+# Water
+
+- <70% of Earth covered by water
+- polar, discrete molecular compound
+- latent heat - measured in kJ / mol or J / kg
+- fusion - solid to liquid (bonds between ice molecules must be broken)
+- vaporisation - liquid to gas
+- relatively high latent heat & heat capacity
+
+## Water cycle
+- clouds
+- rain
+- waterways
+- ocean
+- evaporation (sun)
+
+## Specific heat capacity
+- energy required to heat by one Kelvin
+- $Q=mc \Delta T$
+
+## Density
+- mass per volume (g / cm^3 or kg / m^3)
+- density of water depends on state
+- ice becomes less dense approaching $0^\circ$
+- water becomes less dense approaching $100^\circ$
+- maximum density at $4^\circ$
+
+## Conductivity
+- conductors have electrons which are free to move
+- water has covalent bonding (each atom needs one electron to be stable)
+- this means it has negligible conductance
+- slight conductance is due to self-ionisation
+
+## Hydrogen bonding
+- dispersion forces are weak (only three atoms)
+- H-bonding and covalent bonds hold water molecules together
+- electronegativities (H+, O-) attract H to O
+- since electrons are attracted to O, the other side of H atoms are free from -ve charge
+- +ve side of H atoms are attracted to -ve side of O atoms in other molecules
+- causes higher melting and boiling points, expansion when frozen (hexagonal structure), high latent heat, specific heat capacity, solvent properties
+
+## Solubility
+- "like dissolves like" - polar/non-polar substances
+- miscisble / immiscible
+- ionic crystals (salts) - polar, so soluble in water - "dissociation"
+- polar molecules may react to form ions (which may also dissolve)
+- other polar molecules may form hydrogen bonds
+- polar gases dissolve easily as well e.g. CO2 enables submarine photosynthesis
+- solubility is useful for living organisms (blood etc)
+- surfactants - polar + non-polar ends, dissolve in both oil + water
+
+## Measuring solubility
+- heterogeneous (mixtures) or homogeneous (solutions)
+- (un)saturated solution - maximum amount of solute for volume of solution
+- supersaturated solution - slow cooling of saturated solution
+- aqueous solutions - substance dissolved in water
+- most salts (ionic) are soluble in water to some extent
+- more soluble at higher temperatures
+- solubility measured in g / 100g H2O
+
+## Crystallisation
+- used to isolate substances based on $\Delta$ solubility in substances
+- can cause saturated or supersaturated solutions (unstable)
+- crystals precipitate out of (super)saturated solutions, can be collected by filtration
+- used to purify substances
+- $\Delta T$ can be substituted with pressure
+
+## Concentration
+- amount of solute per volume of solvent - e.g. g / L
+- relative terms - "concentrated" or "dilute"