1# Water 2 3- <70% of Earth covered by water 4- polar, discrete molecular compound 5- latent heat - measured in kJ / mol or J / kg 6- fusion - solid to liquid (bonds between ice molecules must be broken) 7- vaporisation - liquid to gas 8- relatively high latent heat & heat capacity 9 10## Water cycle 11- clouds 12- rain 13- waterways 14- ocean 15- evaporation (sun) 16 17## Specific heat capacity 18- energy required to heat by one Kelvin 19- $Q=mc \Delta T$ 20 21## Density 22- mass per volume (g / cm^3 or kg / m^3) 23- density of water depends on state 24- ice becomes less dense approaching $0^\circ$ 25- water becomes less dense approaching $100^\circ$ 26- maximum density at $4^\circ$ 27 28## Conductivity 29- conductors have electrons which are free to move 30- water has covalent bonding (each atom needs one electron to be stable) 31- this means it has negligible conductance 32- slight conductance is due to self-ionisation 33 34## Hydrogen bonding 35- dispersion forces are weak (only three atoms) 36- H-bonding and covalent bonds hold water molecules together 37- electronegativities (H+, O-) attract H to O 38- since electrons are attracted to O, the other side of H atoms are free from -ve charge 39- +ve side of H atoms are attracted to -ve side of O atoms in other molecules 40- causes higher melting and boiling points, expansion when frozen (hexagonal structure), high latent heat, specific heat capacity, solvent properties 41- hydroxyls (OH) dissolve easily in H2O due to H-bonding 42 43## Solubility 44- "like dissolves like" - polar/non-polar substances 45- miscisble / immiscible 46- ionic crystals (salts) - polar, so soluble in water - "dissociation" 47- polar molecules may react to form ions (which may also dissolve) 48- other polar molecules may form hydrogen bonds 49- polar gases dissolve easily as well e.g. CO2 enables submarine photosynthesis 50- solubility is useful for living organisms (blood etc) 51- surfactants - polar + non-polar ends, dissolve in both oil + water 52- solubility depends on polar / non-polar proportion 53 54## Measuring solubility 55- heterogeneous (mixtures) or homogeneous (solutions) 56- (un)saturated solution - maximum amount of solute for volume of solution 57- supersaturated solution - slow cooling of saturated solution 58- aqueous solutions - substance dissolved in water 59- most salts (ionic) are soluble in water to some extent 60- more soluble at higher temperatures 61- solubility measured in g / 100g H2O 62 63## Heating and cooling 64- solubility $\propto$ temperature, so when substance is cooled, the solute is precipitated out into crystals 65 66## Crystallisation 67- used to isolate substances based on $\Delta$ solubility in substances 68- can cause saturated or supersaturated solutions (unstable) 69- crystals precipitate out of (super)saturated solutions, can be collected by filtration 70- used to purify substances 71- $\Delta T$ can be substituted with pressure 72 73## Concentration 74- amount of solute per volume of solvent - e.g. g / L 75- relative terms - "concentrated" or "dilute" 76- mg / L = ppm = $\mu$g / g