesion is when molecules stick to other molecules of the same kind. In water, this happens because hydrogen bonds make water molecules attract each other strongly.

How Cohesion Creates Surface Tension

One special effect of cohesion is surface tension—this is when the surface of a liquid resists breaking under pressure.

• Water molecules deep inside the liquid are surrounded on all sides, so they evenly bond with their neighbors.

• But at the surface, water molecules are only surrounded on the sides and below, while the top is exposed to air.

• Because of this, the surface molecules form even stronger bonds with their neighbors, creating a tight “skin” on the water.
  
  

4. What is Adhesion?

Adhesion happens when molecules of one kind are attracted to molecules of a different kind. Water has strong adhesion, especially to materials with positive or negative charges (like glass or plant cells).

Capillary Action – How Water Moves Upward

One important effect of adhesion is capillary action, which allows water to move upward against gravity.

• When a thin glass tube (capillary tube) is placed in water, the water sticks to the glass walls because of adhesion.

• At the same time, cohesion helps pull more water molecules up behind them.

• This process makes water climb the tube without needing any force.

Examples of Capillary Action

• Plants use capillary action to pull water from their roots to their leaves.

• Paper towels absorb water because of adhesion between the towel fibers and water molecules. 

• Ink spreads on paper due to capillary action.
  
  

5.Density - Why Ice is Less Dense Than Liquid Water

Water behaves differently from most substances when it freezes. Instead of becoming denser, it actually expands and becomes less dense. This is because of the way hydrogen bonds arrange water molecules when they freeze.

How It Works:

• In liquid water, molecules are close together and constantly moving.

• As water freezes, hydrogen bonds force the molecules into a structured pattern that spreads them farther apart.

• This creates more empty space, making ice less dense than liquid water.

Why This is Important:

• Since ice is less dense, it floats on water instead of sinking. This helps insulate lakes and oceans, keeping the water below warmer and protecting aquatic life during winter. 

• Water expanding when it freezes is why sealed bottles of soda or soup can crack or explode in the freezer!
  
  

6. What is Heat Capacity?

Water has a high specific heat capacity, which means it takes a lot of heat to raise its temperature.

• Specific heat capacity is the amount of heat needed to raise the temperature of 1 gram of a substance by 1°C.

• For water, this amount is called a calorie.
  
  
  

7. What is Evaporative Cooling?

Water has a high heat of vaporization, which means it takes a lot of energy to turn liquid water into gas.

• It takes about 540 calories to convert 1 gram of water into vapor at 100°C (boiling point).

• Even at lower temperatures, some fast-moving (high-energy) water molecules can escape from the surface as vapor.
  
  
  

8. Why is Water the Universal Solvent?

Water is called the universal solvent because it can dissolve many substances. This happens because water is polar, meaning it has a slightly positive side (hydrogen) and a slightly negative side (oxygen).

How Water Dissolves Substances

Water molecules interact with charged particles (like salt or sugar) using electrostatic forces—similar to how magnets attract opposite poles (+ and -).

1. Hydration Shells Form – When a substance dissolves in water, water molecules surround its particles in a three-dimensional sphere called a hydration shell.

2. Dissociation of Salt (NaCl in Water) –

   • Salt (NaCl) is made of positively charged sodium ions (Na⁺) and negatively charged chloride ions (Cl⁻).

   • The oxygen side of water (𝛿⁻) surrounds Na⁺ ions, while the hydrogen side (𝛿⁺) surrounds Cl⁻ ions.

   • This pulls the salt apart and spreads the ions throughout the water.
     
     

9. What Are Acids and Bases?

Solutions can be acidic or basic (alkaline) depending on the amount of hydrogen ions (H⁺) they have compared to pure water.

Understanding pH

The pH scale measures how acidic or basic a solution is. It is based on the concentration of hydrogen ions (H⁺):

• Acidic solutions have more H⁺ ions than pure water (H⁺ concentration > 1 × 10⁻⁷ M) and have a pH below 7.

• Basic (alkaline) solutions have fewer H⁺ ions than pure water (H⁺ concentration < 1 × 10⁻⁷ M) and have a pH above 7.

• Pure water is neutral, with a pH of exactly 7.
  
