Physical & Chemical Properties of Water (HL) (DP IB Environmental Systems & Societies (ESS))

Physical & Chemical Properties of Water

• Water has some unique physical and chemical properties

• These properties make water crucial to life on Earth

Polarity and hydrogen bonding

• Water molecules are polar

  • This means they have a slight positive charge on one side (near the hydrogen atoms) and a slight negative charge on the other (near the oxygen atom)

• This uneven distribution of charge is called polarity

  • Polarity allows water to form hydrogen bonds between its molecules

  • The slightly positive hydrogen atom in one water molecule is attracted to the slightly negative oxygen atom in another

  • This creates weak bonds between water molecules

  • Collectively, however, these bonds are strong enough to give water its unique properties

  

Cohesion

• Cohesion is the result of hydrogen bonding between water molecules

• This makes water molecules 'stick' together, allowing it to form droplets and maintain surface tension

  • For example, water droplets form beads on a waxy surface, and insects like water striders (also known as pond skaters) can move across water without sinking

Adhesion

• Adhesion occurs when water molecules stick to other materials

• This can help water to move through narrow spaces

  • This is seen in capillary action, an import process occurring in the xylem during transpiration

  • This is where water moves up plant stems against gravity, which is essential in order for plants to transport water from their roots to their leaves

Solvent properties

• The polarity of water also makes it an effective solvent

  • This means it can dissolve many substances, especially salts and polar molecules

• This ability is essential for life because it allows water to transport important substances like nutrients, minerals, and gases within living organisms

  • For example, in the human body, water dissolves salts, sugars, and proteins

  • This enables them to be carried in the bloodstream to cells

Transparency

• Water is transparent, meaning light can pass through it

• This transparency is essential for aquatic ecosystems because it allows sunlight to penetrate underwater

• This enables photosynthesis in aquatic plants and algae, which support aquatic food webs

  • In lakes and oceans, the depth to which sunlight penetrates determines the zone where photosynthetic organisms can survive

High specific heat capacity

• Water has a high specific heat capacity

  • This means it can absorb a large amount of heat energy without significantly increasing its temperature

• This property allows water to regulate environmental and body temperatures, maintaining stable conditions for life

  • For example, oceans absorb heat from the sun during the day and release it slowly at night

    • This moderates coastal climates and prevents extreme temperature changes

  • This property is also essential in maintaining body temperature in animals, as water in blood helps distribute heat evenly

    • This prevents rapid overheating or cooling

Density and temperature

• Water has an unusual density pattern; it is most dense at 4 °C

• As it cools below this temperature and freezes, water expands, making ice less dense than liquid water

  • This is why ice floats

• This property is vital for life in cold environments

  • For some species, it provides a valuable habitat, e.g. the floating sea ice enables polar bears to hunt seals

• Also, when ice forms on the surface of lakes or oceans, it acts as an insulating layer

• This protects aquatic life by preventing the water below from freezing completely

  • Without this property, whole bodies of water could freeze solid in cold climates, disrupting ecosystems and killing many species

Gas solubility in water

• Water can dissolve gases like oxygen and carbon dioxide

• The solubility of these gases varies with temperature and pressure.

  • Cold water generally holds more dissolved oxygen than warm water

  • This is why cold aquatic environments can often support more life.

    • For example, the Arctic and Antarctic oceans, with their cold temperatures, support rich ecosystems due to the high oxygen levels in the water

  • When water gets too warm, its ability to hold oxygen decreases

    • This can lead to problems like oxygen depletion in warmer waters, causing "dead zones" where aquatic life struggles to survive