chem / water.mdon commit ethanol practice prac sac (7d1ea8f)
   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
  77
  78## Molarity
  79- special concentration unit in mol / L (abbbreviation M)
  80$$c={n \over v}$$
  81
  82$$c_1v_2=c_2v_2=n\quad\leftarrow(\operatorname{constant})$$
  83
  84| Quantity               | Symbol  | Unit          |
  85| ---------------------- | ------- | ------------- |
  86| molar mass             | $M$     | g / mol       |
  87| molar concentration    | $c$     | mol / L ($M$) |
  88| specific heat capacity | $c$     | J / g / K     |
  89
  90## Precipitation reactions
  91Ions in solution combine to form a new compound.
  92Occurs when concentration exceeds solubility.
  93Precipitation reactions happen when water cannot dissociate ionic lattice structure.
  94Precipitate (product):
  95
  96- generally insoluble in water
  97- forms as solid particles that settle
  98- forms from compounds insoluble in water
  99
 100All group 1 ions are soluble.
 101
 102**Ions swap between reactants**
 103
 104## Ionic equations
 105
 106Used when a salt (ionic) dissolves into constituent ions.
 107"Spectator" ions - ions that do not dissolve or precipitate in reaction. **Not** included in equations.