  

10. What is the pH Scale?

The pH scale measures how acidic or basic (alkaline) a solution is. It usually ranges from 0 to 14:

• pH < 7 → Acidic (More H⁺ ions)

• pH = 7 → Neutral (Pure water)

• pH > 7 → Basic (Alkaline) (Fewer H⁺ ions)

How the pH Scale Works

The pH scale is logarithmic, which means that each step is 10 times stronger than the previous one:

• pH 3 is 10× more acidic than pH 4.

• pH 9 is 10× more basic than pH 8.
  
  
  

11. What Are Buffers?

Buffers are solutions that resist changes in pH. They are important because they help maintain stable hydrogen ion (H⁺) concentrations in biological systems like our bodies.

How Buffers Work

Buffers help keep the pH from rising or falling too much. Here’s how they do it:

• If there are too many H⁺ ions (making the solution too acidic), the buffer will absorb some of them, raising the pH back to normal.

• If there are too few H⁺ ions (making the solution too basic), the buffer will release some of its own H⁺ ions to lower the pH.

Buffer Components

Buffers usually consist of a pair of substances:

• An acid that can donate H⁺ ions.

• A base that can accept H⁺ ions.

This is called a conjugate acid-base pair. The acid and base are closely related, differing only by the presence or absence of a proton (H⁺).

What is Freshwater?

Freshwater is water that doesn’t contain much salt and is essential for life on Earth. It makes up only about 3% of all the water on the planet, which makes it a finite (limited) resource.

What is the Water Cycle?

The water cycle (also known as the hydrologic cycle) is the process through which water circulates continuously between the Earth's surface and the atmosphere. This cycle is essential for distributing water around the planet and supporting life.

Key Processes of the Water Cycle

1. Evaporation – Water from oceans, rivers, lakes, and other bodies of water turns into vapor and rises into the air due to the heat from the Sun. 

2. Transpiration – Plants release water vapor into the air through small openings in their leaves, which also contributes to the movement of water. 

3. Condensation – Water vapor cools down and turns back into liquid water to form clouds in the atmosphere. 

4. Precipitation – When the water in the clouds becomes heavy enough, it falls back to the Earth's surface as rain, snow, sleet, or hail. 

5. Runoff – Water that falls on the Earth’s surface flows into rivers, lakes, and oceans, or it can soak into the ground as groundwater. 

How the Water Cycle Works

• The total amount of water in the water cycle remains roughly the same, but the distribution of water changes as it moves through these processes.

• Water constantly moves from one form to another, from liquid to vapor to solid and back.
  
  
  

The Three Main Processes of the Water Cycle

1. Evaporation

   • Evaporation is when water turns into vapor and rises into the atmosphere.

   • This is the main way water moves from the Earth's surface (oceans, lakes, rivers) into the air.

   • About 90% of the moisture in the atmosphere comes from evaporation from bodies of water, with the remaining 10% coming from plant transpiration (the process where plants release water vapor). 

2. Condensation

   • Condensation is the process where water vapor in the air cools and turns back into liquid water.

   • This process creates clouds as water vapor cools and condenses.

   • Condensation is the opposite of evaporation and plays a key role in forming clouds that will eventually produce precipitation. 

3. Precipitation

   • Precipitation happens when water falls from the clouds to the Earth's surface as rain, snow, sleet, or hail.

   • This is the main way water returns to Earth and re-enters the cycle, either flowing into rivers, lakes, or soaking into the ground.

   • Most precipitation falls as rain.
     
     
     

Types of Water

1. Ocean

   • Oceans are vast bodies of saltwater that cover more than 70% of Earth's surface.

   • Ocean currents play a huge role in controlling the global weather and support an amazing variety of life.

   • Humans rely on the oceans for food, transportation, and climate regulation, but global warming and overfishing are serious threats to ocean health. 

2. Ice/Icebergs

   • Icebergs are large chunks of freshwater ice that break off from glaciers or ice shelves and float in open water.

   • About 90% of an iceberg is submerged below the water's surface.

   • When icebergs drift into shallower waters, they may interact with the seabed, a process called seabed gouging.

   • Icebergs may eventually become pack ice, a type of sea ice. 

