Properties of Water Flashcards
Properties of Water
Density
Liquid at room temperature
Good solvent
Cohesion and surface tension
High specific heat capacity
High latent heat of vaporisation
Reactant
Density:
•If water was less dense aquatic organisms wouldn’t be able to float.
•When water gets cooler it gets denser until 4°C. From 4°C to freezing point, because of its polar nature, water molecules align themselves in a structure that is less dense than liquid water (molecules held further apart).
Note: most liquids just denser as they cool, so their solid forms are denser than liquid.
Therefore, ice is less dense than water:
•Aquatic organisms have a stable environment to live in through the winter (if ice was denser it would sink to the bottom of the pond/lake – killing aquatic organisms).
•Ponds and other bodies of water are insulated against extreme cold. Ice reduces rate of heat loss.
•Provides a surface for arctic animals (e.g. polar bears/seals) to walk on.
Liquid at room temperature:
•The hydrogen bonds between molecules make it more difficult for them to escape and become a gas. Even with its hydrogen bonds water has quite a low viscosity so it can flow easily (as hydrogen bonds constantly form and break).
As it is a liquid at room temp it:
•Provides habitats for living things in rivers, lakes and seas.
•Forms a major component of tissues of living organisms.
•Provides a reaction medium for chemical reactions.
•Provides an effective transport medium (e.g. blood and vascular tissue – xylem/phloem).
Good solvent
•It is a good solvent for ionic solutes (NaCl) and covalent solutes (glucose). Because water is polar the positive and negative parts of the molecule are attracted to the positive and negative parts of the solute. Water molecules cluster around these charged parts of the solute helping to separate them and so they dissolve.
Because Water is a good solvent:
•Molecules and ions can move about and react in water. This happens in the cytoplasm (70% water).
•Molecules and ions can be transported around living things more easily whilst dissolved in water.
Cohesion and surface tension:
•Cohesion: attraction between water molecules due to hydrogen bonding. Makes drops of water spherical.
•Adhesion: attraction to other polar molecules.
•Surface Tension: At the surface of water, the water molecules are hydrogen bonded to molecules below, and hence more attracted to them than the air molecules above. This means the surface of the water contracts (molecules pulled inwards) – giving the surface an ability to resist a force applied to it.
Because of cohesion and surface tension:
•Columns of water in plant vascular tissue are pulled up the tissue together from the roots – cohesion and adhesion to the xylem tissue – transpiration stream
•Insects like pond-skaters can walk on water – surface tension.
•Meniscus forms as glass is polar (water molecules attracted to it – adhesion), air is non-polar to water molecules only attracted to other water molecules resulting in surface contracting.
High specific heat capacity:
•Water temperature is a measure of the KE of the molecules. Water molecules are held tightly due to hydrogen bonds, so lots of heat energy is required to increase KE and temperature (so no rapid temp changes) – specific heat capacity is the heat energy required to raise 1 kg of water by 1 °C (4.2kJ).
Water does not heat up/cool down quickly, it has a high specific heat capacity which is important as
water is the main component of many living things:
•Living things including prokaryotes and eukaryotes, need a stable temperature for enzyme- controlled reactions to happen properly. Aquatic organisms need a stable environment to live in.
High latent heat of vaporisation:
When water evaporates, heat energy, called latent heat of vaporisation helps molecules to break away from each other to become a gas. To break hydrogen bonds, a large amount of energy is required, therefore
water helps cool things and keep their temperature stable:
•Mammals are cooled when sweat evaporates.
•Plants are cooled when water evaporates from mesophyll cells.
Reactant:
Water is a reactant in photosynthesis, and hydrolysis reactions (digestion of starch, lipids and proteins). Its properties as a reactant do not directly draw on its polarity, but its role as a reactant is extremely important in digestion/synthesis of large biological molecules.
