Water Properties PDF

Water - the medium of life Evidence (geological) shows that water has existed on earth at least 3.8 billion years, Earth is 4.5 billion years old Most of earth’s history has been with water, expected to be much earlier than 3.8 billion years ago Life started in w...

Water - the medium of life Evidence (geological) shows that water has existed on earth at least 3.8 billion years, Earth is 4.5 billion years old Most of earth’s history has been with water, expected to be much earlier than 3.8 billion years ago Life started in water and covers 70% of the earth’s surface, living organisms consist of 60-95% water - How much water is found in a human? Water is an important medium for our processes of life in the cell  What is water? Water is a bent molecule, consisting of one oxygen atom and two hydrogen atoms, H2O The oxygen end is slightly negative and hydrogen end, slightly positive → Polar covalent bonds Why? Oxygen has higher electronegativity and “pulls” electrons towards itself Therefore, water forms an electrostatic attraction called hydrogen bond between other water molecules or other “charged” surfaces  Hydrogen bonds - key for its properties These hydrogen bonds give water a range of properties: Cohesion Adhesion Solvent Physical properties Thermal (heat) conductivity Viscosity Buoyancy Specific heat capacity Let’s draw!  Properties of water - cohesion Cohesion = Attractive force between identical molecules Many hydrogen bonds generate a cohesive force that sticks water molecules together  Biological implication of cohesion Cohesion makes it difficult to separate water molecules resulting in high surface tension Enables small animals to walk and transport of water up a tree Vessels called Xylem Properties of water - adhesion Adhesion = Attractive force between different molecules Electrostatic interactions between water and polar surfaces in combination with cohesion enables capillary action Positively charged surface  Biological implication of adhesion Capillary action is the ability of a liquid to flow in narrow spaces in opposition to gravity Capillary action is necessary for water to be transported up plant stems  Properties of water – a good solvent The polarity of water makes it a good solvent for charged or polar substances Water molecules form shells around dissolved particles Interactions between water molecules and dissolved particles overcome the H-bonds between water molecules Water as a solvent - Hydrophilic and hydrophobic Substances or molecules that dissolve well in in water are termed hydrophilic Such as: Ions, smaller molecules with a polarity Substances or molecules that do not dissolve well are termed hydrophobic Such as: Oils/fats, larger molecules, or molecules lacking a polarity Basis for our cell membranes  Physical properties vs. Chemical properties Physical properties are those measurable changes or characteristics of any type of matter that does not change the chemical components of it, such as: Force (N) Length (M) Volume (L) Temperature (C) Water’s prominent physical properties is explained by hydrogen bonding, these bonds are absent in air  Properties of water – specific heat capacity Specific heat capacity = The energy required to raise the temperature of 1 g of substance by 1°C (J·g-1·°C-1) Water has high specific heat capacity (much higher than air) - extensive hydrogen bonding counteracts movement between water molecules  Biological implication of high heat capacity To increase the temperature of water a lot of energy (heat) needs to be absorbed to overcome the H-bonds between water molecules Water resists rapid temperature changes Water is an optimal medium for living organisms that depend on stable conditions  Thermal conductivity Thermal conductivity is the capacity of something to transfer heat Water has much higher thermal conductivity than air, Waterborne mammals like the Ringed seal (Pusa ripida) have “blubber” which prevents loss of heat We also sweat to get rid of heat!  Buoyancy Buoyancy is the upward thrust a liquid exerts upon an object ○ Higher in water than air The seal has poor buoyancy in water due to its weight (density) Needs sheets of ice to steadily rest above the surface ○ Due to the lower density of ice, it stays afloat above water surface  Viscosity Viscosity is the resistance to flow - water has much higher viscosity than air If you live in water you often adapt the following features: Streamlining Flippers/Fins Minimise projections Like the seal!  Black-throated loon (Gavia arctica) The black-throated loon also needs to adapt to the properties just discussed How does it differ from the Ringed seal? Discuss in relation to the following physical properties by comparing adaptations to air and water: Buoyancy Viscosity Thermal conductivity Specific heat capacity