3. Rivers and Lakes

   • A river is a naturally flowing body of freshwater that typically flows toward an ocean, sea, or another river.

   • In some cases, rivers end by flowing into the ground and disappearing without reaching another body of water. 

   • Lakes are bodies of water surrounded by land and can be freshwater or saltwater, providing habitats and water resources.

4. Groundwater

   • Groundwater is water stored underground in spaces between soil, sand, and rock.

   • It is contained in and slowly moves through aquifers, which are geological formations that store water.

   • Groundwater is a crucial source of drinking water and irrigation in many areas.
     
     
     

What is Dissolved Oxygen?

Dissolved oxygen (DO) refers to the amount of gaseous oxygen (O₂) that is dissolved in water. This oxygen is essential for the survival of many aquatic organisms. Oxygen enters water through:

• Direct absorption from the atmosphere (when air mixes with water).

• Rapid movement of water, such as waves, waterfalls, or rapids, which helps mix oxygen into the water.

• Photosynthesis by aquatic plants, which produce oxygen as a by-product.

Water temperature and the movement of water can affect how much oxygen is dissolved in the water. Colder water can hold more oxygen, while warm water holds less.

Why is Dissolved Oxygen Important for Aquatic Life?

Dissolved oxygen is necessary for the survival of many aquatic organisms, including:

• Fish

• Invertebrates (like crabs, oysters, and worms)

• Bacteria

• Aquatic plants

These organisms use oxygen for respiration, just like animals on land. For example:

• Fish and crustaceans get oxygen through their gills.

• Plants and phytoplankton need dissolved oxygen for respiration when there’s no sunlight for photosynthesis.

Different organisms need different amounts of dissolved oxygen:

• Bottom feeders like crabs and oysters need only small amounts of oxygen (1-6 mg/L).

• Shallow water fish require higher levels of dissolved oxygen (4-15 mg/L).

Where Does Dissolved Oxygen Come From?

Dissolved oxygen enters water from:

1. The air – Oxygen can diffuse across the water’s surface from the atmosphere.

2. A plant by-product – Plants release oxygen into the water during photosynthesis.

Aeration (the mixing of oxygen into water) can happen naturally or through human activity:

• Natural aeration: Wind (causing waves), waterfalls, rapids, and groundwater discharge all mix oxygen into the water.

• Man-made aeration: Aquarium air pumps, waterwheels, or dams can also introduce oxygen into the water.

What is Eutrophication?

Eutrophication is the process where a body of water, like a lake or river, becomes overloaded with nutrients, such as nitrogen and phosphorus. This usually happens because of runoff from the land (like fertilizer or sewage), which brings these nutrients into the water.

What is Water Quality?

Water quality refers to the health of a body of water, such as a river, and how well it can support life and ecological processes.

mportant Water Properties

1. Dissolved Oxygen (DO)

   • Dissolved oxygen refers to the amount of oxygen that is dissolved in water. It is a crucial indicator of water quality because it tells us how much oxygen is available for aquatic life like fish and invertebrates.

   • High DO levels indicate healthy water, supporting thriving ecosystems, while low DO levels can harm aquatic life and signal pollution or other issues.

2. pH of Water

   • The pH of water measures how acidic or basic the water is, ranging from 0 (very acidic) to 14 (very basic), with 7 being neutral.

   • The pH affects the solubility and biological availability of chemicals in the water, such as nutrients (like nitrogen, phosphorus, and carbon) and heavy metals (such as lead, copper, cadmium).

   • A balanced pH is necessary for organisms to utilize nutrients and for chemicals to dissolve properly, helping sustain life in the water.

Ocean Water

Ocean water refers to the water from the oceans and seas, which cover more than 70% of the Earth's surface.

• About 97% of Earth's total water is in the oceans, with the remaining 2% stored in glaciers and ice caps.

• Less than 1% of Earth's water is freshwater, which is what humans use for drinking and irrigation.

Composition of Ocean Water

Ocean water contains a variety of minerals, with the largest percentages being:

• Chloride (55%)

• Sodium (31%)

• Magnesium (7.7%)

• Sulfur (3.7%)

• Calcium (1.2%)

• Others (1.4%)

These minerals give ocean water its salty taste and contribute to its unique properties. (Source: NOAA, 2018)

Salinity of Ocean Water

• Salinity refers to the amount of salt dissolved in water. Ocean water's salinity varies depending on factors like temperature and evaporation.

• For example, the Red Sea is the most saline sea, largely due to its high temperatures and limited water circulation, leading to high evaporation. As water evaporates, the salt is left behind, making the remaining water saltier.

Effects of Salinity in Ocean Water

Salinity impacts ocean water in several ways:

1. Phase Change (Freezing Point)

   • Higher salinity lowers the freezing point of ocean water. This means that the more salt there is, the harder it is for the water to freeze, as the salt disrupts the formation of ice crystals.

2. Buoyancy

   • The higher the salinity, the greater the buoyancy of the water. This means that objects are more likely to float in water with higher salinity.

   • For example, in the Red Sea, the high salinity makes it easier for people to float on the surface.

3. Density

   • Higher salinity also increases the density of ocean water. Denser water means it can support larger and heavier objects more easily.

Desalination Processes

Since less than 1% of Earth's surface water is freshwater, scientists have developed ways to desalinate ocean water to provide potable water (water safe for drinking and use in agriculture or industry). This is essential for residential homes, agriculture, and industrial purposes.

What is Desalination?

Desalination is the process of removing excess salt and minerals from seawater to create freshwater. This process makes ocean water suitable for human consumption and irrigation.

Two Methods of Desalination

There are two main methods of desalination:

1. Thermal/Distillation Processes

2. Membrane Processes

Thermal/Distillation Processes

The thermal/distillation process is one of the oldest methods of desalination. It works by heating seawater to create water vapor, then condensing the vapor back into liquid water to produce freshwater.

Types of Thermal/Distillation Processes

1. Multistage Flash Distillation (MSF)

   • Once Through MSF: In this process, ocean water is heated, passes through several chambers, and then is flashed (rapidly evaporated) to produce fresh water. The remaining brine (saltwater) is sent back to the ocean.

   • Brine Recirculation MSF: This method recycles the brine (saltwater) to increase the efficiency of desalination, allowing for higher freshwater production with less energy.

2. Multiple Effect Distillation (MED)

   • This process uses a series of vessels where seawater is evaporated at lower temperatures. The water vapor is then condensed to create fresh water. This method relies on the principle of evaporation and condensation.

3. Vapor Compression (VC)

   • This process is used for small- and medium-scale desalination units.

     • Mechanical Vapor Compression (MVC) uses an electromotor or diesel engine to compress vapor.

     • Thermal Vapor Compression (TVC) uses steam to drive the compression process. Both methods work similarly by compressing and heating the water vapor to remove salt.

Membrane Processes

1. Reverse Osmosis (RO)

   • Reverse Osmosis (RO) is a popular method where seawater is pushed through a semi-permeable membrane.

   • The membrane allows freshwater to pass through but blocks salt and other contaminants. To achieve this, the water is placed under high pressure, which is greater than osmotic pressure, forcing the freshwater to separate from the saltwater.

   • RO is widely used for large-scale desalination and provides a reliable source of freshwater.

The Importance of Water

Water is essential for all living things, and there is not a single organism that can survive without it. Approximately 75% of the Earth is covered by water, but it is crucial to note that most of this water is in the form of saltwater, which constitutes about 72.8% of the total water. Unfortunately, saltwater is not suitable for human consumption or agricultural purposes. The small proportion of water that is freshwater—about 21.2%—is vital for drinking, sanitation, agriculture, and industry. However, not all freshwater is easily accessible:

• About 60% of the freshwater is locked up in glaciers and ice caps, rendering it unusable for day-to-day activities.

• Approximately 30% of the total freshwater is found underground in aquifers and wells, which are crucial for irrigation and drinking water supplies.

• The remaining 10% is found in rivers, lakes, and other surface water bodies, which serve as primary sources for human consumption and ecosystem support.

Water in the Human Body

Water constitutes about 60-70% of the human body, as stated by Khan Academy (2018). This means that water is not only a vital nutrient but also a major component of our physical structure. In addition to humans, most animals and even bacteria are composed largely of water, emphasizing its universal importance across life forms.

PROPERTIES OF WATER

1. Polarity of WaterWater is a polar molecule, characterized by an uneven distribution of electrical charge. The polarity arises from the atomic structure of water, where the oxygen atom pulls electrons more strongly than hydrogen atoms, creating a slight negative charge on the oxygen (𝛿⁻) and a slight positive charge on the hydrogen (𝛿⁺). This polarity is responsible for water's unique properties, including its ability to form hydrogen bonds, enabling it to act like a small magnet.

2. Hydrogen BondingThe polar nature of water leads to the formation of hydrogen bonds between molecules. These weak attractions form when the positive hydrogen atoms of one water molecule are attracted to the negative oxygen atoms of another, an interaction pivotal for maintaining water's structure and properties such as surface tension and capillary action.

3. Cohesion and Surface TensionCohesion is the phenomenon where water molecules stick together due to hydrogen bonding, resulting in surface tension. This property allows water to resist external forces, leading to phenomena such as organisms being able to walk on water without sinking.

4. AdhesionAdhesion is the ability of water to stick to other substances. This property facilitates capillary action, essential for water transport in plants, where water moves upwards through roots and stems due to its attraction to the charged surfaces of plant cells.

5. Density BehaviorWater exhibits a unique behavior when it freezes; it expands and becomes less dense, causing ice to float on liquid water. This critical property helps insulate aquatic ecosystems in winter, as ice sheets prevent heat loss from the water below.

6. Specific Heat CapacityWater has a high specific heat capacity, meaning it requires a significant amount of heat to raise its temperature. This characteristic plays a crucial role in climate regulation and the habitat stability of aquatic systems.

7. Evaporative CoolingThe high heat of vaporization of water results in evaporative cooling, essential for temperature regulation in living organisms and ecosystems. This process helps prevent overheating by allowing superficial layers of water to evaporate, taking heat energy with it.

8. Water as a Universal SolventWater is often called the universal solvent due to its capacity to dissolve a wide range of substances. This property is central to biological systems, as nutrients dissolved in water can be transported within organisms.

9. Acids and BasesThe acidity or basicity of water solutions is measured on the pH scale, a vital aspect determining water quality, biological activity, and chemical availability in natural waters.

10. BuffersBuffers play a critical role in maintaining the pH balance in biological systems, compensating for changes in acidity that could be detrimental to life forms.

11. Freshwater and Its Finite NatureFreshwater is defined as any water source that contains low salt content and is crucial to life on Earth. It constitutes only about 3% of Earth’s total water supply, highlighting its status as a finite resource.

12. Water CycleThe water cycle describes the continuous movement of water through evaporation, condensation, precipitation, and runoff. This cycle is vital for maintaining ecosystem balance and distributing water across the planet. Understanding the water cycle's processes—including evaporation from oceans, transpiration from plants, and precipitation back to Earth—underlines the interconnectedness of water in environmental and human contexts.

13. Types of Water and Their ImportanceDifferent forms of water, such as oceans, icebergs, rivers, and groundwater, each play unique roles in the global ecosystem and human society. For instance, oceans support vast marine biodiversity and climate regulation, while freshwater sources are critical for human consumption and agricultural practices.

14. Dissolved OxygenDissolved oxygen is crucial for aquatic life, affecting the health of ecosystems and the survival of fish and other marine organisms, which depend on it for respiration. Factors like water temperature and movement influence oxygen levels, which can drastically affect biodiversity in aquatic habitats.

15. EutrophicationEutrophication, caused by nutrient overload in water bodies, can degrade water quality and harm aquatic ecosystems. This process often results from agricultural runoff and can lead to severe ecological imbalances.

16. Water QualityWater quality encompasses multiple parameters, including dissolved oxygen levels and pH, which reflect a body of water’s health and its capacity to support life. Ensuring water quality is paramount for environmental sustainability.

Conclusion

Water is an indispensable resource for life, ecosystems, and human society, with its various properties and behaviors influencing environmental conditions and health. Recognizing the finite nature of freshwater and the importance of the water cycle is crucial for sustainable management and preservation of this vital resource.